The overall of an atom is a nucleus (protons and neutrons), and 1 or 2 electrons. The rest are for large atoms: an electron shell, electrons, an electron shell, electrons, an electron shell, electrons, an electron shell, electrons, an electron shell, electrons, an electron shell, electrons, an electron shell, electrons.

The key to "happiness" for an atom is a full outer electron shell. (The outer electron shell is called the valence shell.) There are two conditions that cause a shell not to be full. Either it has only an electron or two (or three) in the outer electron shell or it's short an electron or two in that outer shell. The direct answer to the question is that if an element is chemically active, its outer electron shell is incomplete or is not full.

The seventh electron shell.

The electron shell arrangement of Ceasium is 2.8.8.18.18.1