Chemical properties periodic trends

Chemistry· 5 min read

1. Metallic Character Trend Across Period 3★★☆☆☆⏱ 15 min

📘 Definition

Metallic Character

The ability of an element to lose valence electrons to form positive cations. It is determined by ionisation energy and nuclear attraction for outer electrons.

Example:

Sodium loses one electron easily to form $N a^{+}$ , while chlorine does not form stable positive cations.

Across Period 3 from sodium to argon, nuclear charge increases and atomic radius decreases. This increases attraction for outer electrons, so ionisation energy increases. As a result, metallic character decreases from left to right, while non-metallic character increases.

📐 Worked Example

Arrange the Period 3 elements Si, Na, Cl, Mg in order of increasing metallic character. Explain your reasoning.

1
Recall that the order of elements left to right across Period 3 is Na → Mg → Si → Cl. Metallic character decreases left to right, so the least metallic element comes first for increasing order:
2
$Cl < Si < Mg < Na$
3
Explanation: Nuclear charge increases across the period, pulling outer electrons closer to the nucleus. This makes it harder for elements to lose electrons, so metallic character decreases from left to right across Period 3.

2. Acid-Base Properties of Period 3 Oxides★★★☆☆⏱ 20 min

All Period 3 elements except argon react with oxygen to form oxides. The acid-base character of these oxides follows the trend of metallic character: basic for metallic elements, acidic for non-metallic elements, and amphoteric for the metalloid-like element aluminium.

📘 Definition

Amphoteric Oxide

An oxide that acts as both an acid and a base, reacting with both strong acids and strong alkalis to form salts.

Example:

Aluminium oxide ( $A l_{2} O_{3}$ ) is the key amphoteric oxide tested in CIE A-Level.

📐 Worked Example

Write balanced equations for the reaction of aluminium oxide with (i) dilute hydrochloric acid (ii) hot concentrated sodium hydroxide solution.

1
(i) Aluminium oxide acts as a base when reacting with hydrochloric acid, forming aluminium chloride and water:
2
$Al_2O_3(s) + 6HCl(aq) \to 2AlCl_3(aq) + 3H_2O(l)$
3
(ii) Aluminium oxide acts as an acid when reacting with sodium hydroxide, forming sodium aluminate and water:
4
$Al_2O_3(s) + 2NaOH(aq) \to 2NaAlO_2(aq) + H_2O(l)$

3. Chemical Trends Down Group 2★★★☆☆⏱ 20 min

Group 2 are alkaline earth metals. Down the group, atomic radius increases and shielding increases, so first and second ionisation energies decrease. This makes it easier to lose outer electrons, so metallic character and reactivity increase down the group.

📐 Worked Example

Compare the reaction of magnesium and barium with cold water, and explain the difference in reactivity.

1
Magnesium reacts very slowly with cold water to form magnesium hydroxide and hydrogen:
2
$Mg(s) + 2H_2O(l) \to Mg(OH)_2(aq) + H_2(g)$
3
Barium reacts rapidly and vigorously with cold water to form barium hydroxide and hydrogen:
4
$Ba(s) + 2H_2O(l) \to Ba(OH)_2(aq) + H_2(g)$
5
Explanation: Down Group 2, ionisation energy decreases, so barium loses its two outer electrons more easily than magnesium. This makes barium more reactive, following the trend of increasing metallic character down the group.

✓ Quick check

Which of the following is the correct trend for solubility of Group 2 sulfates down the group?
- A: Increases
- B: Decreases
- C: No change
- D: Increases then decreases
Reveal answer
B —
Correct: Solubility of sulfates decreases down Group 2, while solubility of hydroxides increases down the group.

4. Chemical Trends Down Group 17 (Halogens)★★★☆☆⏱ 15 min

Group 17 halogens are non-metals that gain one electron to form halide anions. Down the group, electronegativity and electron affinity decrease, so the ability to gain an electron decreases. This means reactivity decreases down Group 17, the opposite trend to Group 2 metals.

📐 Worked Example

Predict if a reaction occurs when chlorine gas is bubbled through aqueous potassium bromide. If yes, write the equation and state the observation.

1
Chlorine is higher up Group 17 than bromine, so it is more reactive. A displacement reaction will occur.
2
Chlorine displaces bromide ions from solution, forming potassium chloride and bromine:
3
$Cl_2(g) + 2KBr(aq) \to 2KCl(aq) + Br_2(aq)$
4
Observation: The colourless solution will turn orange-brown, due to the formation of aqueous bromine.

5. Common Pitfalls

Wrong move:

Claiming aluminium oxide is only acidic or only basic

Why:

Aluminium oxide is the only common amphoteric oxide in Period 3, and exam questions specifically test this property

Correct move:

State that aluminium oxide reacts with both acids and bases, so it is amphoteric

Wrong move:

Stating metallic character increases across a period

Why:

Nuclear charge increases across a period, making it harder to lose electrons

Correct move:

Recall metallic character decreases left to right across a period, and increases down all groups

Wrong move:

Claiming Group 2 reactivity decreases down the group

Why:

This confuses the Group 2 metal trend with the Group 17 non-metal trend

Correct move:

Group 2 metal reactivity increases down the group, Group 17 reactivity decreases down the group

Wrong move:

Mixing up Group 2 solubility trends of hydroxides and sulfates

Why:

The opposite trends are often tested in multiple choice questions

Correct move:

Remember: Hydroxide solubility increases Down, Sulfate solubility decreases Down (H I S D)

Wrong move:

Writing $N a_{2} O$ as the product of sodium burning in excess oxygen

Why:

Sodium forms sodium peroxide when burned in excess oxygen, not sodium oxide

Correct move:

The product is $N a_{2} O_{2}$ for combustion in excess oxygen

6. Quick Reference Cheatsheet

Trend	Across Period 3 (left → right)	Down Group 2	Down Group 17
Metallic character	Decreases	Increases	Decreases
Reactivity	Metals: Decreases, Non-metals: Increases	Increases	Decreases
Oxide acid-base character	Basic → Amphoteric → Acidic	All basic	All acidic
Solubility: hydroxides		Increases
Solubility: sulfates		Decreases

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 2
Period 3 oxide acid-base trends
2021 · 1
Group 2 solubility trend question
2023 · 2
Halogen displacement reactions

What's Next

Understanding chemical property trends is the foundation for predicting reaction behaviour in inorganic chemistry, which makes up ~40% of your CIE A-Level Chemistry exam. This sub-topic is often combined with questions on physical periodic trends, bonding, and acid-base chemistry, so mastering it will help you answer a wide range of question types. Next, you can explore related topics to build a complete understanding of periodicity, then dive deeper into the specific chemistry of each major group tested on the syllabus.