Ionic charge and number of bond possibilities

Here is a table of all the elements that we know of. The periodic table. Each box represents one element. Most elements are metals. When metals react with other substances, they want to get rid of electrons.

When they do that, they become positive ions. Some elements are non-metals. Most non-metals want to pick up more electrons, and become negative ions. Non-metals can also take on more electrons in another way, by combining. Then they share electrons with each other, in covalent bonds.

Between the metals and the non-metals, we find elements that don’t quite fit into either of the two categories. These elements are metalloids. This boundary between metals and non-metals is not completely straight, but shaped more like a staircase. And that makes things a bit more complicated. Why is that?

Take a look at this. For some substances, it is easy to see what ionic charge they will get when they lose or when they gain electrons. This is shown by which column they sit in. All atoms in the first column want to lose one electron and form ions with a charge of plus one. The elements in the second column form two-plus-ions, by letting go of two electrons.

For these substances, the transition metals, there is no simple rule. The ions they form can get various charges. Let’s contract the middle of the table to save space. The pattern continues on the other side of the transition metals. In this column, the atoms form ions with a charge of three plus.

But not all of them. Why not? Because here is that “border”, where the metalloids start. And the metalloids don’t follow this rule about ionic charge. It's only the substances underneath the metalloids that get a charge of 3+.

In the columns with 4+ and 5+ ions, it’s the same thing: The ionic charges only apply to the lower rows, underneath the metalloids. Here, the metals are also sometimes able to form ions with a lower charge, but 4+ and 5+ is as high as they get. What about the non-metals, on the right hand side? Here the columns show how many electrons the atoms want to gain. On the extreme right, there are elements that don’t want any electrons at all.

We can say that they get a charge of "zero". One step to the left we find the atoms that want to pick up one electron. When they do that, their charge will be minus one. Non-metals can gain electrons in another way, using covalent bonds. Since these atoms need only one electron, one bond is enough.

Their ‘ability to bond’ – or valency – is one. Is this true for all atoms in this column? No, not the metalloids down here. Another step to the left, the atoms want two more electrons. They can either form two bonds or they can pick up two electrons and become two-minus ions.

But as before, that only goes for the elements above the metalloids. In this column, there are only two non-metals. These get a charge of three minus, or want to form three bonds. And there is only one non-metal that forms four bonds, or can get an ionic charge of four minus. So there is a general rule that tells us the positive charge of metal ions, the negative charge of non-metal ions, and the number of bonds a non-metal will form.

Between the metals and the non-metals, we find the metalloids. They don’t follow this general rule. The boundary between metals and non-metals is shaped like a staircase. That means there are not many metals that get a high positive charge and not many non-metals that get a high negative charge, or that form a high number of covalent bonds.