The periodic table arranges elements in order of increasing atomic (proton) number into horizontal periods (rows 1–7) and vertical groups (columns I to 0). An element's position reveals its electronic structure, bonding behaviour and reactivity.
The notes cover trends across Period 3 and down Group I, Group VII and Group 0, including reactions with water, chlorine and halide salts. Mastering group and period trends is a core skill tested across all O-Level Chemistry topics.
What these notes cover
- •Structure of the Periodic Table
- •Electronic Configuration and Position
- •Trends Across a Period (Period 3: Na to Ar)
- •Group I: Alkali Metals — Physical and Reactivity Trends
- •Group I Reactions with Water and Chlorine
- •Group VII: Halogens — Physical Properties and Reactivity
- •Halogen Displacement Reactions
- •Cyclohexane Extraction Test
- •Group 0: Noble Gases and Their Uses
- •Worked Examples
- •Common Mistakes
- •Summary
Key definitions
- Period
- A horizontal row of the periodic table (rows 1–7). The period number equals the number of occupied electron shells.
- Group
- A vertical column of the periodic table (I to 0). For Groups I–VII, the group number equals the number of valence electrons, and elements in the same group have similar chemical properties.
- Valence electrons
- The electrons in the outer shell of an atom. Elements with the same number of valence electrons share similar chemical properties.
- Halogen displacement reaction
- A reaction in which a more reactive halogen (higher in Group VII) displaces a less reactive halogen from its aqueous salt.
- Noble gases
- Group 0 elements such as He, Ne and Ar with full outer shells (He: 2 electrons; others: 8 electrons). They are chemically inert under normal conditions because they have no tendency to gain, lose or share electrons.
Key formulas
Position in the periodic table
Period number = number of occupied electron shells; Group number (I–VII) = number of valence electrons
Example: sodium (2, 8, 1) has 3 occupied shells and 1 valence electron, so it is in Period 3, Group I.
Group I metal with water
2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)
M = Li, Na or K. Vigour increases down the group: Li (steady fizzing) → Na (rapid, hissing) → K (ignites with lilac flame).
Sodium with chlorine
2Na(s) + Cl₂(g) → 2NaCl(s)
Group I metals form white ionic solids (metal chloride salts) with chlorine. Reactivity order: K > Na > Li.
Chlorine displacing bromine
Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
Chlorine is higher in Group VII and more reactive, so it displaces bromine from its aqueous salt.
Ionic equation for the displacement
Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)
The same reaction with the spectator K⁺ ions removed.
Worked example
Element X has the electron configuration 2, 8, 7. State its period and group. Write an ionic equation for X displacing iodide ions from aqueous potassium iodide.
- 1X has 3 occupied shells, so it is in Period 3; it has 7 valence electrons, so it is in Group VII (a halogen).
- 2Period 3, Group VII identifies X as chlorine (Cl₂).
- 3Chlorine is above iodine in Group VII, so it is more reactive and displaces I⁻: Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq).
- 4The cyclohexane layer turns violet, showing the presence of I₂.
Watch out: Give every species a state symbol and cancel only the spectator K⁺ ions — do not cancel ions that are actually transferred or changed.
Common mistakes
Reversing the Group I and Group VII trends
Wrong: Stating that reactivity decreases down Group I or increases down Group VII.
Right: Reactivity increases down Group I (outer electron lost more easily) but decreases down Group VII (incoming electron gained less easily). Always anchor the explanation to electron gain or loss.
Getting the displacement direction wrong
Wrong: Writing that a halogen lower in the group, such as I₂, displaces Br⁻ or Cl⁻.
Right: A halogen higher in the group always wins: Cl₂ displaces both Br⁻ and I⁻; Br₂ displaces I⁻ only; I₂ displaces neither.
Period 3 melting-point trap
Wrong: Stating that melting point simply rises or falls across Period 3.
Right: The trend is non-linear: Na–Si have giant structures (high melting points) and P–Ar are simple molecular (low melting points). The sudden drop at phosphorus reflects a change in structure type.
Questions students ask
Why does reactivity increase down Group I?
Going down Group I, atomic radius increases, so the outer electron is further from the nucleus and more effectively shielded by inner electron shells. The nuclear attraction holding the valence electron weakens, so it is lost more easily and the metal is more reactive.
Why does reactivity decrease down Group VII?
Each halogen must gain one electron to complete its outer shell. As atomic radius increases, the nucleus has a weaker attraction for an incoming electron because of greater distance and more inner-shell shielding. Gaining the electron becomes harder, so reactivity decreases.
How do you identify which halogen is displaced using cyclohexane?
Cyclohexane is added and the colour of the organic layer is observed. Cl₂ gives a colourless or pale yellow layer, Br₂ gives orange-brown and I₂ gives violet or purple.
How do you find the period and group of an element from its electron configuration?
The period number equals the number of occupied electron shells, and the group number (I–VII) equals the number of valence electrons. For example, potassium (2, 8, 8, 1) has 4 shells and 1 valence electron, so it is in Period 4, Group I.
In the full PDF
- •Table of halogen physical properties: formula, state and colour at room temperature and pressure
- •Table of noble gas uses explained by their inertness
- •Two further worked examples: why potassium reacts more vigorously with water than sodium, and predicting cyclohexane colours when bromine water is added to aqueous KCl and KI
- •The remaining common mistakes on state symbols and ionic equations
- •A five-point summary of the key periodic table trends