What is Atomic orbital, shell, subshell?

Example 1.10 (The third shell)

For $n = 3$: $l = 0$ (3s, one orbital), $l = 1$ (3p, three orbitals), $l = 2$ (3d, five orbitals, $m_l = -2, \dots, 2$): nine orbitals and up to 18 electrons. An electron in 3d has, for instance, the quantum numbers $(3, 2, -1, +\tfrac12)$.

Example 1.18 (Carbon, nitrogen, oxygen, iron)

The method gives, for four atoms:

atom	configuration	last subshell	unpaired
$\ce{C}$ ($Z = 6$)	$[\ce{He}]\,2\text{s}^2 2\text{p}^2$	2p:	2
$\ce{N}$ ($Z = 7$)	$[\ce{He}]\,2\text{s}^2 2\text{p}^3$	2p:	3
$\ce{O}$ ($Z = 8$)	$[\ce{He}]\,2\text{s}^2 2\text{p}^4$	2p:	2
$\ce{Fe}$ ($Z = 26$)	$[\ce{Ar}]\,3\text{d}^6 4\text{s}^2$	3d:	4

For iron the Klechkowski order fills 4s ($Z = 19, 20$) before 3d ($Z = 21$ to 30); the configuration is written with 3d first.