Before we give you the answer—we are almost as bad as "to be continued" TV episodes, huh?—there’s something you need to know about electrons: even though they are negative, they like to have friends. Buddies. Pals. Amigos. Electrons orbit the nucleus in different zones called electron shells, and each shell can hold a certain number of electrons.
For example, the electron shell closest to the nucleus can hold two. It’s the smallest shell, and any more than two electrons would be too crowded. It would be like trying to fit more than one person into your shiny new Peel P50. The next two shells hold eight electrons each. Electrons are happiest when their shell is full to capacity, and happy electrons make for a stable atom. This fact has all kinds of implications in the atomic world.
Electrons fill the innermost shells first, so the outermost electrons are the ones that are most likely to be left with an undesirable number of buddies. Sucks to be them. What is a lonely electron to do? There are actually two options:
- The electron can pack its bags, leave faster than a housemate bailing on the Jersey Shore, and join a more desirable shell/reality show, or
- Invite electrons from other atoms to join its shell. Party time!
The following is an example of electron loss in sodium (Na):
And here is an example of electron gain in chlorine (Cl):
One of the most important ions in biology is calcium. Calcium is used as a signal in the cell and controls lots of processes; it's even involved in programmed cell death. Eek.