The numbers of the main shells, like 1s, arise from the principal quantum number n in the quantum mechanical description of the electrons. The letters designate the sub-shells and follow the historical spectroscopic notation. In general terms, the higher shells have higher energy (less tightly bound) and are on the average further out from the nucleus. This is strictly true for the hydrogen atom where the energy levels depend only upon the principal quantum number (fine structure neglected). But in larger atoms, the energy depends also upon the orbital quantum number so the sub-levels are filled in the order s, p, d, f, etc. This spreading eventually leads to overlap, with the 4s sub-level being lower in energy than the 3d sub-level.

The division into main shells encourages a kind of "planetary model" for the electrons, and while this is not at all accurate as a description of the electrons, it has a certain mnemonic value for keeping track of the buildup of heavier elements.

The buildup process for the first 12 elements of the periodic table gives some hints about the origin of chemical reactivity. The 1s spatial state can have two states of electron spin, which we usually just refer to as "spin up" and "spin down". With two electrons in the 1s state, it is filled and stable, forming the noble gas helium. It is chemically unreactive. Lithium is a very active alkali metal, having just one electron in the second shell. It tends to lose that electron to revert to the stable helium electron structure. The elements from boron to neon in the periodic table are filling the 2p orbital. The three possibilities for 2p orbitals can be associated with spatial directions, say x,y and z. The order of filling will place one electron in each before placing two in any one orbital since the mutually repulsive electrons prefer to be further apart. The filling of the 2p orbital places eight electrons in the second shell, a "stable octet". The particularly stable configuration produces the noble gas neon. But fluorine with one less than a stable octet is very active! It tends to gain one electron while the very active sodium tends to lose one electron, in each case producing the stable octet configuration of neon.