Module 03
The PeriodicTable & Periodicity
118 elements. Patterns everywhere. The periodic table is chemistry's greatest achievement — a single chart that predicts how every element in the universe will behave. Let's unlock its secrets. 🔥
1. Historical Development
Key Term: PeriodicityThe recurrence of similar physical and chemical properties at regular intervals when elements are arranged by increasing atomic number. Think of it like a musical scale that repeats every octave!
1817
Johann Döbereiner — Triads
Law of Triads
Grouped elements into sets of 3 (triads) with similar properties. The middle element's relative atomic mass ≈ average of the outer two. E.g. Li(7) + K(39) ÷ 2 = 23 ≈ Na(23) ✓. Only worked for a few groups — couldn't extend to all known elements.
1865
John Newlands — Law of Octaves
Law of Octaves
Arranged 56 elements by relative atomic mass. Every 8th element had similar properties (like musical octaves). Failed for heavier elements — he was actually mocked by the Royal Chemical Society, with a colleague sarcastically asking whether arranging elements alphabetically might work just as well! He later received the Davy Medal in 1887.
1869
Dmitri Mendeleev — First Periodic Table
Periodic Classification
Arranged by atomic mass; left GAPS for undiscovered elements; occasionally swapped order to keep chemical families together. Predicted the properties of "eka-aluminium" — six years later it was discovered as gallium, matching his predictions almost perfectly. Chemistry's version of predicting a planet before you can see it!
1914
Henry Moseley — Modern Periodic Table
Arrangement by Atomic Number
Used X-ray experiments to show elements should be arranged by atomic number (not atomic mass). This fixed all of Mendeleev's anomalies and is the basis of the table we use today.
2. Structure of the Modern Periodic Table
Elements are arranged in order of increasing atomic number. The table has vertical columns called groups and horizontal rows called periods.
| Group | Special Name | Key Feature |
|---|---|---|
| Group I | Alkali Metals | 1 valence electron; very reactive metals |
| Group II | Alkaline Earth Metals | 2 valence electrons; reactive metals |
| Group VII | Halogens | 7 valence electrons; reactive non-metals |
| Group 0 (Group 8) | Noble Gases | Full outer shell; chemically unreactive |
Magic Rules!
Group number = number of valence electrons (outermost shell electrons)
Period number = number of occupied electron shells
Example: Phosphorus has config (2,8,5) → 5 valence electrons → Group V, 3 shells → Period 3 ✓
Group number = number of valence electrons (outermost shell electrons)
Period number = number of occupied electron shells
Example: Phosphorus has config (2,8,5) → 5 valence electrons → Group V, 3 shells → Period 3 ✓
General Trends
| Direction | Metallic Nature | Non-metallic Nature | Atomic Radius |
|---|---|---|---|
| ↓ Down a group | Increases ↑ | Decreases ↓ | Increases ↑ |
| → Along a period (L→R) | Decreases ↓ | Increases ↑ | Decreases ↓ |
Your Phone Runs on Periodic Trends!Lithium (Group I, Period 2) is used in phone batteries because it has the smallest atomic radius in its group — meaning it loses its single valence electron VERY easily (high reactivity), generating a strong electrical current. Understanding periodicity literally powers your phone!
3. Group II — The Alkaline Earth Metals
Group II contains: Be(2,2), Mg(2,8,2), Ca(2,8,8,2), Sr(2,8,18,8,2), Ba, Ra. All are reactive metals that never occur free in nature — always in compounds.
General Properties
Fairly soft, silvery-white metals. Tarnish quickly in air. All have two valence electrons → form +2 cations (Mg²⁺, Ca²⁺, Ba²⁺). Reactivity INCREASES going DOWN the group.
| Reaction | Magnesium (Mg) | Calcium (Ca) | Barium (Ba) |
|---|---|---|---|
| With Oxygen | Burns with blinding white flame | Brick-red flame | Apple-green flame; very readily |
| With Water | Very slowly; Mg(OH)₂ + H₂↑ | Vigorously; Ca(OH)₂ + H₂↑ | Very vigorously; Ba(OH)₂ + H₂↑ |
| With dilute HCl | Vigorously; MgCl₂ + H₂↑ | Very vigorously; CaCl₂ + H₂↑ | Violently; BaCl₂ + H₂↑ |
Key Equations
2Mg(s) + O₂(g) → 2MgO(s)
Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g)
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Ca(s) + 2HCl(aq) → CaCl₂(aq) + H₂(g)
Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g)
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Ca(s) + 2HCl(aq) → CaCl₂(aq) + H₂(g)
Why Reactivity Increases DOWN Group IIMoving DOWN, each element has MORE electron shells → larger atomic radius → valence electrons are further from the nucleus AND more shielded by inner shells → easier to lose the 2 valence electrons → HIGHER reactivity. Beryllium (smallest) = least reactive. Radium (largest) = most reactive.
Fireworks Chemistry!Group II metals create the vivid colours in fireworks: Mg = brilliant white | Ca = brick red | Sr = crimson red | Ba = apple green. Next time you see fireworks, you're watching Group II chemistry in action!
4. Group VII — The Halogens
Group VII: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Astatine (At). All are reactive non-metals that exist as diatomic molecules (F₂, Cl₂, Br₂, I₂).
Physical Properties at Room Temperature
| Element | Formula | State | Colour | Valence e⁻ |
|---|---|---|---|---|
| Fluorine | F₂ | Gas | Pale yellow | 7 |
| Chlorine | Cl₂ | Gas | Yellow-green | 7 |
| Bromine | Br₂ | Liquid | Red-brown (orange vapour) | 7 |
| Iodine | I₂ | Solid | Grey-black (purple vapour when heated) | 7 |
Displacement Reactions
Definition: Displacement ReactionA MORE REACTIVE halogen displaces a LESS REACTIVE one from its compound. A less reactive halogen CANNOT displace a more reactive one.
| Halogen Added | Solution | Reaction? | Observation | Equation |
|---|---|---|---|---|
| Cl₂ | KBr (aq) | ✓ YES | Turns red-brown (Br₂ produced) | Cl₂ + 2KBr → 2KCl + Br₂ |
| Cl₂ | KI (aq) | ✓ YES | Turns brown (I₂ produced) | Cl₂ + 2KI → 2KCl + I₂ |
| Br₂ | KI (aq) | ✓ YES | Turns brown (I₂ produced) | Br₂ + 2KI → 2KBr + I₂ |
| Br₂ | KCl (aq) | ✗ NO | No colour change | No reaction |
| I₂ | KBr (aq) | ✗ NO | No colour change | No reaction |
Exam Colour Clues!
Orange/red-brown colour → Bromine (Br₂) has been produced
Brown colour → Iodine (I₂) has been produced
Yellow-green colour → Chlorine (Cl₂) gas is present
Reactivity order (most → least reactive): F > Cl > Br > I > At
Oxidising strength order: At < I < Br < Cl < F
Orange/red-brown colour → Bromine (Br₂) has been produced
Brown colour → Iodine (I₂) has been produced
Yellow-green colour → Chlorine (Cl₂) gas is present
Reactivity order (most → least reactive): F > Cl > Br > I > At
Oxidising strength order: At < I < Br < Cl < F
Why Reactivity INCREASES Going UP Group VIIMoving UP, atoms have FEWER electron shells → smaller atomic radius → valence shell is CLOSER to the nucleus → atom attracts an extra electron MORE strongly → easier to GAIN 1 electron → HIGHER reactivity. Fluorine (smallest) = most reactive. Astatine (largest) = least reactive.
Chlorine Saves Lives!Chlorine is added to swimming pools and drinking water as a powerful oxidising agent. It kills bacteria by accepting electrons from their cell membranes. The WHO estimates that water chlorination prevents millions of deaths annually from cholera, typhoid, and dysentery. Group VII literally saves lives every day!
5. Trends in Period 3
Period 3: Na, Mg, Al, Si, P, S, Cl, Ar. All have 3 occupied electron shells. Moving left → right, metallic nature decreases and non-metallic nature increases.
(2,8,1)
(2,8,2)
(2,8,3)
(2,8,4)
(2,8,5)
(2,8,6)
(2,8,7)
ArNoble Gas(2,8,8)
Physical Properties: Metals vs Non-metals
| Property | Metals (Na, Mg, Al) | Non-metals (P, S, Cl, Ar) |
|---|---|---|
| State at 25°C | Solid | Gas (Cl, Ar); S is solid |
| Electrical conductivity | Good conductors | Insulators (Si = semiconductor) |
| Appearance | Shiny, silvery | Dull |
| Melting/boiling point | High | Low |
| Behaviour in reactions | Lose electrons (1–3 e⁻) | Gain or share electrons |
Why Trends Change Across Period 3Moving L→R, each element adds one more PROTON and one more VALENCE ELECTRON, but the number of shells stays the same (3). More protons = stronger nuclear pull = smaller atomic radius.
METALS: harder to lose electrons → reactivity decreases: Na > Mg > Al
NON-METALS: easier to gain electrons → reactivity increases: P < S < Cl
METALS: harder to lose electrons → reactivity decreases: Na > Mg > Al
NON-METALS: easier to gain electrons → reactivity increases: P < S < Cl
Silicon — The Special Case (Metalloid)Silicon (Group IV) is a metalloid — it has 4 valence electrons and SHARES them (covalent bonding) rather than losing or gaining. It's a SEMICONDUCTOR — conductivity between metals and non-metals. This makes it the material of choice for transistors, microchips, and solar cells. The 2020 Apple M1 chip had 16 billion silicon transistors on a chip smaller than your fingernail!
Table Salt — Period 3's Most Famous Reaction!Sodium (highly reactive metal, left of Period 3) reacts violently with chlorine (highly reactive non-metal, right of Period 3) to form sodium chloride — ordinary table salt: 2Na(s) + Cl₂(g) → 2NaCl(s). Two dangerous substances combine to make something essential for life. Chemistry is full of surprises!
6. Simulations & Videos
7. CSEC Practice Questions
Attempt each question first, then click to reveal the step-by-step answer. Remember the 4-point reactivity explanation for full marks! 🎯
The electronic configuration of element X is (2,8,5). (a) State the group and period of X. (b) Is X a metal or non-metal? Explain. (c) When X reacts, does it lose or gain electrons? How many? [5 marks]
5 Marks
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Step-by-Step Answer
1Part (a) — Group: Count valence electrons (outermost shell) = 5 → Group V. Count occupied shells = 3 (shells with electrons: 2, 8, 5) → Period 3. [1 mark]
2Part (b) — Metal or non-metal? X is in Period 3 with 5 valence electrons — it is on the right side of Period 3 (after Si), so it is a NON-METAL. [1 mark] Explanation: Non-metals have more than 4 valence electrons and tend to gain/share electrons; they are poor conductors and have low melting points. [1 mark]
3Part (c): X is a non-metal with 5 valence electrons. It GAINS electrons to achieve a full outer shell of 8. It gains 8 − 5 = 3 electrons. [2 marks]
✅ (a) Group V, Period 3 | (b) Non-metal — right side of Period 3, gains electrons | (c) Gains 3 electrons → forms a 3− ion (X is Phosphorus, P³⁻)
Explain why calcium reacts more vigorously than magnesium with dilute hydrochloric acid. [4 marks]
4 Marks
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Step-by-Step Answer — Use All 4 Points!
1Number of shells: Calcium has MORE occupied electron shells than magnesium (Ca = 4 shells; Mg = 3 shells). [1 mark]
2Atomic radius: Because Ca has more shells, its atomic radius is LARGER — the valence electrons are further from the nucleus. [1 mark]
3Shielding: Ca's valence electrons are more shielded by inner electron shells from the positive pull of the nucleus. [1 mark]
4Ease of ionisation: Therefore it is EASIER for calcium to lose its 2 valence electrons → calcium is more reactive → reacts more vigorously with HCl. [1 mark]
✅ 4-point answer: More shells → larger atomic radius → more shielding → easier to lose electrons → more reactive
When chlorine gas is bubbled into potassium bromide solution, the solution turns orange-brown. Explain why, and write a balanced equation for the reaction. [4 marks]
4 Marks
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Step-by-Step Answer
1Identify what's happening: This is a displacement reaction. Chlorine is a more reactive halogen than bromine. [1 mark]
2Explain why Cl₂ is more reactive than Br₂: Chlorine has fewer electron shells → smaller atomic radius → valence shell is closer to nucleus → less shielding → atom attracts electrons more strongly → higher oxidising power/reactivity. [1 mark]
3What happens: Chlorine displaces bromide ions from potassium bromide, releasing bromine gas. The orange-brown colour is due to bromine (Br₂) being produced. [1 mark]
4Balanced equation: Cl₂(g) + 2KBr(aq) → 2KCl(aq) + Br₂(aq) [1 mark — check atoms balanced: 2 Cl, 2 K, 2 Br each side ✓]
✅ Cl₂ is more reactive (higher oxidising strength) → displaces Br⁻ from KBr → Br₂ produced = orange-brown colour. Equation: Cl₂ + 2KBr → 2KCl + Br₂
List the Group VII elements in order of increasing oxidising strength. Explain the trend. [3 marks]
3 Marks
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Step-by-Step Answer
1Order (weakest → strongest): At < I < Br < Cl < F [1 mark]
2Trend explanation — direction: Oxidising strength INCREASES going UP Group VII (i.e. from At to F). [1 mark]
3Why: Going UP Group VII, atoms have fewer electron shells → smaller atomic radius → valence shell is closer to the nucleus → stronger nuclear attraction on the extra electron → element can attract and gain electrons more easily → stronger oxidising agent. Fluorine (top, smallest) = strongest oxidising agent. [1 mark]
✅ Order: At < I < Br < Cl < F. Increases UP because smaller radius → stronger nuclear attraction → easier to gain electrons
Element W has an electronic configuration of (2,8,18,8,2). Identify the group and period of W, and predict its reaction with water. [3 marks]
3 Marks
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Step-by-Step Answer
1Group: Valence electrons = last shell = 2 → Group II [1 mark]
2Period: Count occupied shells: 2, 8, 18, 8, 2 → 5 shells → Period 5. This element is Strontium (Sr). [1 mark]
3Reaction with water: W is a Group II metal in Period 5 — it is BELOW calcium in the group, so it is MORE reactive than calcium. It will react vigorously with water: W + 2H₂O → W(OH)₂ + H₂↑, producing strontium hydroxide (a white precipitate) and hydrogen gas. [1 mark]
✅ Group II, Period 5 (Strontium, Sr). Reacts vigorously with water: Sr + 2H₂O → Sr(OH)₂ + H₂↑
Give THREE physical differences between sodium (a metal) and chlorine (a non-metal) found in Period 3. [3 marks]
3 Marks
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Step-by-Step Answer — Any THREE of the following:
1State at room temperature: Sodium is a solid; chlorine is a gas. [1 mark]
2Electrical conductivity: Sodium is a good electrical conductor; chlorine does not conduct electricity. [1 mark]
3Appearance: Sodium is shiny/silvery (metallic lustre); chlorine is a dull yellow-green gas. [1 mark]
4Melting/boiling point: Sodium has a higher melting/boiling point than chlorine. [1 mark — bonus]
5Density: Sodium (solid metal) has a higher density than chlorine (gas). [1 mark — bonus]
✅ Any 3 of: State (solid vs gas) | Conductivity | Appearance (shiny vs dull) | Melting point | Density