BASIC CHEMISTRY VII

BASIC CHEMISTRY VII

SCOPE OF THE OCTET RULE
It must be emphasized that the octet rule does not describe the electronic configuration of all compounds. The very existence of any compounds of the noble gases is evidence that the octet rule does not apply in all cases. Other examples of compounds that do not obey the octet rule are BF3, PF5, and SF6. But the octet rule does summarize, systematize, and explain the bonding in so many compounds that it is well worth learning and understanding. Compounds in which atoms attain the configuration of helium (the duets) are considered to obey the octet rule, despite the fact that they achieve only the duet characteristic of the complete first shell of electrons.

DISTINCTION BETWEEN IONIC AND COVALENT BONDING
The word bonding applies to any situation in which two or more atoms are held together in such close proximity that they form a characteristic species which has distinct properties and which can be represented by a chemical formula. In compounds consisting of ions, bonding results from the attractions between the oppositely charged ions. In such compounds in the solid state, each ion is surrounded on all sides by ions of the opposite charge. In a solid ionic compound, it is incorrect to speak of a bond between specific pairs of ions, and ionic compounds do not form molecules. In contrast, covalent bonding involves the sharing of electron pairs between two specific atoms, and it is possible to speak of a definite bond. For example, in molecules of HCl and CH4 there are one and four covalent bonds per molecule, respectively.

BASIC CHEMISTRY V

BASIC CHEMISTRY V

CHEMICAL FORMULAS

Chemical formulas yield the following information:

1. Which elements are present
2. The ratio of the number of atoms of each element to the number of atoms of each other element
3. The number of atoms of each element per formula unit of compound
4. The fact that all the atoms represented are bonded together in some way

You cannot tell from a formula how many atoms of each element are present in a given sample of substance, because there might be a little or a lot of the substance present. The formula tells the ratio of atoms of each element to all the others, and the ratio of atoms of each element to formula units as a whole.

THE OCTET RULE
The elements helium, neon, argon, krypton, xenon, and radon, known as the noble gases, occur in nature as monatomic gases. Their atoms are not combined with atoms of other elements or with other atoms like themselves. The charge on the nucleus and the number of electrons in the valence shell determine the chemical properties of the atom. The electronic configurations of the noble gases (except for that of helium) correspond to a valence shell containing eight electrons, a very stable configuration called an octet. Atoms of other main group elements tend to react with other atoms in various ways to achieve the octet. The tendency to achieve an octet of electrons in the outermost shell is called the octet rule. If the outermost shell is the first shell, that is, if there is only one shell occupied, then the maximum number of electrons is two. A configuration of two electrons in the first shell, with no other shells occupied by electrons, is stable.