Chemical Periodicity of Other Elements

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Predict the characteristic properties of an element in a given group by using knowledge of chemical periodicity
  2. Deduce the nature, possible position in the Periodic Table and identity of unknown elements from given information about physical and chemical properties

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What is Chemical Periodicity?

Periodicity means that certain properties of elements repeat in a regular pattern as you move through the Periodic Table. This happens because elements are arranged by their atomic number (number of protons), and elements in the same group (vertical column) have the same number of electrons in their outer shell.

Because elements in the same group have similar outer electron arrangements, they show similar chemical and physical behaviour. This predictable pattern is what we call periodicity.

Key point: If you know how one element in a group behaves, you can predict how other elements in that same group will behave.

Two-Way Predictions Using Periodicity

Periodicity allows you to make predictions in two directions:

Direction 1: Position Known → Predict Properties

  • If you know where an element is in the Periodic Table, you can predict its physical and chemical properties
  • You do this by comparing it to other elements in the same group

Direction 2: Properties Known → Predict Position

  • If you know the properties of an unknown element, you can work out where it belongs in the Periodic Table
  • You do this by matching its behaviour to the patterns shown by different groups

Understanding Periodic Trends

To make accurate predictions, you need to understand how certain properties change across the Periodic Table. Here are the key trends:

Atomic Radius (Size of Atoms)

Across a period (left to right):

  • Atomic radius decreases
  • Why? The number of protons increases, creating a stronger pull on the electrons. The electrons are pulled closer to the nucleus, making the atom smaller.
  • The number of electron shells stays the same across a period.

Down a group (top to bottom):

  • Atomic radius increases
  • Why? Each new period adds an extra electron shell, so the outer electrons are further from the nucleus.
  • Even though there are more protons, the extra distance and shielding (inner electrons blocking the pull from the nucleus) means outer electrons feel less attraction.

Example: Sodium (Na) has a radius of 0.186 nm, while chlorine (Cl) at the end of Period 3 has only 0.099 nm.

Ionic Radius (Size of Ions)

When atoms form ions, their size changes:

Positive ions (cations):

  • Smaller than their parent atoms
  • Why? They've lost electrons (often a whole outer shell), and have more protons than electrons, so the remaining electrons are pulled in tighter.
  • Example: K atom = 200 pm, but K⁺ ion = 138 pm

Negative ions (anions):

  • Larger than their parent atoms
  • Why? They've gained electrons, increasing electron-electron repulsion. This pushes the electron cloud outward.
  • Example: Cl atom is smaller than Cl⁻ ion

Ionisation Energy

First ionisation energy is the energy needed to remove one electron from a gaseous atom.

Across a period:

  • Ionisation energy generally increases
  • Why? Atoms get smaller and nuclear charge increases, so it's harder to remove an electron.

Down a group:

  • Ionisation energy decreases
  • Why? The outer electron is further from the nucleus and shielded by more inner electrons, so it's easier to remove.

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