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NDA Chemistry · Atomic Structure and Periodic Classification

Electron Configuration and Valence Shells

Electrons fill shells from the inside out, each shell holding up to 2n² electrons; the outermost (valence) shell decides how the atom bonds.

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Why this matters

4 PYQs built on one rule — the maximum electrons a shell can hold is 2n². From that rule the bank asks the cap of a named shell (K, L, M, N), or works out the valence-shell count of an element from its atomic number. Learn 2n² and you cover the whole subtopic.

Concept 1 of 2: Maximum electrons in a shell (the 2n² rule)

Electrons live in shells labelled K, L, M, N (n = 1, 2, 3, 4). Each shell can hold at most 2n² electrons — so the more we go outward, the more room there is. The first shell holds 2, the next 8, then 18, then 32.

Definition

The shell-capacity rule and the resulting caps:

  • The maximum number of electrons a shell can hold is 2n², where n is the shell number.
  • K shell (n = 1) → 2(1)² = 2 electrons.
  • L shell (n = 2) → 2(2)² = 8 electrons.
  • M shell (n = 3) → 2(3)² = 18 electrons.
  • N shell (n = 4) → 2(4)² = 32 electrons.

Maximum electrons per shell

Maximum electrons=2n2\text{Maximum electrons} = 2n^2
  • nshell number (K=1, L=2, M=3, N=4)

Worked example

What is the maximum number of electrons that the N shell can hold?
Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2020 · Q72Easy

Example 1 · Atomic Structure and Periodic Classification · Electron Configuration and Valence Shells

The number of maximum electrons in N Shell is

2n² is a CAP, not the actual filling

The 2n² rule gives the maximum a shell can hold. In real atoms the outermost shell never holds more than 8 before the next shell starts filling — but the capacity question asks for the cap (M = 18, N = 32).
Drill 1 more on maximum electrons in a shell (the 2n² rule)

Concept 2 of 2: Valence-shell electrons and bonding from the configuration

Write the configuration by filling 2, 8, 8, 18… from the inside, and the electrons left in the outermost shell are the valence electrons. Those decide the valency — how many electrons the atom gains, loses or shares to reach a full octet.

Definition

How to read bonding from the configuration:

  • Fill shells in order 2, 8, 8, 18 (using the Bohr filling scheme); the outermost shell's electrons are the valence electrons.
  • Bromine (Z = 35): config 2, 8, 18, 7 → valence shell has 7 electrons.
  • An atom with 6 valence electrons (Z = 8, oxygen: 2, 6) needs to gain 2 to complete its octet — so it gains 2 electrons (e.g. when bonding with sodium to form Na₂O).
  • Valency = electrons gained, lost or shared to reach 8 in the outer shell.

Electrons gained/lost to reach an octet

Valency=8−(valence electrons)   or   (valence electrons)\text{Valency} = 8 - (\text{valence electrons}) \;\text{ or }\; (\text{valence electrons})

Worked example

An element has atomic number 35. Using the Bohr filling scheme, how many electrons are in its valence shell?
Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Apr 2022 · Q84Moderate

Example 2 · Atomic Structure and Periodic Classification · Electron Configuration and Valence Shells

For an element with atomic number 35, which one of the following will be the correct number of electrons in its valence shell based on Bohr's model of an atom?

Valence electrons live in the OUTERMOST shell only

For Z = 35 the configuration is 2, 8, 18, 7 — the M shell holds 18, but the valence shell is the N shell with 7. Count only the outermost shell, not the largest.
Drill 1 more on valence-shell electrons and bonding from the configuration

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

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