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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 H2 Chemistry 9476-2027
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Periodic Trends: Radius, Ionisation Energy and Electronegativity: Orientation

Trend questions are the fastest marks in the paper if you use the same explanation skeleton every time. This lesson trains the “drivers → attraction → property” chain for radius, ionisation energy and electronegativity, plus the isoelectronic shortcut for ionic radii.

Definitions (Must Know)

A. Atomic radius

The atomic radius is an operational measure of atomic size, commonly taken as half the internuclear distance between neighbouring atoms of the same element. The measurement model must be compared consistently.

B. Ionic radius

The ionic radius is the radius of an ion in an ionic lattice (as inferred from interionic distances).

C. First ionisation energy, IE₁

The 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⁻

D. Electronegativity

Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond.

E. Shielding

Shielding is the reduction in attraction between the nucleus and an outer electron due to repulsion by inner-shell electrons.

Detailed Explanations

A. Electronic configurations across and down

  • Across Period 3, electrons are added to the third shell: Na is [Ne]3s¹, Mg is [Ne]3s², then the 3p subshell fills from Al, [Ne]3s²3p¹, to Cl, [Ne]3s²3p⁵.
  • Down Group 2, each atom has two outer-shell electrons, ns².
  • Down Group 17, each atom has seven outer-shell electrons, ns²np⁵.

The value of n increases down a group, so an additional occupied shell is present at each step.

B. Across a period (why the “attraction” increases)

  • Proton number increases, so nuclear charge increases.
  • The added electron enters the same principal shell, so the outer electrons are at a similar distance from the nucleus.
  • Inner shells do not change across a period, so shielding changes little.

Therefore the attraction between the nucleus and the outer electrons increases.

This stronger attraction:

  • pulls the electron cloud in → atomic radius decreases
  • holds the outer electron more strongly → IE₁ generally increases
  • attracts the bonding pair more strongly → electronegativity increases

Mini example (compare two Period 3 atoms):

  • Cl has a higher Z than S but the same number of shells, so it attracts the bonding pair more strongly → higher electronegativity.

C. Down a group (why the “attraction” decreases)

  • Outer electrons occupy a higher shell, so they are further from the nucleus (greater distance).
  • There are more inner shells, so there is more shielding.

Therefore attraction to the outer electrons decreases, so atomic radius increases, IE₁ decreases, and electronegativity decreases.

D. Ionic radius comparisons (workflow)

  1. Check if the species are isoelectronic (same number of electrons).
  2. If isoelectronic: higher Z → smaller radius (stronger attraction for the same electron count).
  3. If not isoelectronic: compare shells first (more shells usually means bigger), then apply the attraction idea.

Mini example (isoelectronic shortcut): Na⁺ \ and\ Mg²⁺

  • Both have 10 electrons, so they are isoelectronic.
  • Mg²⁺ has higher Z, so it attracts the same electron cloud more strongly → Mg²⁺ is smaller.

Across Period 3 there are two useful series:

  • Na⁺ > Mg²⁺ > Al³⁺ in radius: all have 10 electrons, so increasing Z contracts the electron cloud.
  • P³⁻ > S²⁻ > Cl⁻ in radius: all have 18 electrons, so increasing Z again contracts the electron cloud.

There is a large increase from Al³⁺ to P³⁻ because the anions have an occupied third shell whereas the cations have lost theirs.

Worked Examples

Modelled example 1

Rank Isoelectronic Ionic Radii

Core

Problem

Arrange Na⁺, Mg²⁺ and Al³⁺ in order of increasing ionic radius.
Study the worked solution

  1. Compare electron counts

    Method

    Recognise that all three ions contain ten electrons.

    Reason

    Isoelectronic species have the same number of occupied shells and a directly comparable electron cloud.

    Working

    Na⁺, Mg²⁺ and Al³⁺: 10 electrons each.

  2. Compare nuclear charge

    Method

    Put the ion with the greatest proton number first in increasing-radius order.

    Reason

    A larger nuclear charge attracts the same electron cloud more strongly and makes it smaller.

    Working

    Al³⁺ < Mg²⁺ < Na⁺.

Guided practice 2

Order Period 3 Atomic Radii

About 5 min

Problem

Arrange Na, Mg, Al, Si, P, S and Cl in order of decreasing atomic radius.

Choose the direction before explaining

Decreasing-radius direction

Hints

Hint 1: shells
The added electrons enter the same principal shell.
Hint 2: attraction
Nuclear charge increases while shielding changes comparatively little.
View solution step by step

  1. Identify the driver

    Method

    Use increasing nuclear charge across the period.

    Reason

    The same occupied shell and similar shielding allow the nucleus to attract the outer electrons more strongly.

    Working

    Atomic radius decreases from left to right.

  2. Write the order

    Method

    List sodium first and chlorine last.

    Reason

    The question asks for decreasing radius.

    Working

    Na > Mg > Al > Si > P > S > Cl.

Common misconception 3

Explain Chlorine versus Sulfur Electronegativity

Find and correct the mistake

Learner claim

A learner says chlorine is more electronegative than sulfur because chlorine has an extra occupied electron shell. Correct the explanation.

Try this before viewing the solution

Occupied-shell comparison

View solution step by step

  1. Remove the false premise

    Method

    State that both atoms use the same outer principal shell.

    Reason

    Moving from sulfur to chlorine does not add a new electron shell.

    Working

    Both are Period 3 atoms.

  2. Give the correct cause

    Method

    Use chlorine’s greater nuclear charge with similar shielding.

    Reason

    Chlorine attracts a shared pair of bonding electrons more strongly.

    Working

    EN(Cl) > EN(S).

Challenge 4

Compare Magnesium and Calcium Ionisation Energies

Minimal support

Down-group transfer

Explain why the first ionisation energy of Mg is higher than that of Ca.

Try this before viewing the solution

Hints

Hint 1: outer shell
Calcium’s outer electron occupies a higher principal shell.
Hint 2: net attraction
Combine greater distance with greater shielding before comparing removal energy.
View solution step by step

  1. Compare the outer electrons

    Method

    Place calcium’s outer electron farther from the nucleus and behind more inner shells.

    Reason

    Moving down Group 2 adds an occupied shell and increases shielding.

    Working

    Ca has a 4s outer electron; Mg has a 3s outer electron.

  2. Link attraction to energy

    Method

    State that calcium’s outer electron is less strongly attracted and easier to remove.

    Reason

    Greater distance and shielding outweigh the increase in nuclear charge.

    Working

    IE₁(Mg) > IE₁(Ca).

Mind Stretchers

Mind stretcher 1Extension

Arrange N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺ in order of increasing ionic radius.

Show Answer

Mark scheme:

  • All are isoelectronic (10 electrons).
  • Increasing Z pulls the same electron cloud in more strongly → smaller radius.
  • Increasing radius: Al³⁺ < Mg²⁺ < Na⁺ < F⁻ < O²⁻ < N³⁻.