1-3 Periodicity (edited 2025, v2)

Module 1: Properties and Structure of Matter

Part 3: Periodicity

Inquiry Question:

• How do the properties of substances help us to classify and separate them?

1.3 Properties of Elements

1.3.1 Key Trends

• Trends in Physical and Chemical Properties

  • State of matter at room temperature

  • Electronic configurations and atomic radii

  • First ionisation energy and electronegativity

  • Reactivity with water

Understanding Periodicity

• Periodicity can be defined as periodic trends:

  • Period Trends: Changes observed left to right across a period (row).

  • Group Trends: Changes observed top to bottom down a group (column).

• It refers to the regular recurrence of properties in the periodic table.

Groups and Periods

Groups (Families)

• Elements in the same group share similar properties and have the same number of electrons in their outer shell.

Periods

• The numbering of periods from 1 to 7 indicates the number of electron shells of its elements.

  • Example: Magnesium (Mg) and Chlorine (Cl) are both in period 3 with electronic configurations of 1s²2s²2p⁶3s² and 1s²2s²2p⁶3s²3p⁵ respectively.

States of Elements at Room Temperature

Room Temperature Defined

• Melting Point (MP): The temperature at which an element transitions from solid to liquid.

• Boiling Point (BP): The temperature at which an element transitions from liquid to gas.

• Classification of States:

  • Solid: MP > 25°C

  • Liquid: MP < 25°C < BP

  • Gas: BP < 25°C

Atomic Properties

Property 1: Atomic Radius

• Trends in Atomic Radius:

  • Decreases from left to right across a period.

  • Increases down a group due to additional electron shells.

  • Explanation:

    • Down a Group: More shells added, increasing distance between nucleus and valence electrons, thus increasing radius.

    • Across a Period: Increase in protons leads to greater electrostatic attraction, pulling electrons closer and decreasing radius.

Property 2: Electronegativity

• Defined as the ability of an atom to attract bonding electrons.

• Trends:

  • Decreases down a group due to increased shielding and distance from nucleus.

  • Increases across a period as effective nuclear charge increases.

• Maximum Electronegativity: Observed in Group 17 (Halogens); Noble Gases (Group 18) have an electronegativity of zero.

Ionisation Energy

Property 3: Ionisation Energy

• Defined as the energy required to remove an electron from a gaseous atom.

• Trends:

  • Increases across a period as nuclear charge increases.

  • Decreases down a group due to increased electron shielding.

• Example Reactions:

  • First Ionisation Energy: M(g) → M+(g) + e–

Reactivity with Water

Property 4: Reactivity with Water

• Alkali Metals: Reactivity increases down the group due to lower ionisation energy.

  • Reaction Example: Lithium + Water → Lithium Hydroxide + Hydrogen

• Non-metals: Do not react with water but become more reactive across a period due to increasing electronegativity.

• Reactive metals show more vigorous reactions as you proceed down Group 1.

Summary of Periodic Trends

• Atomic Radius: Increases down a group, decreases across a period.

• Ionisation Energy: Decreases down a group, increases across a period.

• Electronegativity: Decreases down a group, increases across a period.

• Reactivity of Metals: Increases down Group 1 and Group 2, while generally decreases across other groups.