We now know that the number of protons is what defines the kind of atom or element we are dealing with, and if you change the number of protons, you get a totally different element. We also know that changing the number of neutrons does not necessarily change the kind of atom or element, though it will make the atom heavier or lighter. What happens if we change the number or electrons? The suspense is killing us!
Before we give you the answer—wow, we can be as bad as a TV episode ending with "to be continued," 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 electron buddies.
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 shells are 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 pals. Sucks to be them. What can such lonely electrons do? There are two options:
Either strategy will throw off that perfect balance between positive and negative charges because, even if electrons leave or join an atom, there will still be the same number of positively charged protons. Regardless of the path taken, the result will be an ion, or a charged particle.
If one or more electrons ditch their home atom, the resulting ion will be positive since the protons will outnumber remaining electrons. If electrons join a partially empty shell, the resulting ion will be negative since the electrons will outnumber the protons. Why you gotta gang up on the protons like that, electrons? Not cool.
The following is an example of electron loss in sodium:
And here is an example of electron gain in chlorine:
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.