The elements in Period 3 (sodium, magnesium, aluminium, silicon, phosphorus, and sulfur) exhibit a clear transition in physical and chemical properties from left to right. This transition reflects the change from highly reactive, electropositive metals on the left to less reactive non-metals on the right. In this lesson, we study their reactions with water and oxygen, including the balanced chemical equations with state symbols required by the AQA specification.
🔑 Key Principle: Metallic to Non-metallic character
As you move across Period 3, elements lose their metallic character. The oxides they form transition from ionic basic oxides on the left, to an amphoteric oxide in the middle, to covalent acidic oxides on the right.
1. Reactions of Period 3 Elements with Water
Only the metals at the start of Period 3 show significant reactions with water. The reactivity decreases rapidly from sodium to aluminium.
Sodium (\(\text{Na}\)) with Cold Water
Sodium reacts vigorously and rapidly with cold water. When placed in water, it melts into a shiny ball (due to the highly exothermic nature of the reaction), fizzes (effervesces) as hydrogen gas is produced, floats, and moves rapidly across the water surface. It may ignite with a characteristic yellow flame. The resulting solution is strongly alkaline due to the formation of sodium hydroxide:
Magnesium (\(\text{Mg}\)) with Cold Water and Steam
Magnesium has two distinct reactions with water depending on the temperature:
- With Cold Water: Magnesium reacts extremely slowly. A very slow stream of hydrogen bubbles is produced, and a thin, insoluble layer of magnesium hydroxide coating forms on the metal, which stops further reaction: \[ \text{Mg}(\text{s}) + 2\text{H}_2\text{O}(\text{l}) \rightarrow \text{Mg}(\text{OH})_2(\text{s}) + \text{H}_2(\text{g}) \]
- With Steam: When heated, magnesium reacts vigorously with steam. It burns with a brilliant white light (flame) to form white solid magnesium oxide and hydrogen gas: \[ \text{Mg}(\text{s}) + \text{H}_2\text{O}(\text{g}) \rightarrow \text{MgO}(\text{s}) + \text{H}_2(\text{g}) \]
Aluminium (\(\text{Al}\)) with Water
Aluminium does not react visibly with water. Although it is a highly reactive metal, it is protected by a thin, tough, impervious layer of aluminium oxide (\(\text{Al}_2\text{O}_3\)) on its surface, which prevents water molecules from coming into contact with the metal beneath.
Passivation is the formation of a protective, unreactive oxide layer on the surface of a metal (such as aluminium) that prevents further chemical reaction or corrosion of the metal underneath.
Do not write that aluminium does not react with water because it is unreactive. Aluminium has a high negative standard electrode potential, making it very reactive. It is the protective oxide layer that prevents the reaction. If this layer is removed (for example, by reacting it with mercury ions), aluminium reacts vigorously with water.
2. Reactions of Period 3 Elements with Oxygen
All Period 3 elements from sodium to sulfur react with oxygen when heated, forming their corresponding oxides. You must know the balanced equations, state symbols, and reaction observations:
Sodium (\(\text{Na}\))
Sodium burns vigorously in oxygen with a bright yellow flame to produce a white solid mixture of sodium oxide and some sodium peroxide:
\[ 4\text{Na}(\text{s}) + \text{O}_2(\text{g}) \rightarrow 2\text{Na}_2\text{O}(\text{s}) \]
Magnesium (\(\text{Mg}\))
Magnesium burns vigorously with a brilliant white flame to produce a white powder of magnesium oxide:
\[ 2\text{Mg}(\text{s}) + \text{O}_2(\text{g}) \rightarrow 2\text{MgO}(\text{s}) \]
Aluminium (\(\text{Al}\))
Aluminium burns in oxygen if heated strongly and powdered, showing a white flame to produce white solid aluminium oxide:
\[ 4\text{Al}(\text{s}) + 3\text{O}_2(\text{g}) \rightarrow 2\text{Al}_2\text{O}_3(\text{s}) \]
Silicon (\(\text{Si}\))
Silicon reacts slowly with oxygen only when heated strongly, producing silicon dioxide (silica), which is a white solid:
\[ \text{Si}(\text{s}) + \text{O}_2(\text{g}) \rightarrow \text{SiO}_2(\text{s}) \]
Phosphorus (\(\text{P}\))
White phosphorus reacts spontaneously with oxygen (or when ignited in air) with a brilliant white flame, producing thick clouds of white smoke, which is solid phosphorus(V) oxide:
\[ 4\text{P}(\text{s}) + 5\text{O}_2(\text{g}) \rightarrow \text{P}_4\text{O}_{10}(\text{s}) \]
A substance that can behave chemically as both an acid and a base. It reacts with both strong acids and strong bases to form salts.
Sulfur (\(\text{S}\))
Sulfur burns gently with a blue flame when ignited, forming colourless, pungent sulfur dioxide gas. A small amount of sulfur trioxide is also formed in excess oxygen:
\[ \text{S}(\text{s}) + \text{O}_2(\text{g}) \rightarrow \text{SO}_2(\text{g}) \]
Make sure you write the molecular formula for phosphorus oxide as \(\text{P}_4\text{O}_{10}\), not the empirical formula \(\text{P}_2\text{O}_5\). The molecular structure consists of \(\text{P}_4\text{O}_{10}\) molecules, and this is what AQA marks expect. Also, pay close attention to states: \(\text{SO}_2\) is a gas, whereas \(\text{P}_4\text{O}_{10}\) is a solid.
3. Trend in Oxide Character
The nature of the bonds changes from ionic on the left to covalent on the right. This leads to a corresponding change in chemical behaviour:
Answer:
- Reaction with cold water: \[ \text{Mg}(\text{s}) + 2\text{H}_2\text{O}(\text{l}) \rightarrow \text{Mg}(\text{OH})_2(\text{s}) + \text{H}_2(\text{g}) \]
- Reaction with steam: \[ \text{Mg}(\text{s}) + \text{H}_2\text{O}(\text{g}) \rightarrow \text{MgO}(\text{s}) + \text{H}_2(\text{g}) \]
- Magnesium hydroxide (\(\text{Mg}(\text{OH})_2\)) is thermally unstable. When magnesium reacts with steam, the temperature is very high. At high temperatures, magnesium hydroxide thermally decomposes into magnesium oxide and water: \[ \text{Mg}(\text{OH})_2(\text{s}) \rightarrow \text{MgO}(\text{s}) + \text{H}_2\text{O}(\text{g}) \] This is why the oxide, rather than the hydroxide, is formed when steam is used.
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