Electron Configuration: Quick Reference (Shells, Subshells, and Periodic Table Blocks)

Shells = distance from nucleus; denoted by the principal quantum number $n$ (e.g., $n=1,2,3,4$ ).
Subshells = shape within a shell; letters $s, p, d, f$ indicate shape: $s$ = spherical, $p$ = dumbbell, $d$ = cloverleaf, $f$ = more complex.
Orbitals = orientation/directionality within a subshell.
In an electron configuration, you get information about the shell and subshell; the orbital links (exact orientation) are not always specified.
The exponent (superscript) on each $n ext{l}$ designation gives the number of electrons in that shell–subshell combination: e.g., $1s^2$ means 2 electrons in the 1s subshell.
Example interpretation: in $3s^2 3p^6$ within the same shell ( $n=3$ ), there are 2 electrons in the $3s$ subshell and 6 electrons in the $3p$ subshell, totaling $8$ electrons in the third shell’s subshells.

Use the periodic table to build configurations by blocks:
- S-block = left side; P-block = right side; D-block = middle transition metals; F-block = separate bottom rows.
- The blocks correspond to subshell types: $s, p, d, f$ .
How to fill:
- Move across a period left to right, then drop down to the next row when you can’t go further, continuing until you reach the target element.
- For every element you pass, add one electron to the configuration in the correct subshell as you go.
- The row number tells the shell $n$ ; the block tells the subshell ( $s, p, d, f$ ).

s-block: leftmost columns; p-block: rightmost columns; d-block: middle; f-block: bottom two rows (lanthanides/actinides).
Long-form rule (Aufbau-style): fill in order as you move through the table, respecting the available subshell capacities.
Common shorthand: use noble gas core to compress the configuration: $[ ext{NobleGas}] ext{remaining electrons}$ (e.g., $[ ext{Ar}] 3d^8 4s^2$ for neutral Ni).
Exponents per subshell follow maximum capacities: $s: 2, p: 6, d: 10, f: 14$ .

Nickel has atomic number 28 (neutral = 28 electrons).
Long-form configuration (stepwise as you move across):
$1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^2 \, 3d^8$
Total electrons in third shell (n=3) equals $3s^2 + 3p^6 = 8$ , all within the third shell; another portion is in the fourth shell (4s^2) and a 3d subshell with 8 electrons.
Neutral Ni (Ni): $28$ electrons; configuration can also be written in shorthand as $[ ext{Ar}] 3d^8 4s^2$ .
Ions (quick notes):
- Ni^{2+} (two electrons removed) typically loses 4s electrons first, giving $[ ext{Ar}] 3d^8$ (26 electrons total).
- Higher or lower charges will adjust by removing or adding electrons accordingly; concept to remember: electrons are negative, so higher positive charge means fewer electrons.

Replace the noble gas core with its symbol in brackets, then continue with the remaining electrons:
- Example: $[ ext{Ar}] 3d^8 4s^2$ for neutral Ni.
For elements beyond the noble gas core, continue with the appropriate subshell sequence (including possible 4f, 5d, etc., as needed).

Don’t confuse charge with the number of electrons: a positive charge means fewer electrons, not more.
The order of filling follows the general pattern across periods and down periods; the explicit order can be compressed into shorthand with noble gas cores.
The f-block is real but is less commonly tested for basic exams; be prepared to identify it as the lanthanide/actinide region.
The notation $n ext{l}^{m}$ (e.g., $3d^8$ ) captures the shell, subshell, and electron count for that segment of the configuration.