A Level Chemistry: [Shortcut] Determine no. of paired/ unpaired e

Determine total e^– count of the species.
If electron count is less than 18, it is faster to just write out the electronic configuration.
If electron count is more than 18, subtract e^– count by 18, 36 or 54; the largest number that will give you a positive result.
If the remainder is less than or equal to 10, fill them up in d orbitals. Ignore the quantum shell.
If the remainder is more than 10, subtract 10 from the remainder and fill up the leftover e^– in s and p orbitals. Ignore the quantum shell.

Example:

Determine the number of unpaired electrons in (a) Mn²⁺ (b) Cu⁺ (c) Se^– (d) Te⁺.

(a) Mn²⁺: 23e^–

e^– remaining = 23 – 18 = 5

Mn²⁺: […] nd⁵ → 5 unpaired e^–

(b) Mo²⁺ : 40e^–

e^– remaining = 40 – 36 = 4

Mn²⁺: […] nd⁴ → 4 unpaired e^–

e^– remaining = 35 – 18 = 17 (> 10!)

e^– remaining = 17 – 10 = 7

Se^–: […] ns²np⁵→ 1 unpaired e^–

(d) Te⁺: 51e^–

Remaining electrons = 51 - 46 = 5 ns2np3: 3 unpaired electrons

e^– remaining = 51 – 36 = 15 (> 10!)

e^– remaining = 15 – 10 = 5

Te⁺: […] ns²np³→ 3 unpaired e^–