○○○
The 1st shell of an atom, as with helium, has only 2 electrons. The 2nd shell is full at 8 electrons. Since negatively charged electrons are attracted to their positively charged nucleus, electrons in an atom will generally occupy an outermost shell only when the shells within have been filled.
The general formula for filling shells is 2 x 2n, where n is the shell number. Hence: 2 = 2 x 12; 8 = 2 x 22; 18 = 2 x 32; 32 = 2 x 42.
Atoms with an incomplete outer shell naturally bond with other atoms to form molecules. These bonds involve sharing electrons.
The valence of an atom tells how many electrons an atom needs to fill its outermost shell. The chemical properties of an atom are defined by its outermost shell: the valence shell.
The simple story is that the valence shell determines how reactive an atom is. More accurately, the electrons traveling farthest from the nucleus – the most energetic ones – determine how an atom reacts chemically. These valence electrons are necessarily in the valence shell.
Oxygen, at 2|6, has a valence of 2, and so readily combines with 2 atoms of hydrogen gas to make H2O: water (6 (from oxygen) + 2 (from hydrogen) = 8 (stable valence shell)).
The 5 most abundant elements in the solar system are: hydrogen (1), helium (2), oxygen (2|6), carbon (2|4), and nitrogen (2|5). Only helium is not prone to forming molecules.
Helium is rare on Earth and has no role in organic chemistry. The other abundant atomic species – carbon, nitrogen, oxygen, and hydrogen – all play active roles in organic chemistry precisely because they are reactive.
