Two sodium atoms are walking along when one of them suddenly drops to the ground and starts feeling around frantically.
Sodium atom 2: What’s wrong?
Sodium atom 1: I lost an electron!
Sodium atom 2: Are you sure?
Sodium atom 1: I’m positive!
Atoms are the building blocks of everything around us, but atoms are made of even smaller particles called protons, neutrons, and electrons. The protons and neutrons are located in the nucleus – or center – of the atom, while the electrons zip around outside the nucleus.
Protons and electrons have a property called charge. Charge comes in two varieties: positive and negative. Each proton bears a unit of positive charge. For simplicity’s sake we’ll call it +1. Each electron bears a unit of negative charge: -1. Neutrons do not have any charge; as their name implies, they are neutral.
If an atom has the same number of protons and electrons, then their charges balance out and the atom as a whole is neutral. Consider a sodium atom, for example. A neutral sodium atom has 11 protons and 11 electrons. In any neutral atom, the number of electrons is equal to the number of protons.
Atoms don’t generally gain or lose protons (except in nuclear reactions, which is a discussion for another day), but electrons can come and go. If a neutral atom gains one or more electrons, then it is no longer neutral: it has more electrons than protons. It has an overall negative charge due to the excess electrons. Non-metal atoms like chlorine and oxygen tend to gain electrons.
If a neutral atom loses one or more electrons, it has fewer electrons than protons. Now it has an overall positive charge. Metal atoms like sodium and iron tend to lose electrons. Thus, when our anthropomorphic sodium atom lost an electron, he (she? it?) truly became positive.