Inorganic Chemistry / Main-group chemistry
Period 3 elements and their compounds
Trends across Na to Ar: reactions with oxygen and water, and the acid-base behaviour of the oxides and chlorides.
A-LevelIn a nutshell
Period 3 runs from sodium to argon, and walking across it shows the periodic table's logic in action. On the left are reactive metals that form ionic compounds; in the middle is silicon, which builds giant covalent networks; on the right are non-metals that form small molecules. You can see this in what happens when each element burns in oxygen and how its oxide behaves in water. Sodium oxide dissolves to give a strongly alkaline solution, aluminium oxide reacts with both acids and alkalis, and sulfur trioxide makes sulfuric acid. The pattern follows directly from how bonding changes as electronegativity rises across the period. These trends are a staple of A-level inorganic chemistry and a useful way to connect structure, bonding and reactivity.
Reactions with oxygen
- Na burns with a yellow flame: \mathrm{4Na + O_2 \rightarrow 2Na_2O}.
- Mg burns with a brilliant white flame to MgO; Al forms Al_2O_3 (powder burns).
- Si gives SiO_2; P burns with a white flame to P_4O_{10}; S burns with a blue flame to SO_2.
Reactions with water
Sodium reacts vigorously with cold water to give NaOH and H_2 (pH about 14). Magnesium reacts very slowly with cold water but quickly with steam: \mathrm{Mg + H_2O \rightarrow MgO + H_2}.
Oxides: structure and acid-base behaviour
- Na_2O, MgO (ionic, basic): Na_2O gives NaOH(aq), pH about 14; MgO is only slightly soluble, pH about 9. Both react with acids.
- Al_2O_3 (ionic with covalent character, amphoteric): insoluble, but reacts with acids and with hot NaOH(aq), forming the aluminate [Al(OH)_4]^-.
- SiO_2 (giant covalent, acidic): insoluble, but reacts with hot concentrated NaOH to give silicate.
- P_4O_{10}, SO_2, SO_3 (molecular, acidic): give H_3PO_4, H_2SO_3 and H_2SO_4 solutions (pH about 0 to 3).
Melting points peak at the giant ionic MgO and Al_2O_3 and giant covalent SiO_2, then drop sharply for the molecular oxides, whose molecules are held only by weak intermolecular forces.
Chlorides
- NaCl, MgCl_2 (ionic): dissolve to give roughly neutral solutions (MgCl_2 slightly acidic).
- AlCl_3 (covalent, Al_2Cl_6 dimer): the hydrated ion releases H^+, giving pH about 3.
- SiCl_4, PCl_5 (molecular): hydrolyse violently, releasing HCl fumes: \mathrm{SiCl_4 + 2H_2O \rightarrow SiO_2 + 4HCl}.
Why the trend
Electronegativity increases across the period, so bonding to oxygen and chlorine changes from ionic to covalent, and the oxide changes from a source of O^{2-} (base) to a covalent molecule attacked by water (acid). Argon, with a full outer shell, forms no oxide.
Key points
- Across Period 3, oxides change from basic (Na, Mg) through amphoteric (Al) to acidic (Si, P, S).
- Bonding in oxides changes from ionic to giant covalent (SiO_2) to simple molecular (P_4O_{10}, SO_2, SO_3).
- Na reacts vigorously with cold water; Mg reacts slowly with cold water but readily with steam.
- Al_2O_3 reacts with both acids and hot alkali, forming aluminate with NaOH.
- Covalent chlorides (AlCl_3, SiCl_4, PCl_5) hydrolyse to give acidic solutions.
Check yourself
5 questions
1Which Period 3 oxide gives the most alkaline solution in water?
Na_2O reacts fully with water to form NaOH, pH about 14; MgO is only slightly soluble.
2Which equation shows the amphoteric behaviour of aluminium oxide?
Amphoteric means reacting both as a base (with acid) and as an acid (with alkali).
3Why does SiO_2 have a much higher melting point than P_4O_{10}?
Melting SiO_2 requires breaking strong Si–O covalent bonds; melting P_4O_{10} only overcomes weak forces between molecules.
4What do you observe when SiCl_4 is added to water?
SiCl_4 hydrolyses vigorously to SiO_2 (white solid) and HCl, which fumes in moist air.
5Why does an aqueous solution of AlCl_3 have a pH of about 3 while NaCl solution is neutral?
[Al(H_2O)_6]^{3+} ⇌ [Al(H_2O)_5(OH)]^{2+} + H^+; Na^+ is too weakly polarising to do this.
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Recent preprints on this topic
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Matched by keyword from ChemRxiv via Crossref. Not peer-reviewed; read critically.