Reactions of Period 3 Elements
This lesson covers:
- Comparing the reactivity of sodium and magnesium with water
- How period 3 elements react with oxygen to form oxides
Sodium is more reactive than magnesium
The first two elements in period 3 are sodium (Na) and magnesium (Mg).
When reacting, sodium forms a 1+ ion by losing one electron, whereas magnesium forms a 2+ ion by losing two electrons.
- Sodium loses electrons more readily than magnesium because it takes less energy to lose one electron versus two.
- So, sodium is the more reactive element.
This difference in reactivity is demonstrated in how sodium and magnesium react with water:
Sodium reacts vigorously even with cold water. This produces a molten ball that floats on the surface of the water and fizzes as hydrogen gas is given off. The solution formed is highly alkaline (pH 12-14) sodium hydroxide.
- 2Na(s) + 2H2O(l) ➔ 2NaOH(aq) + H2(g)
Magnesium reacts slowly with cold water. It makes a weakly alkaline solution (pH ≈10) containing some magnesium hydroxide, which has low solubility.
- Mg(s) + 2H2O(l) ➔ Mg(OH)2(aq) + H2(g)
Magnesium reacts faster with steam to form magnesium oxide. The increased heat energy drives the reaction to completion.
- Mg(s) + H2O(g) ➔ MgO(s) + H2(g)
Reactions of period 3 elements with oxygen
When period 3 elements encounter oxygen gas, oxidation reactions occur readily (except for Al and Si). The elements form oxides where they adopt their highest oxidation states, equal to their group number, with sulfur as the exception. The oxidation state of the period 3 element in its oxide generally increases as we move from left to right across the period.
| Element | Oxide formula | Oxidation state | Reaction with air |
|---|---|---|---|
| Sodium | Na2O | +1 | Vigorous |
| Magnesium | MgO | +2 | Vigorous (faster if powdered) |
| Aluminium | Al2O3 | +3 | Slow |
| Silicon | SiO2 | +4 | Slow |
| Phosphorus | P4O10 | +5 | Spontaneously combusts |
| Sulfur | SO2 / SO3 | +4 / +6 | Burns steadily / Requires heat and catalyst |
The reactions between period 3 elements and oxygen can be summarised as follows:
Sodium - Burns brightly in air with a characteristic yellow flame to form white sodium oxide.
2Na(s) + 1⁄2O2(g) ➔ Na2O(s)
Magnesium - Burns in oxygen with a bright white flame, forming white magnesium oxide powder.
2Mg(s) + O2(g) ➔ 2MgO(s)
Aluminium - Its powder burns brightly when heated in oxygen, producing white aluminium oxide powder.
4Al(s) + 3O2(g) ➔ 2Al2O3(s)
Silicon - When heated strongly in oxygen, silicon reacts to form silicon dioxide.
Si(s) + O2(g) ➔ SiO2(s)
Phosphorus - White phosphorus ignites spontaneously in air, giving off white smoke of phosphorus pentoxide.
4P(s) + 5O2(g) ➔ P4O10(s)
Sulfur - Its powder burns in oxygen with a blue flame to produce sulfur dioxide (SO2) gas. At high temperatures and with a catalyst, sulfur trioxide (SO3) gas is also produced.
S(s) + O2(g) ➔ SO2(g)
S(s) + 3⁄2O2(g) ➔ SO3(g)