Ionic charge in the periodic table of elements

Many atoms can get rid of electrons and become positively charged ions. Other atoms can pick up electrons and become negatively charged ions. Positive ions and negative ions can form ionic compounds. In an ionic compound, the positive and the negative charges balance. To figure out how many ions of each kind are needed, we need to know the charge of each ion.

But how can we know the ionic charge? You can learn all the ions’ charges by heart, of course. But for some elements, there is an easier way. Maybe you’ve seen one of these before? It’s a table of all the elements the periodic table.

Let’s follow one row of the periodic table and see what charge each atom gets, when it becomes an ion. Furthest to the left is sodium. A sodium atom has one electron in its outermost shell one valence electron. How does the atom become an ion? By getting rid of its single valence electron!

The ionic charge becomes plus one. Next to sodium, in the second column, is magnesium. This atom has two electrons in its outermost shell. That means magnesium atoms form ions with a charge of plus two, as they lose their valence electrons. There is a pattern here.

The next element is over here. Aluminum, with three valence electrons. This ion gets a charge of plus three. Each step to the right means one more valence electron. The next element – silicon – has four valence electrons.

However, silicon doesn’t want to let go of its electrons. That means, on this row, plus three is as high as the charge gets. What about the negative ions? Those are found on the right side of the periodic table. At the extreme right is argon.

Argon has eight valence electrons a “full outer shell”. These atoms don’t want to form ions at all. We can think of them as having a charge of “zero”. One step to the left of argon is chlorine. Chlorine atoms need only one more electron to get a full outer shell.

The ion that it forms – the chloride ion – gets a charge of minus one. One step further to the left is sulfur. Sulfur is two steps away from having a full outer shell. That means sulfur picks up two electrons. The ions formed are called sulfide ions, with a charge of minus 2.

Phosphorous follows this pattern, picking up three electrons to get a full valence shell. This ion, the phosphide ion, gets a charge of minus three. This way we can figure out the ionic charge of a substance, just by finding it in the periodic table. From the left, the charge is plus one, plus two and plus three. From the right, the charge is minus one, minus two and minus three. "zero", In the middle of the row, we find silicon.

Silicon can only form one-minus and two-minus ions, so it doesn’t follow the simple rule for ionic charge that the others do. Let’s choose two elements: magnesium and phosphorus. What will the chemical formula be for magnesium phosphide? Start by finding magnesium and phosphorus in the periodic table. Magnesium is in the second column, which means the ion has a charge of plus two.

Phosphorus is three steps from the “zero-column” furthest right, so the phosphide ion has a charge of minus three. In the ionic compound, we need the same number of positive charges as negative. In order to get that, you will need to use three magnesium ions, for a total positive charge of six and two phosphide ions, for a total negative charge of six. The formula for magnesium phosphide will be Mg3P2. For now, we’ve only considered the elements in the third row.

The other rows in the periodic table follow the same rule in general, but there are a few exceptions. We’ll save those for next time.