An atom can be considered unstable in one of two ways. If it picks up or loses an electron, it becomes electrically charged and highly reactive. Such electrically charged atoms are known as ions. Instability can also occur in the nucleus when the number of protons and neutrons is unbalanced.

What makes an atom unstable or stable?

A stable atom is an atom that has enough binding energy to hold the nucleus together permanently. An unstable atom does not have enough binding energy to hold the nucleus together permanently and is called a radioactive atom.

How do you know which ion is more stable?

The larger the atom, and the further the electrons from the nucleus, the more polarizable it is. The more polarizable the atom, the more stable the anion. Within a row of the periodic table, the more electronegative an atom, the more stable the anion.