Reactions of Period 3 Elements with Water & Oxygen (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching A-Level Chemistry, and this video is about how sodium and magnesium react with water, and how the Period 3 elements react with oxygen.

Moving across Period 3, elements become less metallic, sodium reacts violently with water while magnesium only reacts slowly. And that same trend continues into their oxides, which shift from ionic to covalent as the elements become less metallic.

Sodium and magnesium are both metals, but they react with water quite differently: sodium reacts vigorously with cold water, while magnesium reacts only very slowly with cold water, and much faster with steam. That same falling reactivity continues across the rest of Period 3: sodium, magnesium, aluminium, silicon, phosphorus and sulfur all combine with oxygen to form oxides, and as the elements become less metallic, the bonding in those oxides shifts from ionic to covalent.

This video covers sodium and magnesium's reactions with water first, then the oxides formed across Period 3, before finishing with the bonding trend that links them together.

Sodium reacts vigorously and exothermically with cold water, floating on the surface, fizzing rapidly and melting from the heat produced, to form sodium hydroxide with a pH of around 13 to 14. Magnesium reacts with cold water extremely slowly — only a small number of bubbles form on the magnesium ribbon — producing magnesium hydroxide, which is only partially soluble and gives a much less alkaline pH of around 10. Heated magnesium reacts far faster with steam, burning with a bright white flame, but the product is different too: magnesium oxide forms instead of magnesium hydroxide.

Sodium, magnesium, aluminium, silicon, phosphorus and sulfur all combine with oxygen, each forming an oxide in its highest oxidation state: sodium oxide, magnesium oxide, aluminium oxide, silicon dioxide, and phosphorus(V) oxide. Sulfur is unusual in forming two oxides — sulfur dioxide forms directly, while sulfur trioxide needs a catalyst and a very high temperature to form.

Sodium oxide, magnesium oxide and aluminium oxide are all ionic, since sodium, magnesium and aluminium are metals combining with the non-metal oxygen. Silicon dioxide has a giant covalent structure, like diamond, while phosphorus oxide and sulfur dioxide are simple covalent molecules. That progression — ionic at the metal end of the period, covalent at the non-metal end — is the same falling metallic character that made sodium react so much faster with water than magnesium.

Sodium reacts fast with water; magnesium reacts slowly with cold water, and only quickly once heated with steam, forming magnesium oxide rather than the hydroxide. Across the rest of Period 3, the elements form oxides from sodium oxide through to sulfur trioxide, and the bonding in those oxides shifts from ionic to covalent as the elements become less metallic — the same trend that shaped their reactions with water.