# Chemical properties periodic trends

> Chemistry · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9701-u9-chemical-properties-periodic-trends/

This module explores trends in chemical properties across periods and down groups in the periodic table, focusing on Period 3 elements, Group 2 alkaline earth metals and Group 17 halogens, linking trends to electronic structure and bonding.

**Prerequisites:** [Periodic table structure and electron configuration](https://www.owlsprep.com/study/cie-9701-u1-atomic-structure/); [Types of bonding and structure](https://www.owlsprep.com/study/cie-9701-u2-bonding-structure/)

## Learning objectives

- Describe trends in chemical properties across Period 3 and down main groups
- Explain trends in reactions of elements with oxygen, water and acids
- Relate chemical property trends to electronic structure and bonding
- Predict reactivity of elements based on their position in the periodic table

## Metallic Character Trend Across Period 3

**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 $Na^+$, 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.

## Acid-Base Properties of Period 3 Oxides

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.

**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 ($Al_2O_3$) 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. $$Al_2O_3(s) + 2NaOH(aq) \to 2NaAlO_2(aq) + H_2O(l)$$

> **tip**
>
> You will almost certainly be asked about amphoteric aluminium oxide in your exam. Don't confuse it with other Period 3 oxides!

## Chemical Trends Down Group 2

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.

**Check your understanding**

1. 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

   *Why:* Correct: Solubility of sulfates decreases down Group 2, while solubility of hydroxides increases down the group.

## Chemical Trends Down Group 17 (Halogens)

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.

> **tip**
>
> Halogen displacement reactions are a very common practical and multiple choice question topic for CIE.

## Common pitfalls

- **Wrong:** Claiming aluminium oxide is only acidic or only basic
  - Why it fails: Aluminium oxide is the only common amphoteric oxide in Period 3, and exam questions specifically test this property
  - Correct: State that aluminium oxide reacts with both acids and bases, so it is amphoteric
- **Wrong:** Stating metallic character increases across a period
  - Why it fails: Nuclear charge increases across a period, making it harder to lose electrons
  - Correct: Recall metallic character decreases left to right across a period, and increases down all groups
- **Wrong:** Claiming Group 2 reactivity decreases down the group
  - Why it fails: This confuses the Group 2 metal trend with the Group 17 non-metal trend
  - Correct: Group 2 metal reactivity increases down the group, Group 17 reactivity decreases down the group
- **Wrong:** Mixing up Group 2 solubility trends of hydroxides and sulfates
  - Why it fails: The opposite trends are often tested in multiple choice questions
  - Correct: Remember: Hydroxide solubility increases Down, Sulfate solubility decreases Down (H I S D)
- **Wrong:** Writing $Na_2O$ as the product of sodium burning in excess oxygen
  - Why it fails: Sodium forms sodium peroxide when burned in excess oxygen, not sodium oxide
  - Correct: The product is $Na_2O_2$ for combustion in excess oxygen

## 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 | - |

## 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.

- [Physical properties periodic trends](https://www.owlsprep.com/study/cie-9701-u9-physical-properties-periodic-trends/)
- [Group 2 and Group 17](https://www.owlsprep.com/study/cie-9701-u10-overview/)
- [Group 2 properties and reactions](https://www.owlsprep.com/study/cie-9701-u10-group-2-properties-and-reactions/)

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