# Atoms become ions

How many electrons can the valence shell of a large atom hold?

## Atoms become ions

Sometimes, we draw atoms as balls with smooth surfaces. But they are a bit more complex than that. There is a nucleus in the center, and a cloud of electrons swarming around it. And the electron cloud itself is divided into electron shells. The outermost electrons - the valence electrons - are the reason why chemical reactions occur at all.

Smaller atoms, with only one electron shell, can hold a maximum of two electrons. In the larger atoms the valence shell can hold a maximum of eight electrons, no matter how large the atoms get. There is something special with having exactly eight valence electrons. It gives the atom a kind of stability. Here is a chlorine atom.

It starts out with seven electrons in its outer shell. If it could only get one more, it would reach that magical number of stability. It wants an extra electron so much -- that it can pull it away from almost any other substance. That is why chlorine is so reactive. The chlorine atom is neutral to start with.

It has the same amount of positive protons and negative electrons. When it gets that extra electron, it becomes a negative ion, with a charge of minus 1. We write it as C-L-minus As an ion, it's not reactive at all. Now that it has eight electrons in its outer shell, it doesn't need to steal any more electrons. Some atoms change their name slightly when forming ions.

The ion of chlorine is called a chloride ion. Here's another atom: oxygen. It starts out with six electrons in its outer shell, so it wants to get two more to reach the magic number of eight. When it picks up two electrons, it gets a charge of minus two. The ion is called an oxide ion.

We write it like this: O-two-minus. So, when an atom has an almost full outer shell, it wants to take up more electrons. But what about atoms that start out with just a few electrons in their outer shell? Let's look at aluminium. It has three electrons in its outer shell.

You'd think that getting five more would do the trick. But there are limits to how many extra electrons atoms can get. What the aluminium atom can do instead is get rid of its valence electrons. This is how all the metals form ions. Does that mean that it now has zero electrons in its outer shell?

No, look: as the electrons from the third shell go away, the second shell becomes the outer shell. And that had eight electrons to start with. When an atom loses electrons, as metal atoms do, it becomes a positively charged ion. The aluminium atom loses three electrons, so its charge becomes plus three. We write it like this: A-L-three-plus.

Atoms that form positive ions don't change their names, so this is simply called an aluminium ion. Many of the elements around us usually exist in their ionic form. And ions are just like atoms, only with extra - or fewer - electrons.