electrons than it is for it to lose six, so sulfur forms negative anions with a charge
of negative two, represented by S2−. Every other element in the same group as
sulfur (like oxygen and selenium) will also form ions with a charge of negative
two.
The halogens are one group to the right. The easiest way for the halogens to
achieve a full octet is by gaining one electron. So, a fluorine ion would have a
charge of negative one, represented by F−. Going in the other direction, the
elements in the nitrogen group (such as phosphorous) typically form ions with a
charge of negative three.
If we take an element like magnesium, we see that it has two valence electrons.
It is going to be easier for a magnesium atom to lose those two electrons, leaving
behind a full octet in the second shell, than it would be for that atom to gain the
six electrons that would be necessary to fill the octet in the third shell. So,
magnesium atoms form ions with a charge of positive 2, or Mg2+. The same
would be true for all other alkaline earth elements (such as Be or Ca). Alkali
metals (such as Li or Na) form ions with a charge of positive one, and the metals
in aluminum’s group form ions with a charge of positive 3.
The elements in the carbon group all have four valence electrons, and it would
be equally easy for them to lose all four or gain an additional four to complete
the nearest octet. However, carbon and silicon in particular do not like to form
ions—they are much more inclined to share their four valence electrons with
other elements, forming covalent bonds (discussed in the next section).
However, the metals in carbon’s group (notably, lead and tin) typically form ions
with a charge of +4.
Looking at the transition metals in the d and f blocks, there is no fast and easy
way to determine the charge of those ions. It is possible to determine that they
will lose electrons to form cations, as is the case with all metals. However, the
number of electrons they will lose can vary, and these transition metals often
form ions with multiple possible charges. Copper atoms are typically +1 or +2,
iron atoms are +2 or +3, and so on. There is no easy way to determine the charge
on transition ions using the Periodic Table.
To determine the formula of an ionic compound, the important rule to remember
is that the total charge on any stable compound must be zero. When sodium
forms an ion, it loses an electron to become Na+. When sulfur forms an ion, it