CHEMICAL FORMULAS

Students, the atomic structure of elements and the position of each element in the periodic table are very important in learning chemical bond. By using these two basic concepts, you will understand chemical bonding. You have to be careful that there is a variety of bonds which will be difficult if you don't understand in dept.
As you know, in general, elements can be classify as metal and non metal. Certainly the combination of bonds is metal - metal, metal - non metal, and non metal - non metal.
The bond between metal - metal is metallic bond. The effect of this bond gives certain properties which are different with the properties of non metal elements. For example, metal can conduct the electricity, because its valence electron can move fastly as a current. The atom becomes positive ion because its valence electron moves from the ion to the other ions. We can say that metallic bond is a bond between positive ion of metal with its electron current. Its mean that the positive ions of metal are closely and link together by the electron current. This can be classified as a giant structure and forms a metallic crystal. The melting point is relative high. The formula of metal is only written as its symbol. For example, iron is Fe. This is an empirical formula rather than a molecular formula. It doesn't mean that iron consists of atoms; remember the metallic bond which have been discussed. Fe is not a real formula. Empirical formula is only tell us the kind of elements and the ratio of their components.
Metal - non metal bond is named ionic bond. When metal reacts with non metal, there is an electron transfer from metal to non metal. The metal becomes positive ion (cation) and the non metal forms negative ion (anion). Each cation surrounded by anions and each anion surrounded by cations. They are link together closely. This is a giant structure of ionic crystal. The melting point is relative high. The chemical formula is an empirical formula. As metal, this isn't a real formula. For example, table salt, NaCl. The ration of Na+ : Cl- = 1 : 1. This ionic compound doesn't conduct electricity, because the cations and anions are very close, they can not move freely.
Non metal - non metal is bonded as a covalent bond and coordinated bond. The first bond is formed if the shared electron pair comes from both atoms. The second is the shared electron pair comes from one atom only. The melting point of the covalent compounds is relative low, because these compounds have simple structure and we call as a molecular structure. This compound is also called as a molecule. For example H2O and CO2.
If the same atoms link together, for example H2 or O2, the bond is non polar covalent bond. H2O or CO2 can be classified as a polar covalent bond, because the electronegativify of these two atoms is different. All covalent compounds that have simple structures have molecular formulas, the chemical formulas that indicate the type of elements and show all the number of atoms. This is a real formula. H2O means that each molecule of it consists of 2 hydrogen atoms and one oxygen atoms.
The important point in this discussion is the difference between empirical formula and molecular formula. If a simple structure, its okay, the formula is molecular one, but if the structure is a giant, this is a crystal rather than a molecule, so the formula is empirical formula.