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Periodic Trends Radius Ie En

Learn and apply Periodic Trends Radius Ie En in the published Chemistry course sequence.

  • GCE A-Level H1 Chemistry 8873-2027
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H1 Periodic Trends: Radius, Ionisation Energy and Electronegativity: Orientation

Periodic trends are causal explanations, not lists to memorise. For every comparison, identify the change in nuclear charge, distance and shielding, then link the resulting attraction to the measured property.

H1 8873 scope
  • H1 applies down-group trend reasoning to Group 17, not the H2 Group 2 strand.
  • The Period 3 and ionic-radius reasoning remains fully required.

Definitions (Must Know)

  • Atomic radius is an operational measure of atomic size, commonly half the internuclear distance between neighbouring atoms of the same element.
  • Ionic radius is the radius assigned to an ion from interionic distances in a lattice.
  • First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1 + ions:

X(g) → X⁺(g) + e⁻

  • Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond.
  • Shielding is the reduction in nuclear attraction for an outer electron caused by repulsion from inner-shell electrons.

Detailed Explanations

A. Across Period 3

The outer configurations develop from 3s¹ in sodium to 3s²3p⁵ in chlorine. Each successive atom has one more proton, while the added electron enters the third shell. Inner-shell electron count is unchanged, so shielding changes little. The increasing effective nuclear attraction pulls the electron cloud closer, increases the energy required to remove an electron and increases attraction for a bonding pair.

First-ionisation-energy data contain local deviations because subshell energy and electron pairing also matter. State the overall attraction trend first, then explain a supplied anomaly using the electron configuration.

B. Down Group 17

The outer pattern remains ns²np⁵, but n increases. The outer electrons are farther from the nucleus and more shielded. Although proton number rises, distance and shielding dominate, so atomic radius increases while first ionisation energy and electronegativity decrease.

C. Ionic-radius workflow

  1. Count electrons and occupied shells.
  2. If the ions are isoelectronic, compare proton number directly.
  3. Otherwise compare occupied shells before refining the answer with charge and attraction.

For Na⁺, Mg²⁺ and Al³⁺, all three ions have ten electrons. Increasing proton number contracts the same electron cloud, so Al³⁺ is smallest.

Worked Examples

Modelled example 1

Compare Sodium and Chlorine Atomic Radii

Core

Problem

Explain why chlorine has a smaller atomic radius than sodium.
Study the worked solution

  1. Hold shell and shielding constant

    Method

    State that both outer electron clouds occupy the third principal shell and experience broadly similar shielding.

    Reason

    Moving across Period 3 does not add another occupied shell.

    Working

    Na and Cl are both Period 3 atoms.

  2. Use nuclear charge

    Method

    Give chlorine the greater nuclear charge.

    Reason

    Its nucleus attracts the outer electron cloud more strongly and pulls it closer.

    Working

    r(Cl) < r(Na).

Common misconception 2

Correct a Charge-Only Ionic-Radius Rule

Find and correct the mistake

Learner claim

A learner orders Al³⁺ < Mg²⁺ < Na⁺ but says, “A more positive ion is always smaller.” Assess the order and replace the shortcut with a valid explanation.

Try this before viewing the solution

Electron count in each ion
Increasing radius

Hints

Hint 1: common feature
Confirm the three species are isoelectronic before comparing sizes.
Hint 2: changing feature
Compare proton number acting on the same electron cloud.
View solution step by step

  1. Establish the common cloud

    Method

    Count ten electrons in every ion.

    Reason

    This fixes occupied shells and electron–electron shielding for the comparison.

    Working

    Isoelectronic species.

  2. Rank nuclear attraction

    Method

    Place the greatest proton number first in increasing-radius order.

    Reason

    Aluminium contracts the common cloud most strongly and sodium least strongly.

    Working

    Al³⁺ < Mg²⁺ < Na⁺.

Mind Stretchers

Attempt each unfamiliar application before opening the hint, then compare your reasoning chain with the solution.

Mind stretcher 1: Interpreting a local anomalyExtension

Question. The first ionisation energy falls slightly from magnesium to aluminium despite the general Period 3 increase. Explain using electron configuration.

Show Hint

Compare the subshell from which the electron is removed.

Show Answer

Magnesium loses a 3s electron, whereas aluminium loses a higher-energy 3p electron. The 3p electron is easier to remove, so aluminium has the slightly lower first ionisation energy despite its greater nuclear charge.

Mind stretcher 2: Six-ion deductionExtension

Question. Arrange N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺ and Al³⁺ in increasing radius.

Show Hint

They have the same electron count; order them by proton number.

Show Answer

All have ten electrons. More protons contract the same electron cloud more strongly. Increasing radius is Al³⁺ < Mg²⁺ < Na⁺ < F⁻ < O²⁻ < N³⁻.