My Science School

A compound is a substance that is created when two or more elements are bonded by a chemical reaction. It is difficult to split a compound back into its original elements. Compounds do not necessarily take on the characteristics of the elements that form them. For example, sodium is a metal and chlorine is a gas. Together they form a compound called sodium chloride, which is not like either of them. In fact, sodium chloride is the chemical name for the salt that we put on our food.

In chemistry, a compound is a substance that results from a combination of two or more different chemical elements, in such a way that the atom s of the different elements is held together by chemical bonds that are difficult to break. These bonds form as a result of the sharing or exchange of electron s among the atoms. The smallest unbreakable unit of a compound is called a molecule.

A compound differs from a mixture, in which bonding among the atoms of the constituent substances does not occur. In some situations, different elements react with each other when they are mixed, forming bonds among the atoms and thereby producing molecules of a compound. In other scenarios, different elements can be mixed and no reaction occurs, so the elements retain their individual identities. Sometimes, when elements are mixed, the reaction occurs slowly (as when iron is exposed to oxygen); in other cases it takes place rapidly (as when lithium is exposed to oxygen). Sometimes, when an element is exposed to a compound, a reaction occurs in which new compounds are formed (as when pure elemental sodium is immersed in liquid water).

Often, a compound looks and behaves nothing like any of the elements that comprise it. Consider, for example, hydrogen (H) and oxygen (O). Both of these elements are gases at room temperature and normal atmospheric pressure. But when they combine into the familiar compound known as water, each molecule of which contains two hydrogen atoms and one oxygen atom (H 2 O), the resulting substance is a liquid at room temperature and normal atmospheric pressure.

The atoms of a few elements do not readily bond with other elements to form compounds. These are called noble or inert gases: helium, neon, argon, krypton, xenon, and radon. Certain elements readily combine with other elements to form compounds. Examples are oxygen, chlorine, and fluorine.

1: Pure water is a compound made from two elements - hydrogen and oxygen. The ratio of hydrogen to oxygen in water is always. Each molecule of water contains two hydrogen atoms bonded to a single oxygen atom.

2: Pure table salt is a compound made from two elements - sodium and chlorine. The ratio of sodium ions to chloride ions in sodium chloride is always.

3: Pure methane is a compound made from two elements - carbon and hydrogen. The ratio of hydrogen to carbon in methane is always.

4: Pure glucose is a compound made from three elements - carbon, hydrogen, and oxygen. The ratio of hydrogen to carbon and oxygen in glucose is always.

Elements do not usually exist on their own. In the natural world, they are found in combination with other elements. By understanding how elements combine, scientists have been able to make new combinations, creating molecules that are not found in nature. These combinations are not made simply by mixing two or more substances together. Brown sugar and salt can be stirred together, for example, but this does not create a new substance. Each little particle is either a grain of sugar or a grain of salt — they have remained separate. Mixtures can usually be separated again, but when elements are chemically joined together, they are said to be bonded and have created a new substance.

Bonding is caused by a chemical reaction. Most chemical reactions need some form of energy to start them. Usually, this energy is supplied in the form of heat. Many compounds are made by heating two or more substances together until their molecules are moving so fast that they react with each other.

Energy plays a key role in chemical processes. According to the modern view of chemical reactions, bonds between atoms in the reactants must be broken, and the atoms or pieces of molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds, and energy is evolved as bonds are made. In some reactions the energy required to break bonds is larger than the energy evolved on making new bonds, and the net result is the absorption of energy. Such a reaction is said to be endothermic if the energy is in the form of heat. The opposite of endothermic is exothermic; in an exothermic reaction, energy as heat is evolved. The more general terms exoergic (energy evolved) and endoergic (energy required) are used when forms of energy other than heat are involved.

 A great many common reactions are exothermic. The formation of compounds from the constituent elements is almost always exothermic. Formation of water from molecular hydrogen and oxygen and the formation of a metal oxide such as calcium oxide (CaO) from calcium metal and oxygen gas are examples. Among widely recognizable exothermic reactions is the combustion of fuels (such as the reaction of methane with oxygen mentioned previously).

The formation of slaked lime (calcium hydroxide, Ca (OH)2) when water is added to lime (CaO) is exothermic. This reaction occurs when water is added to dry Portland cement to make concrete, and heat evolution of energy as heat is evident because the mixture becomes warm.

Not all reactions are exothermic (or exoergic). A few compounds, such as nitric oxide (NO) and hydrazine (N2H4), require energy input when they are formed from the elements. The decomposition of limestone (CaCO3) to make lime (CaO) is also an endothermic process; it is necessary to heat limestone to a high temperature for this reaction to occur. The decomposition of water into its elements by the process of electrolysis is another endoergic process. Electrical energy is used rather than heat energy to carry out this reaction.

Generally, evolution of heat in a reaction favours the conversion of reactants to products. However, entropy is important in determining the favorability of a reaction. Entropy is a measure of the number of ways in which energy can be distributed in any system. Entropy accounts for the fact that not all energy available in a process can be manipulated to do work.

A chemical reaction will favour the formation of products if the sum of the changes in entropy for the reaction system and its surroundings is positive. An example is burning wood. Wood has low entropy. When wood burns, it produces ash as well as the high-entropy substances carbon dioxide gas and water vapour. The entropy of the reacting system increases during combustion. Just as important, the heat energy transferred by the combustion to its surroundings increases the entropy in the surroundings. The total of entropy changes for the substances in the reaction and the surroundings is positive, and the reaction is product-favoured.

When we cook food, chemical reactions take place as het energy is supplied to the ingredients. New compounds are formed, so that the cooked dish usually has a different appearance, texture and taste from the mixed raw ingredients.

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The amount of space that a substance takes up is called its volume. It is measured in cubic units. For example, a cube measuring one metre on each side has a volume of one cubic metre or 1m3. But a cubic metre of lead has a much greater mass than a cubic metre of wood. That is because the lead has a much higher density than the wood. Its particles are more tightly packed together. The density of an object is calculated by dividing its mass by its volume and is expressed as kilograms per cubic metre (kg/m3) or pounds per cubic foot (lb/ft3).

Density is a measure of how compact the mass in a substance or object is. The density of an object or substance can be calculated from this equation: density in kilograms per meter cubed is equal to mass in kilograms, divided by volume in meters cubed. Or in other words, density is mass spread out over a volume. Or in other, other words, it's the number of kilograms that 1 meter cubed of the substance weights. If each meter cubed weighs more, the substance is denser.

As we'll discuss in other lessons, density is super important because it relates to whether things rise or sink. Less dense materials tend to rise above more dense materials, particularly in the case of liquids and gases. So understanding density has major implications for the motions of materials and gases in the atmosphere and objects floating (or sinking) in water. Density is the reason some objects sink and other objects float. And it's the reason that some clouds are high in the sky, while others are low down.

Density means that if you take two cubes of the same size made out of different materials and weigh them, they usually won't weigh the same. It also means that a huge cube of Styrofoam can weigh the same as a tiny cube of lead.

Examples of dense materials include iron, lead, or platinum. Many kinds of metal and rock are highly dense. Dense materials are more likely to 'feel' heavy or hard. Although a sparse material (sparse is the opposite of dense) can feel heavy if it's really big. Examples of sparse materials would be Styrofoam, glass, soft woods like bamboo, or light metals like aluminum.

In general, gases are less dense than liquids and liquids are less dense than solids. This is because solids have densely-packed particles, whereas liquids are materials where particles can slide around one another, and gases have particles free to move all over the place.

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The mass of a substance is the amount of matter it contains. This is different from its weight, which is a measurement of the pull of gravity on this mass. For example, an astronaut would have the same mass on Earth as on the Moon, but his weight would be much less in the Moon’s gravity than in the Earth’s.

We use the word mass to talk about how much matter there is in something. (Matter is anything you can touch physically.) On Earth, we weigh things to figure out how much mass there is. The more matter there is, the more something will weigh. Often, the amount of mass something has is related to its size, but not always. A balloon blown up bigger than your head will still have less matter inside it than your head (for most people, anyhow) and therefore less mass.

The difference between mass and weight is that weight is determined by how much something is pulled by gravity. If we are comparing two different things to each other on Earth, they are pulled the same by gravity and so the one with more mass weighs more. But in space, where the pull of gravity is very small, something can have almost no weight. It still has matter in it, though, so it still has mass.

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As well as heating or cooling, changing the pressure acting on a substance can also cause it to change state. If the pressure on the molecules in a substance is increased, it becomes harder for them to move apart from each other, so the temperature at which they become a liquid is increased. Similarly, at low pressure, changes happen at lower temperatures. It is impossible to make a good cup of tea or coffee' at the top of Everest, for. example, because water boils at a temperature almost 30°C (50°F) less than at sea level.

All matter can move from one state to another. It may require extreme temperatures or extreme pressures, but it can be done. Sometimes a substance doesn't want to change states. You have to use all of your tricks when that happens. To create a solid, you might have to decrease the temperature by a huge amount and then add pressure. For example, oxygen (O2) will solidify at -361.8 degrees Fahrenheit (-218.8 degrees Celsius) at standard pressure. However, it will freeze at warmer temperatures when the pressure is increased.

Some of you know about liquid nitrogen (N2). It is nitrogen from the atmosphere in a liquid form and it has to be super cold to stay a liquid. What if you wanted to turn it into a solid but couldn't make it cold enough to solidify? You could increase the pressure in a sealed chamber. Eventually you would reach a point where the liquid became a solid. If you have liquid water (H2O) at room temperature and you wanted water vapor (gas), you could use a combination of high temperatures or low pressures to solve your problem.

One winter day, you sit by a window inside your warm home. You watch the snow pile up on the ground. You see small animals slide across a frozen pond in your backyard. You can see their hot breath as steam clouds in the cold air. You are drinking a cup of cocoa. You see steam rising from the mug, and you know it is too hot to drink. So you add an ice cube to the cup and wait for the melting ice to cool your cocoa. Solids, liquids, and gases are all around you. The solid ice in the pond, the liquid cocoa, and the steamy air are different states of matter. What is matter? How are solids, liquids, and gases different? Why did the solid ice cube melt into liquid when you put it into your cocoa?

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When molecules are heated, they gain heat energy in addition to the kinetic energy they already have. If the molecules in a solid gain enough energy, they can break free of each other and become liquid. This is called melting. If they gain even more heat energy, the liquid becomes a gas.

Water freezes, or becomes solid, at temperature of 0oC (32oF) or below. If the temperature outside drops to this level, the water on the surface of ponds and lakes will freeze, although the water below may hold enough heat to remain liquid.

When solid water (ice) is heated, it melts to become liquid. Generally speaking, we think of water as being liquid at a “room temperature” of 200C (68oF), or, in other words, under normal conditions, Copper, however, is a solid under such conditions, because it needs a temperature of 1083oC (1981oF) to melt into a liquid.

When water is heated and boils, it turns into a gas. We can see this when a kettle boils. In fact, it is not the billowing steam that is the gas – that is the water turning back in tiny droplets of liquid as it comes into contact with cool air. The real steam is invisible. It can be “seen” in the gap between the spout of the kettle and the visible vapour.

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In nature, it is rare to find one atom on its own. Atoms are usually grouped together in larger structures called molecules. A molecule is the smallest particle of a substance that can exist by itself. The atoms in a molecule are chemically bonded together. They may be atoms of the same element or they may be of different elements. A molecule of carbon dioxide, for example, has two atoms of oxygen and one of carbon.

For millennia, scientists have pondered the mystery of life – namely, what goes into making it? According to most ancient cultures, life and all existence was made up of the basic elements of nature – i.e. Earth, Air, Wind, Water, and Fire. However, in time, many philosophers began to put forth the notion that all things were composed of tiny, indivisible things that could neither be created nor destroyed (i.e. particles).

However, this was a largely philosophical notion, and it was not until the emergence of atomic theory and modern chemistry that scientists began to postulate that particles, when taken in combination, produced the basic building blocks of all things. Molecules, they called them, taken from the Latin “moles” (which means “mass” or “barrier”). But used in the context of modern particle theory, the term refers to small units of mass.

By its classical definition, a molecule is the smallest particle of a substance that retains the chemical and physical properties of that substance. They are composed of two or more atoms, a group of like or different atoms held together by chemical forces.

It may consist of atoms of a single chemical element, as with oxygen (O2), or of different elements, as with water (H2O). As components of matter, molecules are common in organic substances (and therefore biochemistry) and are what allow for life-giving elements, like liquid water and breathable atmospheres.

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Many substances can exist in three different states of matter: as solids, liquids and gases. In each state, the substance has the same chemical make-up — the elements in its molecules have not changed, but the way in which they are connected to each other has. Scientists think of all matter as being constantly in motion. The atoms and molecules of which it is made have energy, called kinetic energy.

Solids have a set shape and volume. In a solid, the particles are more closely arranged. The particles have less net movement. Solids are rigid, meaning that the particles are “locked” into place. Solids are not easily compressible, given that they have a set volume. Solids do not flow easily, because the particles are rigidly connected and do not move past one another. In solids, the energy is not strong enough for the particles to break free of the attraction they have for each other. It is as though they are vibrating but not moving from their positions.

Liquids have a set volume, but they can change shape depending on what container they are in. The particles in a liquid move more freely within the set volume. Liquids do not form structures like solids, because of the intermolecular movement. Liquids are also not easily compressible, because of the set volume. Liquids flow easily because the particles are able to move past each other easily. The molecules in a liquid have more energy and can move away from neigh-boring molecules, so that a liquid will flow to cover as wide an area as it can.

Gases can change shape and volume. Gas particles will spread out as far as they need to in order to fill the container. They will take the shape and volume of whatever container they're in. They are easy to compress because there is a lot of space between the particles. It also flows easily because of the amount of space between particles. The molecules of a gas have most kinetic energy. They will move apart from each other until they fill the space in which they are contained.

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Although the idea for the machine had already been suggested by other scientists, the Geiger counter was perfected by a German physicist called Hans Geiger (1882-1945).

The German physicist Hans Wilhelm Geiger is best known as the inventor of the Geiger counter to measure radiation. In 1908, Geiger introduced the first successful detector of individual alpha particles. Later versions of this counter were able to count beta particles and other ionizing radiation. The introduction in July 1928 of the Geiger-Muller counter marked the introduction of modern electrical devices into radiation research.

Geiger, the eldest of 5 children of a professor of philology, was born on September 30, 1882, in Neustadt an der Hardt, Rhineland-Palatinate state in western Germany (about 20 miles southwest of Mannheim). He studied physics at the universities of Munich and Erlangen in Bavaria, Germany, and received the PhD degree from the latter university in 1906. At the University of Erlangen, he worked with Eilhard Wiedemann (1852-1928) and wrote a thesis on electrical discharges through gases.

Geiger, the eldest of 5 children of a professor of philology, was born on September 30, 1882, in Neustadt an der Hardt, Rhineland-Palatinate state in western Germany (about 20 miles southwest of Mannheim). He studied physics at the universities of Munich and Erlangen in Bavaria, Germany, and received the PhD degree from the latter university in 1906. At the University of Erlangen, he worked with Eilhard Wiedemann (1852-1928) and wrote a thesis on electrical discharges through gases.

In 1913, Geiger was joined by two physicists, Walther Bothe (1891-1957), later the 1954 Nobel Prize winner in physics, and James Chadwick (1891-1974), later Sir James Chadwick and winner of the 1935 Nobel Prize in physics. Bothe investigated alpha scattering, and Chadwick counted beta particles. The work was interrupted in 1914, with the beginning of World War I (1914-1918). Geiger served in the German army in the field artillery.

After the war, Geiger returned to his work, and in 1924, he used his device to confirm the Compton Effect, namely, the increase in wavelength of electromagnetic radiation, especially of an X-ray or gamma-ray photon, scattered by an electron. The Compton Effect was discovered by the American physicist Arthur Holly Compton (1892-1962), for which he was awarded the 1927 Nobel Prize in physics.

In 1925, Geiger accepted his first teaching position, which was at the University of Kiel, Germany. Here, he and Walther Müller improved the sensitivity, performance, and durability of the counter, and it became known as the “Geiger-Müller counter.” It could detect not only alpha particles but also beta particles (electrons) and ionizing photons. The counter was essentially in the same form as the modern counter.

In 1929, Geiger moved to the University of Tübingen (Germany), where he was named professor of physics and director of research at the Institute of Physics. In 1929, while at the Institute, Geiger made his first observations of a cosmic-ray shower. Geiger continued to investigate cosmic rays, artificial radioactivity, and nuclear fission after accepting a position in 1936 at the Technische Hochschule in Berlin, a position he held until his death. In 1937, with Otto Zeiller, Geiger used the counter to measure a cosmic-ray shower.

During World War II (1939-1945), Geiger participated briefly in Germany’s abortive attempt to develop an atomic bomb. In June 1945, Geiger fled the Russian occupation of Berlin and went to nearby Potsdam, where he died on September 24, 1945, at the age of 62 years, less than 2 months after the American atomic bomb was dropped on Hiroshima, Japan.

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A Geiger counter measures the radiation being given off by a substance. It has both a dial, giving a reading, and a loudspeaker that transmits a regular clicking sound if radiation is detected. The faster the clicking, the more radiation there is.

A Geiger counter is a metal cylinder filled with low-pressure gas sealed in by a plastic or ceramic window at one end. Running down the center of the tube there's a thin metal wire made of tungsten. The wire is connected to a high, positive voltage so there's a strong electric field between it and the outside tube.

When radiation enters the tube, it causes ionization, splitting gas molecules into ions and electrons. The electrons, being negatively charged, are instantly attracted by the high-voltage positive wire and as they zoom through the tube collide with more gas molecules and produce further ionization. The result is that lots of electrons suddenly arrive at the wire, producing a pulse of electricity that can be measured on a meter and (if the counter is connected to an amplifier and loudspeaker) heard as a "click." The ions and electrons are quickly absorbed among the billions of gas molecules in the tube so the counter effectively resets itself in a fraction of a second, ready to detect more radiation. Geiger counters can detect alpha, beta, and gamma radiation.

The sound of a Geiger counter is often associated with nuclear weapons and fallout. While it is useful in these situations, it is also used every day for the detection and control of nuclear waste, by-products and exposure in nuclear power plants, hospitals and even mines.

These ingenious devices allow anyone to detect potentially harmful radiation around them, using the power of electrons and the degradation of unstable radioactive atoms.

The detector is the main part of the Geiger counter. It is responsible for capturing, detecting and then signalling that a radioactive particle, known as a radioactive isotope, has passed through the detector.

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Nuclear fusion releases so much energy that it is hard to control. At the moment, only nuclear fission is used to give nuclear power. In power stations with pressurized water reactors, a radioactive substance, such as uranium, is bombarded with neutrons so that its atoms split and release energy. This energy heats water. The resulting steam turns a turbine to create electricity. Nuclear power has also been used to power submarines. One problem with nuclear power is that the waste material left behind is still radioactive and must be disposed of safely.

Nuclear energy is the energy in the nucleus, or core, of an atom. Atoms are tiny units that make up all matter in the universe, and energy is what holds the nucleus together. There is a huge amount of energy in an atom's dense nucleus. In fact, the power that holds the nucleus together is officially called the "strong force."

Nuclear energy can be used to create electricity, but it must first be released from the atom. In the process of nuclear fission, atoms are split to release that energy.

A nuclear reactor, or power plant, is a series of machines that can control nuclear fission to produce electricity. The fuel that nuclear reactors use to produce nuclear fission is pellets of the element uranium. In a nuclear reactor, atoms of uranium are forced to break apart. As they split, the atoms release tiny particles called fission products. Fission products cause other uranium atoms to split, starting a chain reaction. The energy released from this chain reaction creates heat.

The heat created by nuclear fission warms the reactor's cooling agent. A cooling agent is usually water, but some nuclear reactors use liquid metal or molten salt. The cooling agent, heated by nuclear fission, produces steam. The steam turns turbines, or wheels turned by a flowing current. The turbines drive generators, or engines that create electricity.

Rods of material called nuclear poison can adjust how much electricity is produced. Nuclear poisons are materials, such as a type of the element xenon, that absorb some of the fission products created by nuclear fission. The more rods of nuclear poison that are present during the chain reaction, the slower and more controlled the reaction will be. Removing the rods will allow a stronger chain reaction and create more electricity.

As of 2011, about 15 percent of the world's electricity is generated by nuclear power plants. The United States has more than 100 reactors, although it creates most of its electricity from fossil fuels and hydroelectric energy. Nations such as Lithuania, France, and Slovakia create almost all of their electricity from nuclear power plants.

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Nuclear weapons were first used in the Second World War. Two bombs were dropped on the Japanese cities of Nagasaki and Hiroshima, killing hundreds of thousands of people.

On August 6, 1945, the United States dropped an atomic bomb on the Japanese city of Hiroshima. It killed or wounded nearly 130,000 people. Three days later, the United States bombed Nagasaki. Of the 286,00 people living there at the time of the blast, 74,000 were killed and another 75,000 sustained severe injuries. Japan agreed to an unconditional surrender on August 14, 1945; it also resulted in the end of World War II. Scientists at Los Alamos had developed two distinct types of atomic bombs by 1945—a uranium-based design called “the Little Boy” and a plutonium-based weapon called “the Fat Man.”

While the war in Europe had ended in April, fighting in the Pacific continued between Japanese forces and U.S. troops. In late July, President Harry Truman called for Japan’s surrender with the Potsdam Declaration. The declaration promised “prompt and utter destruction” if Japan did not surrender.

On August 6, 1945, the United States dropped its first atomic bomb from a B-29 bomber plane called the Enola Gay over the city of Hiroshima, Japan. The “Little Boy” exploded with about 13 kilotons of force, leveling five square miles of the city and killing 80,000 people instantly. Tens of thousands more would later die from radiation exposure.

When the Japanese did not immediately surrender, the United States dropped a second atomic bomb three days later on the city of Nagasaki. The “Fat Man” killed an estimated 40,000 people on impact.

Nagasaki had not been the primary target for the second bomb. American bombers initially had targeted the city of Kokura, where Japan had one of its largest munitions plants, but smoke from firebombing raids obscured the sky over Kokura. American planes then turned toward their secondary target, Nagasaki.

Citing the devastating power of “a new and most cruel bomb,” Japanese Emperor Hirohito announced his country’s surrender on August 15—a day that became known as ‘V-J Day’—ending World War II.

In subsequent years, the United States, the Soviet Union and Great Britain conducted several nuclear weapons tests. In 1954, President Jawaharlal Nehru of India called for a ban on nuclear testing. It was the first large-scale initiative to ban using nuclear technology for mass destruction.

In 1958, nearly 10,000 scientists presented to United Nations Secretary-General Dag Hammarskjold a petition that begged, “We deem it imperative that immediate action be taken to effect an international agreement to stop testing of all nuclear weapons.”

France exploded its first nuclear device in 1960 and China entered the "nuclear arms club" in October 1964 when it conducted its first test. The United States, Soviet Union and some sixty other countries signed a treaty to seek the ends of the nuclear arms race and promote disarmament on July 1, 1968. The treaty bars nuclear weapons states from propagating weapons to other states and prohibits states without nuclear weapons to develop or acquire nuclear arsenal. It permits the use of nuclear energy for peaceful purposes. It entered into force in 1970 and was extended indefinitely and unconditionally on May 11, 1995.

In 1974, India conducted its first nuclear test: a subterranean explosion of a nuclear device (not weapon). India declared it to be a "peaceful" test, but it announced to the world that India had the scientific know-how to build a bomb.

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The half-life of a radioactive substance is a measure of the rate at which the nuclei of its atoms are breaking up or decaying. It is the time it takes for half the atoms in a sample to decay. Thorium, for example, has a half-life of 24 days, while radium-221 has a half-life of only 30 seconds. Uranium has a half-life of 4.5 thousand million years. Of course, as each isotope of an element has a certain number of protons and neutrons in its nucleus, it changes as it decays, forming other elements. For example, plutonium-242 decays to become uranium-238, which in turn breaks down to become thorium-234.

The half-life of a radioactive substance is a characteristic constant. It measures the time it takes for a given amount of the substance to become reduced by half as a consequence of decay, and therefore, the emission of radiation.

Archeologists and geologists use half-life to date the age of organic objects in a process known as carbon dating. During beta decay, carbon 14 becomes nitrogen 14. At the time of death organisms stop producing carbon 14. Since half-life is a constant, the ratio of carbon 14 to nitrogen 14 provides a measurement of the age of a sample.

In the medical field, the radioactive isotope Cobalt 60 has been used for radiotherapy to shrink tumors that will later be surgically removed, or to destroy cancer cells in inoperable tumors. When it decays to stable nickel, it emits two relatively high-energy gamma rays. Today it is being replaced by electron beam radiation therapy systems.

The half-life of isotopes from some sample elements:

Oxygen 16 – infinite

uranium 238 – 4,460,000,000 years

uranium 235 – 713,000,000 years

carbon 14 – 5,730 years

cobalt 60 – 5.27 years

silver 94 - .42 seconds

In the illustration above, 50% of the original mother substance decays into a new daughter substance. After two half-lives, the mother substance will decay another 50%, leaving 25% mother and 75% daughter. A third half-life will leave 12.5% of the mother and 87.5% daughter. In reality, daughter substances can also decay, so the proportions of substance involved will vary.

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The isotope called carbon-14 has a half-life of 5730 years. All living things on our planet contain this form of carbon, but they stop taking it in when they die. Scientists can examine ancient substances to see how much the carbon in it has decayed. They can then give a fairly accurate date for when the substance was alive. This is particularly useful for archaeologists and historians, who can date objects they find, helping to build up a picture of the past.

Radiocarbon dating is a method of what is known as “Absolute Dating”. Despite the name, it does not give an absolute date of organic material - but an approximate age, usually within a range of a few years either way. The other method is “Relative Dating” which gives an order of events without giving an exact age: typically artifact typology or the study of the sequence of the evolution of fossils.

There are three carbon isotopes that occur as part of the Earth's natural processes; these are carbon-12, carbon-13 and carbon-14. The unstable nature of carbon 14 (with a precise half-life that makes it easy to measure) means it is ideal as an absolute dating method. The other two isotopes in comparison are more common than carbon-14 in the atmosphere but increase with the burning of fossil fuels making them less reliable for study; carbon-14 also increases, but its relative rarity means its increase is negligible. The half-life of the 14C isotope is 5,730 years, adjusted from 5,568 years originally calculated in the 1940s; the upper limit of dating is in the region of 55-60,000 years, after which the amount of 14C is negligible. After this point, other Absolute Dating methods may be used.

Today, the amount of carbon dioxide humans are pumping into Earth’s atmosphere is threatening to skew the accuracy of this technique for future archaeologists looking at our own time. That’s because fossil fuels can shift the radiocarbon age of new organic materials today, making them hard to distinguish from ancient ones. Thankfully, research published yesterday in the journal Environmental Research Letters offers a way to save Libby’s work and revitalize this crucial dating technique: simply look at another isotope of carbon.

Carbon-12 is a stable isotope, meaning its amount in any material remains the same year-after-year, century-after-century. Libby's groundbreaking radiocarbon dating technique instead looked at a much rarer isotope of carbon: Carbon-14. Unlike Carbon-12, this isotope of carbon is unstable, and its atoms decay into an isotope of nitrogen over a period of thousands of years. New Carbon-14 is produced at a steady rate in Earth's upper atmosphere, however, as the Sun's rays strike nitrogen atoms.

Radiocarbon dating exploits this contrast between a stable and unstable carbon isotope. During its lifetime, a plant is constantly taking in carbon from the atmosphere through photosynthesis. Animals, in turn, consume this carbon when they eat plants, and the carbon spreads through the food cycle. This carbon comprises a steady ratio of Carbon-12 and Carbon-14.

When these plants and animals die, they cease taking in carbon. From that point forward, the amount of Carbon-14 in materials left over from the plant or animal will decrease over time, while the amount of Carbon-12 will remain unchanged. To radiocarbon date an organic material, a scientist can measure the ratio of remaining Carbon-14 to the unchanged Carbon-12 to see how long it has been since the material's source died. Advancing technology has allowed radiocarbon dating to become accurate to within just a few decades in many cases.

Carbon dating is a brilliant way for archaeologists to take advantage of the natural ways that atoms decay. Unfortunately, humans are on the verge of messing things up. The slow, steady process of Carbon-14 creation in the upper atmosphere has been dwarfed in the past centuries by humans spewing carbon from fossil fuels into the air. Since fossil fuels are millions of years old, they no longer contain any measurable amount of Carbon-14. Thus, as millions of tons of Carbon-12 are pushed into the atmosphere, the steady ratio of these two isotopes is being disrupted. In a study published last year, Imperial College London physicist Heather Graven pointed out how these extra carbon emissions will skew radiocarbon dating.

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