Question

What do valence electrons have to do with bonding?

Answer 1

The valence electrons are the electrons that determine the most typical bonding patterns for an element.

These electrons are found in the s and p orbitals of the highest energy level for the element.

Sodium `1s^2 2s^2 2p^6 3s^1`
Sodium has 1 valence electron from the 3s orbital

Phosphorus `1s^2 2s^2 2p^6 3s^2 3p^3`
Phosphorus has 5 valence electrons 2 from the 3s and 3 from the 3p

Lets take the ionic formula for Calcium Chloride is `CaCl_2`

Calcium is an Alkaline Earth Metal in the second column of the periodic table. This means that calcium `s^2` has 2 valence electrons it readily gives away in order to seek the stability of the octet. This makes calcium a Ca+2 cation.

Chlorine is a Halogen in the 17th column or `s^2p^5` group.
Chlorine has 7 valence electrons. It needs one electron to make it stable at 8 electrons in its valence shells. This makes chlorine a `Cl^(−1)` anion.

Ionic bonds form when the charges between the metal cation and non-metal anion are equal and opposite. This means that two `Cl^(−1)` anions will balance with one `Ca^(+2)` cation.

This makes the formula for calcium chloride, `CaCl_2`.

For the example Aluminum Oxide `Al_2O_3`

Aluminum `s^2p^1` has 3 valence electrons and an oxidation state of +3 or `Al^(+3)`
Oxygen `s^2p^4` has 6 valence electrons and an oxidation state of -2 or `O^(−2)`

The common multiple of 2 and 3 is 6.
We will need 2 aluminum atoms to get a +6 charge and 3 oxygen atoms to get a -6 charge. When the charges are equal and opposite the atoms will bond as `Al_2O_3`.

In molecular (covalent) compounds these same valence electrons are shared by atoms in order to satisfy the rule of octet.

I hope this is helpful.
SMARTERTEACHER