Answer:
The number of valence electrons typically becomes 8 (octet rule) with notable exceptions for H, He, Al, B
Explanation:
We should separate this question into two parts for complete clarity. Both principles, however, will be based on the fact that generally atoms tend to have their valence shells filled with octets (8 valence electrons).
- In molecular compounds, those formed between nonmetals, electron pairs are shared between two atoms. The two or more atoms with any number of valence electrons bond together in a fashion that allows each of them have a total of 8 electrons (except hydrogen and helium, they would need 2 as a maximum, as well as Al and B which form 3 single bonds). This is achieved by forming multiple bonds if required (for example, double or triple bonds rather than just single bonds). Let's say we form a nitrogen gas molecule. Each nitrogen has 5 valence electrons, so they need 3 more electrons to gain an octet. A triple bond will be formed: the three electrons of each nitrogen will be donated into single bonds. This will leave a lone pair on each nitrogen. However, notice that 6 electrons in the 3 bonds yield a total of 6 electrons. This and a lone pair on nitrogen (2 electrons) produce 8 electrons for each nitrogen, as electrons are shared equally.
- In ionic compounds, those formed between metals and nonmetals, metals tend to lose their valence electrons and nonmetals gain them, again, to have octets. Let's look at a simple example, NaCl. Sodium has one valence electron and chlorine has 7. Sodium would rather lose 1 electron than gain 7 more, as it's energetically more favorable, to gain an octet. This electron lost is accepted by chlorine that initially has 7 electrons and obtains a total of 8 electrons upon acceptance. Now notice that sodium lost its whole shell after giving its 1 electron to chlorine, and its previous shell is already filled with 8 electrons, so each now have an octet.
