The number of naturally occurring stable elements on the earth is 92 only. Although scientists have so far discovered 107 elements in all, but 15 of these have been artificially made in the laboratories. These artificial elements are unstable in nature. The atoms of these 92 elements are also of 92 kinds only. Do you know how millions of substances are made from these elements?
All substances available in the universe are made by the combination of atoms of these 92 elements. The atoms of different elements combine with one another in various proportions and keep on forming countless substances. Some of the important elements are: iron, gold, silver, copper, aluminium, sodium, potassium (metallic elements), oxygen, nitrogen, chlorine, carbon, sulphur (non-metallic elements). All the elements consist of atoms and the atoms of the same elements are alike. Two or more atoms combine with each other to make molecules. For example, two atoms of hydrogen combine with one atom of oxygen to make one molecule of water. Even a very small quantity of water consists of innumerable molecules. Similarly one atom of sodium combines with one atom of chlorine to make one molecule of the common salt sodium chloride.
Now the question arises: how do the atoms combine with one another to form compounds? Mainly they combine in two ways. To understand the processes of combination, we have to know the structure of an atom. The central portion of every atom is positively charged and is called the nucleus. The electrons of the atom revolve round the nucleus in different orbits. Electrons are negatively charged particles. In the first kind of combination, one atom donates some of its electrons to the other making a bond between them and this result in the formation of a new material. In the second type of combination one atom makes a bond with another atom by mutual sharing of electrons. In both these combinations only the electrons of the outermost orbits participate. Now the question arises: how does the exchange of sharing of electrons create new materials?
In the formation of common salt, one electron of the sodium atom is donated to one atom of chlorine. This transfer of electron makes sodium atom a positive ion and chlorine atom a negative ion. There exists an electrostatic force of attraction between these ions. This force of attraction binds the two atoms together and a molecule of common salt is formed. Similarly in the formation of water, oxygen and hydrogen atoms have a mutual sharing of electrons by which a force of attraction exists between them which binds the atoms together. As a result the molecules of water are formed.
For example, one carbon atom combines with two oxygen atoms to make one molecule of carbon dioxide.
Similarly one atom of nitrogen combines with three atoms of hydrogen to make one large molecule of ammonia gas. Carbon is one such element which combines with large number of elements and makes the maximum number of compounds – called organic materials. The compounds which do not contain carbon are called inorganic.
The Russian scientist, Dmitri Mendeleyev, classified all elements into several groups and in 1989 he published the “Periodic Table” showing all the elements in their respective groups.