Electronic configuration of atoms

The complete distribution of an atom's electrons among its various orbitals is called its electronic configuration. Once the Aufbau, Pauli and Hund rules are all in hand, writing it down for any atom is mechanical.

Notation. An electronic configuration is written using the shorthand $nl^x$, where $n$ is the principal quantum number of the shell, $l$ is written using the subshell's LETTER designation rather than its numeric value ($s$ for $l=0$, $p$ for $l=1$, $d$ for $l=2$, $f$ for $l=3$), and $x$ is the number of electrons actually occupying that subshell. For hydrogen's single electron, which by the Aufbau principle occupies the lowest-energy orbital available (1s), this gives $n=1$, subshell letter $s$, and $x=1$ - so the configuration is written $1s^1$ and read aloud as "one-ess-one." Its orbital-box diagram is a single box containing one upward arrow, labelled $1s^1$.

Building up to neon. Table 2.4 works through every element from hydrogen ($Z=1$) to neon ($Z=10$), applying the three rules in combination at each step: the shells fill $1s$ then $2s$ then $2p$ in that order (Aufbau); each orbital box takes a maximum of two electrons of opposite spin before the next orbital is touched (Pauli); and within the three 2p orbitals, each gets one electron before any of them gets a second (Hund's rule) - visible in the table as boron through nitrogen picking up one 2p electron at a time in three separate orbitals, before oxygen through neon start pairing those same three orbitals up. …