Atoms, Elements and Compounds

Atoms, Elements and Compounds
It would be difficult to study metallurgy meaningfully without relating mechanical properties to the elementary forces acting between the atoms of which a metal is composed. We shall study the structures of atoms later in the chapter but it suffices at this stage to regard these atoms as tiny spheres held close to one another by forces of attraction. If in a substance all of these atoms are of the same type then the
substance is a chemical element. Thus, the salient property of a chemical element is that it cannot be split up into simpler substances whether by mechanical or chemical means. Most of the elements are chemically reactive, so that we find very few of them in their elemental state in the Earth s crust—oxygen and nitrogen mixed together in the atmosphere are the most common, whilst a few metals such as copper, gold and silver, also occur uncombined. Typical substances occurring naturally contain atoms of two or more kinds
It was mentioned above that chemical combination between two atoms is governed by the number of electrons in the outer electron shell of each. Moreover, it was pointed out that those elements whose atoms had eight electrons in the outer shell (the noble gases neon and argon) had no inclination to combine with other elements and therefore had no chemical affinity. It is therefore reasonable to suppose that the completion of the octet of electrons in the outer shell of an atom leads to a valence of zero. The noble gas helium, with a completed duplet of electrons in the single shell, behaves in a similar manner. As far as the simpler atoms we have been discussing are concerned the tendency is for them to attempt to attain this noble-gas structure of a stable octet (or duplet) of electrons in the outer shell. Their chemical properties are reflected in this tendency. With the more complex atoms, the situation is not quite so simple, since these atoms possess larger outer shells, which are generally sub-divided, to the extent that electrons may begin to fill a new outer sub-shell before the penultimate sub-shell has been completed. As mentioned above this would explain the existence of groups of metallic elements the properties of which are transitional between those of one well-defined group and those of the next. The broad principles of the electronic theory of valence mentioned here in connection with the simpler atoms will apply. On these general lines, three main forms of combination exist.
In this, type of combination a metallic atom loses the electrons, which constitute its outer shell (or sub-shell), and the number of electrons so lost are equivalent to the numerical valence of the element. These lost electrons are transferred to the outer electron shells of the non-metallic atom (or atoms) with which the metal is combining. In this way, a complete shell of electrons is left behind in the metallic particle whilst a hitherto incomplete shell is filled in the non-metallic particle. Let us consider the combination, which takes place between the metal sodium and the non-metal chlorine to form sodium chloride (common salt). The sodium atom has a single electron in its outer shell and these transfers to join the seven electrons in the outer shell of the chlorine atom. When this occurs each resultant particle is left with a complete octet in the outer shell. (The sodium particle now has the same electron structure as the noble gas neon, and the chlorine particle has the same electron structure as the noble gas argon.) The balance of electrical charges, which existed between protons and electrons in the original atoms, is, however, upset. Since the sodium atom has lost a negatively charged particle (an electron), the remaining sodium particle must now possess a resultant positive charge. Meanwhile the chlorine atom has gained this electron so the resultant chlorine particle must carry a negative charge. These charged particles, derived from atoms in this manner, are called ions. In terms of symbol sthe sodium ion is written thus, Na+, and the chlorine ion, Cl".