Main energy states:

It is customary to speak of electron shells around the nucleus and the shells are referred to by number. The first, or No. 1, shell is the one closest the nucleus; the second, or No. 2, shell is further; then the third, or No. 3, shell; and many more in numerical sequence.  In common, electrons closer to the nucleus have a lower energy state. Atomic electrons always find the lowest energy state obtainable.

The electron shells represent main energy states of electrons. Every shell holds one or more subshells called orbitals, each along with a slightly variant energy. In sequence of increasing energy, a orbitals are designated through the small letters s, p, d, f, g, h.

No two shells consist of the similar number of orbitals. A first shell holds only one orbital, an s orbital. The second shell holds s and p orbitals. In common, every higher shell holds a new category of orbital:

the first shell contains an s orbital,

the second shell contains s and p orbitals,

the third shell contains s, p, and d orbitals,

the fourth shell contains s, p, d, and f orbitals,

and many more. Every orbital could hold a definite maximum number of electrons. There is additionally a limit to the number of electrons in every shell and the limit increases as one goes to higher shells.