How do properties change across a period, and why?
Periodicity of atomic radius, ionisation energy and melting point across Period 3, the structures and bonding of the Period 3 elements and oxides, and the trends in the acid-base character of the oxides.
A CCEA A-Level Chemistry answer on periodicity, covering the trends in atomic radius, ionisation energy and melting point across Period 3, the structures and bonding of the Period 3 elements, and the trend from basic to acidic character in the Period 3 oxides.
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What this dot point is asking
CCEA wants you to describe and explain the periodic trends in atomic radius, ionisation energy and melting point across Period 3, relate them to the structures and bonding of the elements, and describe the trend in the acid-base character of the Period 3 oxides.
Atomic radius and ionisation energy
The ionisation energy trend has two dips. It falls from magnesium to aluminium because aluminium's outer electron is in a higher-energy sub-shell, and it falls from phosphorus to sulfur because sulfur's has a pair of electrons in one orbital that repel, making one easier to remove.
Melting points and structure
The structures across Period 3 explain the melting point trend:
- Na, Mg, Al: giant metallic structures; melting point rises as the charge on the ion and number of delocalised electrons increase.
- Si: a giant covalent structure with strong bonds throughout, giving the highest melting point.
- P, S, Cl: simple molecular structures with weak van der Waals forces, so low melting points.
- Ar: monatomic, with the weakest forces and lowest melting point.
Acid-base character of the oxides
The reason behind this trend is the change in bonding across the period. On the left, the difference in electronegativity between the metal and oxygen is large, so the oxides are ionic and basic. Sodium oxide dissolves to give a strongly alkaline solution (). On the right, the electronegativity difference is small, so the oxides are covalent and acidic. Sulfur trioxide dissolves to give sulfuric acid (). Aluminium oxide sits on the borderline and is amphoteric, reacting with acids (as a base) and with hot concentrated alkali (as an acid).
Examples in context
Example 1. Anodising aluminium. Aluminium is protected by a thin layer of aluminium oxide. Because this oxide is amphoteric, manufacturers can thicken and colour it by anodising, an electrolytic process that exploits the oxide's ability to react in both acidic and basic conditions. The corrosion resistance of aluminium window frames and aircraft skins rests on this amphoteric Period 3 oxide, exactly the character the dot point asks candidates to explain.
Example 2. Silicon in semiconductors. Silicon sits at the giant-covalent peak of the Period 3 melting point trend, and that same strong, directional covalent bonding underlies its use in microchips. Its oxide, silicon dioxide, is acidic and extremely high melting, which is why it is used as an insulating layer on silicon chips and as the basis of glass. Linking silicon's position in the period to its bonding, melting point and oxide character is a complete CCEA periodicity argument.
Try this
Q1. Explain why the atomic radius decreases across Period 3. [2 marks]
- Cue. Nuclear charge increases while electrons enter the same shell, so they are pulled closer.
Q2. Explain why silicon has the highest melting point in Period 3. [2 marks]
- Cue. It has a giant covalent structure, so many strong covalent bonds must be broken.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20216 marksThe first ionisation energies of the Period 3 elements generally increase from sodium to argon, but there are dips at aluminium and at sulfur. Explain the general increase and account for each dip in terms of electron configuration.Show worked answer →
Markers want the general trend explained, then each dip explained separately by sub-shell or pairing effects.
The general increase: across Period 3 the nuclear charge increases while electrons are added to the same outer shell, so the shielding stays roughly constant and the outer electron is held more strongly. More energy is therefore needed to remove it, so the first ionisation energy rises overall.
The dip from magnesium to aluminium: magnesium loses an electron from a full sub-shell, but aluminium's outermost electron is in a higher-energy sub-shell, which is slightly further out and slightly shielded by the electrons. It is therefore easier to remove, so aluminium's first ionisation energy is lower than magnesium's.
The dip from phosphorus to sulfur: phosphorus has three electrons, one singly in each orbital. Sulfur has four, so one orbital now holds a pair. The two paired electrons repel each other, making one easier to remove, so sulfur's first ionisation energy is lower than phosphorus's.
Markers reward the general explanation (nuclear charge up, shielding roughly constant), the -above- reason for the Al dip, and the electron-pairing repulsion reason for the S dip.
CCEA 20184 marksDescribe and explain the trend in the acid-base character of the oxides of the Period 3 elements from sodium to sulfur, giving the formula of one oxide that is amphoteric.Show worked answer →
A trend-and-explanation question on the oxides.
Across Period 3 the oxides change from basic, through amphoteric, to acidic. Sodium oxide () and magnesium oxide () are basic metal oxides that react with acids and (for sodium oxide) dissolve to give alkaline solutions. Aluminium oxide () is amphoteric: it reacts with both acids and bases. The non-metal oxides, silicon dioxide (), phosphorus(V) oxide () and the sulfur oxides (, ), are acidic and react with bases.
The explanation is the change in bonding: the metal oxides on the left are ionic and basic, while the non-metal oxides on the right are covalent and acidic, with aluminium oxide on the borderline showing both characters.
The amphoteric oxide is . Markers reward the basic-to-acidic trend, the link to ionic versus covalent bonding, and naming aluminium oxide as amphoteric.
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Sources & how we know this
- CCEA GCE Chemistry specification — CCEA (2016)