BASIC CHEMISTRY V
Chemical formulas yield the following information:
- Which elements are present
- The ratio of the number of atoms of each element to the number of atoms of each other element
- The number of atoms of each element per formula unit of compound
- 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.