Atomic Shells

Shells as Energy Levels

In the simple Bohr model, electrons are shown in atomic shells, which represent specific energy levels around the nucleus. A shell closer to the nucleus has lower energy. A shell farther from the nucleus has higher energy.

OpenStax explains that the Bohr model uses orbits with specific energies to explain the line spectrum of hydrogen. The reference is available at The Bohr Model from OpenStax.

Picture a building with floors. The first floor is near the ground, the second floor is higher, and so on. In an atom, the "floor" is not a physical floor. It is an energy level. So, a circular shell diagram is an energy-level sketch, not a rigid track that electrons follow like a train.

Shells are named with letters:

Shell	Shell number

$\mathrm{K}$	$n = 1$
$\mathrm{L}$	$n = 2$
$\mathrm{M}$	$n = 3$
$\mathrm{N}$	$n = 4$
$\mathrm{O}$	$n = 5$
$\mathrm{P}$	$n = 6$
$\mathrm{Q}$	$n = 7$

Shell	Shell number	Maximum electrons from $2n^2$
$\mathrm{K}$	$1$	$2(1)^2 = 2$
$\mathrm{L}$	$2$	$2(2)^2 = 8$
$\mathrm{M}$	$3$	$2(3)^2 = 18$
$\mathrm{N}$	$4$	$2(4)^2 = 32$
$\mathrm{O}$	$5$	$2(5)^2 = 50$
$\mathrm{P}$	$6$	$2(6)^2 = 72$
$\mathrm{Q}$	$7$	$2(7)^2 = 98$

Statement	Accurate meaning
Atomic shells are drawn as circles.	The drawing helps show energy levels. It is not a rigid physical path.
The formula $2n^2$ gives maximum capacity.	It is not the complete filling order for every element. Heavier elements need the subshell and orbital model.

Atomic Shells

Shells as Energy Levels

Atomic Shells

Shells as Energy Levels

Maximum Electron Capacity

Shell Lab for Light Elements

Why Calcium Is Two Eight Eight Two

Calcium Shell Filling Steps

Why Not Two Eight Ten

Limits of the Model