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by Maurice Goldsmith

An element is made up of atoms of one kind only. For example, a piece of copper (Cu) is made up of atoms which are all identical in chemical terms; the element Carbon is made up of atoms with a distinct structure. When these atoms of the same or any kind combine they form molecules. Thus, two Hydrogen atoms combine to form a Hydrogen molecule. Most atoms can also combine with atoms of different kinds to form compounds of molecules. An atom of Carbon will combine with two atoms of Oxygen to form carbon dioxide (CO2); or two atoms of Hydrogen will combine with one atom of Oxygen to form water (H20). Notice that we have had to give specific numbers of atoms; also that, in combining, the elements themselves may change, and the properties of the products may be amazingly different from those of the elements. For instance, Hydrogen, a highly inflammable gas, combines with another gas, Oxygen, to form the liquid water; or the highly poisonous metal Sodium (Na) combines with the equally poisonous gas Chlorine to form essential-to-life salt (NaCl).

To understand the elements we must know about the atoms that make them up. An atom may be considered as made up of a central nucleus, and revolving around it are electrons. The nucleus is made up of protons and neutrons. The proton has a positive electric charge, the neutron has no electric charge, and the electron has a negative charge. The number of protons in the atomic nucleus is equal to the number of electrons revolving around that nucleus. This means that the positive electric charge is balanced by the negative. The atom is therefore electrically neutral. The atomic number states the number of protons (therefore, electrons) in an atom of that element. For example, the atomic number of Carbon is 6: this means that there are six positively charged protons in the nucleus, and six negatively charged electrons whirling around.

The atomic weight is the relative weight of an atom of the element. At one stage this was compared with Hydrogen as unity, then with Oxygen as 16. Thus, as the atom of Helium has a weight of 4, the Oxygen atom is four times as heavy. The matter of the atom - in other words, its mass - is contained in the nucleus, which is extremely small. To take an example, the size of a Carbon atom is about one-one hundred millionth of an inch. A pencil dot on a piece of paper contains more carbon atoms than there are people on earth. The diameter of an atom is about ten thousand times its nucleus, yet the latter has no less than 99.9 per cent of the mass of the atom. The atoms which make up matter are mostly empty space.

(from The Young Scientist's Companion, Souvenir Press, London, 1961)