Electrons, energy levels and atomic orbitals

AS Level Chemistry · Atomic structure. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Electrons are arranged in shells (principal quantum number n = 1, 2, 3 ...). Each shell is split into sub-shells called s, p and d, and each sub-shell is made of orbitals. One orbital holds at most two electrons, with opposite spins.

In the ground state, electrons fill the lowest energy sub-shells first: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Inside a sub-shell, electrons go into separate orbitals first and pair up only when they have to, because paired electrons repel.

An s orbital is spherical. A p orbital is dumb-bell shaped. A free radical is a species with one or more unpaired electrons.

Rules to remember

Common mistake

For ions of the elements scandium to zinc, students remove 3d electrons first. Write the atom first, then take electrons from 4s before 3d.

Worked example

What is the electronic configuration of the Mn2+ ion (proton number 25)?

  1. The atom has 25 electrons: 1s2 2s2 2p6 3s2 3p6 3d5 4s2, or [Ar] 3d5 4s2.
  2. A 2+ ion has lost two electrons, leaving 23.
  3. The two 4s electrons are lost first, so 3d5 stays.

Answer: [Ar] 3d5, with five unpaired electrons.

Practice questions

Try each one, then open the answer.

1. In the electrons-in-boxes diagram of an oxygen atom (proton number 8), how many unpaired electrons are shown?

  1. A
    2
  2. B
    1
  3. C
    0
  4. D
    3
Show answer

Answer: A. Oxygen is 1s2 2s2 2p4. The four 2p electrons fill the three boxes as one pair and two single electrons, so two are unpaired.

2. Which describes the shapes of s and p orbitals?

  1. A
    s is dumb-bell shaped; p is spherical
  2. B
    both s and p orbitals are spherical, but p is larger
  3. C
    s is spherical; p is dumb-bell shaped
  4. D
    s is spherical; each p orbital is a flat ring around the nucleus
Show answer

Answer: C. An s orbital is spherical around the nucleus. Each p orbital has two lobes on opposite sides of the nucleus (a dumb-bell), and the three p orbitals lie along the x, y and z axes.

3. Which species is a free radical?

  1. A
    Cl−
  2. B
    Cl2
  3. C
    CH3+
  4. D
    •CH3
Show answer

Answer: D. A free radical has one or more unpaired electrons. The methyl radical •CH3 has seven electrons around carbon, one of them unpaired. Ions and Cl2 have all electrons paired.

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