Most of chemisty deals not with individual atoms but with combinations of atoms of different elements. These combinations of atoms are called compounds and have completely different chemical and physical properties than the consitutent elements. For example, a common general chemistry laboratory experiment involves the addition of tin metal to iodine. Both of these are in their elemental forms, tin being a metal and iodine being a molecular solid. As shown in Figure 25, tin is grey, while iodine is purple (you may have used tincture of iodine as an antiseptic for cuts). When the two elements are heated, an orange solid forms. This orange solid is tin tetraiodide and consists of tin atoms bonded to iodine atoms in a compound that has the formula SnI4. This compound melts at 144 °C and will dissolve in many organic solvents, whereas metallic tin melts over 300 °C and is insoluble in organic solvents. If we take another metal such as sodium and allow it to react with elemental iodine, not only will we get a much more violent reaction, but we will also obtain a white product that is not very soluble in organic solvents but highly soluble in water. Moreover, this compound, sodium iodide, has a melting point of 651 °C.
Figure 25. Elemental tin, iodine and SnI4.
Generally, this kind of difference in physical properties is indicative of something quite different about the compounds at the atomic level. In fact, compounds are placed into two general categories--ionic and covalent. Ionic compounds are formed between metals and nonmetals, while covalent compounds are formed between two nonmetals, or, as is the case with tin tetraiodide, between an element that is a borderline metal (notice the position of tin on the periodic chart) and a nonmetal like iodine. In ionic compounds, the atoms are present as ions; that is, as charged particles. For example, in NaI, the sodium is present as sodium ions and the iodine as iodide ions. The sodium ion, you will remember, has 12 protons in the nucleus, but only 11 extranuclear electrons. Consequently, it has one more positive charge than negative change and therefore has an overall plus one charge (Na+). The iodide ion has just the opposite situation: the number of electrons is greater than that of protons by one and is negatively charged (I-). In some ionic compounds there are plus two or plus three cations and minus two or minus three anions. Magnesium carbonate contains a plus two magnesium ion (Mg2+) and a minus two carbonate ion (CO32-).
In covalent compounds there are no charged ions; the atoms are held together by covalent bonds. These covalent bonds are believed to be a result of the sharing of electron density between two atoms. Covalent compounds either contain molecules or a large network of atoms linked together. A representation of both compounds is shown in Figure 26.
Figure 26. Two types of covalent compounds (a) molecular (NH3) and (b) covalent network (a portion of the diamond structure).