Structure of the Atom • Topic 2 of 3

Bohr's Model & Electronic Configuration

To fix the stability problem of Rutherford's model, Neils Bohr put forward a new picture of the atom in 1913.

Bohr's postulates

  • Electrons revolve around the nucleus only in certain fixed circular paths called orbits, shells or energy levels.
  • Each shell is associated with a definite (discrete) amount of energy, so these are also called energy levels.
  • As long as an electron stays in a particular shell, it does not radiate energy. This is why the atom is stable.
  • Energy is absorbed or released only when an electron jumps from one shell to another.

Naming the shells

The shells are numbered outward from the nucleus by the shell number n = 1, 2, 3, 4, ... and are also given letters: the first shell (n = 1) is the K shell, the second (n = 2) is the L shell, the third (n = 3) is the M shell, and the fourth (n = 4) is the N shell. Shells closer to the nucleus have lower energy; energy increases as we move outward (K < L < M < N).

The 2n2 rule

The maximum number of electrons that a shell can hold is given by the formula 2n2, where n is the shell number. This gives:

  • K shell (n = 1): 2 × 12 = 2 electrons
  • L shell (n = 2): 2 × 22 = 8 electrons
  • M shell (n = 3): 2 × 32 = 18 electrons
  • N shell (n = 4): 2 × 42 = 32 electrons

The outermost-shell limit

There are two extra rules for filling shells. First, the outermost shell of any atom can hold a maximum of 8 electrons (the octet), even though its 2n2 capacity may be larger. Second, electrons are not placed in an outer shell until the inner shells are filled in a stepwise manner. These rules are why, for the first 18–20 elements, we fill K, then L, then M (up to 8), and only then begin N.

Drawing electronic configurations

The electronic configuration is the way electrons are distributed in the shells, written K, L, M, N from inside out. For example, sodium (Z = 11) is 2, 8, 1 and chlorine (Z = 17) is 2, 8, 7. Calcium (Z = 20) is 2, 8, 8, 2 — the M shell stops at 8 (not 18 yet) and the next two electrons start the N shell, following the outermost-shell and stepwise-filling rules.

Bohr model of magnesium (Z = 12): K shell 2, L shell 8, M shell 2+12K = 2L = 8M = 2Magnesium (Z = 12): 2, 8, 2
Solved Examples
1
Worked Example
State the maximum number of electrons in the K, L, M and N shells using the 2n2 rule.
Solution
  1. K shell (n = 1): 2 × 12 = 2.
  2. L shell (n = 2): 2 × 22 = 8.
  3. M shell (n = 3): 2 × 32 = 18.
  4. N shell (n = 4): 2 × 42 = 32.

Answer: K = 2, L = 8, M = 18, N = 32.

2
Worked Example
Write the electronic configuration of sodium (Z = 11).
Solution
  1. Sodium has 11 electrons (Z = 11).
  2. Fill K first: 2 electrons. Remaining = 9.
  3. Fill L next: 8 electrons. Remaining = 1.
  4. The last electron goes into the M shell: 1.

Answer: Na (Z = 11): 2, 8, 1.

3
Worked Example
Write the electronic configuration of chlorine (Z = 17).
Solution
  1. Chlorine has 17 electrons.
  2. K = 2 (remaining 15), L = 8 (remaining 7).
  3. The remaining 7 go into the M shell.

Answer: Cl (Z = 17): 2, 8, 7.

4
Worked Example
Write the electronic configuration of calcium (Z = 20) and explain why the M shell does not hold 10 electrons.
Solution
  1. Calcium has 20 electrons.
  2. K = 2 (remaining 18), L = 8 (remaining 10).
  3. By the outermost-shell rule, an outer shell takes at most 8 before the next shell starts, so M takes 8 (remaining 2).
  4. The last 2 electrons begin the N shell.

Answer: Ca (Z = 20): 2, 8, 8, 2 — M stops at 8 and the next two start N.

5
Worked Example
An atom has the configuration 2, 8, 8. Identify it and state its atomic number.
Solution
  1. Add up the electrons: 2 + 8 + 8 = 18.
  2. The number of electrons in a neutral atom equals its atomic number.
  3. So Z = 18, which is argon (Ar).

Answer: The atom is argon (Ar), Z = 18.

6
Worked Example
Why does an atom not radiate energy as long as the electron stays in a fixed shell?
Solution
  1. According to Bohr, the shells are fixed energy levels with definite energy.
  2. While an electron revolves within one shell, its energy does not change.
  3. Energy is absorbed or emitted only when the electron jumps from one shell to another.

Answer: Because each shell is a fixed energy level; the electron neither gains nor loses energy while it stays in the same shell, which keeps the atom stable.

Key Points

  • Bohr's model: electrons revolve in fixed circular orbits (shells / energy levels) and do not radiate energy while staying in a shell, which explains the atom's stability.
  • Shells are named K (n = 1), L (n = 2), M (n = 3), N (n = 4) outward from the nucleus, with increasing energy.
  • The maximum electrons a shell can hold is 2n2: K = 2, L = 8, M = 18, N = 32.
  • The outermost shell holds a maximum of 8 electrons, and electrons fill inner shells stepwise before outer ones.
  • Electronic configuration lists electrons shell by shell, e.g. Na (Z = 11) = 2, 8, 1 and Ca (Z = 20) = 2, 8, 8, 2.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.The maximum number of electrons that the M shell (n = 3) can hold is:
Explanation: By the 2n² rule, the M shell holds 2 × 3² = 18 electrons.
Q2.The electronic configuration of sodium (Z = 11) is:
Explanation: Filling K (2), then L (8), leaves 1 for the M shell: 2, 8, 1.
Q3.The maximum number of electrons in the outermost shell of any atom is:
Explanation: The outermost shell can hold a maximum of 8 electrons (the octet), even if its 2n² capacity is larger.
Q4.According to Bohr, an electron radiates energy when it:
Explanation: Energy is absorbed or emitted only when an electron jumps between shells, not while it stays in a fixed shell.
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