How do I determine the valency of a main group element?

Dec 30, 2016

Usually by looking at how many columns away from the noble gases the element is, on the periodic table.

The valency is basically how many bonds it can make, which you can find by drawing the Lewis dot structure and finding which electrons are unpaired at the moment.

Take oxygen ($\text{O}$) for example (atomic number $8$), which has $\boldsymbol{6}$ valence electrons (two $2 s$ and four $2 p$ electrons), because it is $\boldsymbol{2}$ columns away from the noble gases ($8 - 2 = 6$).

Drawing in the valence electrons one by one, the first four can be drawn like this:

$\cdot \stackrel{.}{\text{O}} \cdot$
color(white)(.')""^(.)

Drawing the remaining two, it doesn't really matter where you place them; you'll find that there are two unpaired electrons when you're done:

$\cdot \stackrel{. .}{\text{O}} :$
color(white)(a.)""^(.)

Or, maybe you could have:

$\cdot \stackrel{. .}{\text{O}} \cdot$
color(white)('')""^(..)

Either way, the Lewis structure tells us that the valency of oxygen atom is $2$, so it can make $\boldsymbol{2}$ bonds via electron sharing.

Again, these covalent bonds are made by pairing up each electron with another electron from the other atom, such as carbon or hydrogen.

Two examples are water:

or even something like formaldehyde:

in which it also makes $\boldsymbol{2}$ bonds (they are to the same atom, but there are still the same two electrons shared with carbon).

As a result, you typically see oxygen making two bonds; otherwise, it ends to have a charge, like in ${\text{OH}}^{-}$ (hydroxide), or ${\text{H"_3"O}}^{+}$ (hydronium).