Ch 3 · Chemical periodicity of other elements · AS & A Level Chemistry 9701

Topic 3 of 3 in The Periodic Table: chemical periodicity — AS & A Level Chemistry 9701 — 48 lessons.

Introduction

Periodicity is not confined to Period 3: the same reasoning, from position in the Periodic Table to structure, bonding and reactivity, lets you predict properties of any element from its group and period, or identify an unknown element from a set of data. Cambridge questions on this subtopic present unfamiliar elements and ask you to reason from patterns rather than recall facts.

Key ideas — Predicting properties from group position

Elements in the same group have the same number of outer-shell electrons, so they show similar chemical behaviour and a graded (usually smooth) change in physical properties down the group. Down a group, atomic radius increases (extra occupied shells outweigh the increase in nuclear charge), first ionisation energy decreases (the outer electron is further from the nucleus and more shielded, despite the higher nuclear charge), and metallic character generally increases. If you know the properties of two elements in a group, you can predict, by interpolation or by extending the trend, the properties of another member: for example, reactivity of Group 1 metals with water increases down the group, so an element below caesium would be predicted to react even more violently than caesium.

🧑‍🏫 Teacher's explanation

Think of a group in the Periodic Table like a family of siblings. They all share the same 'DNA'—the number of electrons in their outermost shell. Because they have the same number of outer electrons, they tend to react in very similar ways, like siblings who all have the same stubborn personality. However, as you go down the group, the 'family members' get physically larger because they are adding more layers (shells) of electrons.

📖 Definitions

🔍 Worked example — Easy

Predict the trend in atomic radius for Group 2 elements from Magnesium (Mg) to Barium (Ba).

🔍 Worked example — Standard

An unknown element X is in Group 1. It reacts with water to form a metal hydroxide and hydrogen gas. Element Y is directly above X in Group 1. If Y reacts vigorously with water, how would you predict X reacts?

🔍 Worked example — Exam-style

The first ionisation energy of Lithium (Li) is 520 ext kJ mol^-1 and Sodium (Na) is 496 ext kJ mol^-1. Predict the trend for the first ionisation energy of Potassium (K) relative to Na.

⚠️ Common errors

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🎯 Exam question

Key ideas — Comparing periodic patterns across a group

Comparing several elements in the same group reveals which properties change smoothly and which change abruptly. Melting point, boiling point, density and first ionisation energy usually vary smoothly down a group, since they depend mainly on atomic size and nuclear attraction. Some properties change more sharply at particular points, for example where structure changes from molecular to metallic (as in Group 14, from covalent carbon and silicon to metallic tin and lead) or where a new sub-shell becomes involved in bonding. Recognising a smooth trend allows confident prediction; recognising a discontinuity flags an element whose structure or bonding differs from its neighbours.

🧑‍🏫 Teacher's explanation

Think of a group in the periodic table like a family of people. Most family traits, like height or weight, change gradually from parent to child. If you know the height of the father and the grandfather, you can make a very good guess at the son's height. This is like 'smooth' trends in melting points or atomic radii down a group.

📖 Definitions

🔍 Worked example — Easy

Predict the trend in atomic radius for Group 1 elements (Li, Na, K) and explain why.

🔍 Worked example — Standard

An unknown element in Group 14 has a melting point significantly higher than Carbon and Silicon. What does this suggest about its structure?

🔍 Worked example — Exam-style

The first ionisation energies (kJ/mol) for the following elements are: Li (520), Na (496), K (419). Calculate the percentage decrease from Li to K.

⚠️ Common errors

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🎯 Exam question

Key ideas — Using ionisation energy and atomic radius to place an element in a group

A plot or table of successive ionisation energies for an element reveals its electron configuration: a large jump in ionisation energy occurs whenever an electron is removed from a shell closer to the nucleus, so the position of the first large jump after n small increases shows that the element has n electrons in its outer shell. That number of outer electrons identifies the group. Comparing the first ionisation energy and atomic radius of the unknown element with known elements of that group then places it at the correct point in the period, since both properties increase up a group and (for atomic radius) decrease across a period.

🧑‍🏫 Teacher's explanation

Think of an atom like a multi-story building where each floor is a shell. The ground floor is easy to access, but as you go higher, the floors get further away and the stairs get harder to climb. However, once you reach the top floor and try to remove an electron from the 'basement' (the inner shell), you have to break through the entire structural foundation of the building. This requires a massive, sudden burst of energy.

📖 Definitions

🔍 Worked example — Easy

An element X has the following first four ionisation energies (kJ/mol): 500, 4500, 6200, 8100. Identify the group of element X.

🔍 Worked example — Standard

Element Y has a first ionisation energy of 501 ext kJ/mol and an atomic radius of 228 ext pm. Element Z has a first ionisation energy of 578 ext kJ/mol and an atomic radius of 196 ext pm. If Y and Z are in the same group, which one is further down the group?

🔍 Worked example — Exam-style

An unknown element has the following successive ionisation energies (kJ/mol): 738, 1450, 7730, 10500, 13600. Determine the element's group and its likely period (assume it is in Period 3).

⚠️ Common errors

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🎯 Exam question

(a) Determine the group of element X. [2] (b) Explain why the fourth ionisation energy is significantly higher than the third. [2] [4 marks]

Key ideas — Deducing an unknown element from physical and chemical data

To identify an unknown element, work systematically: use the pattern of ionisation energies to fix the group (number of outer electrons and hence typical oxidation states and formulae of compounds); use atomic radius, melting point and electrical conductivity, compared with known trends, to judge the period and therefore the identity of the element; use chemical behaviour, for example the type of oxide formed (basic, amphoteric or acidic) or the reactivity with water or dilute acid, to check the deduction, since these follow the same periodic patterns as Period 3. An element with a giant covalent structure, a very high melting point and an amphoteric or weakly acidic oxide, for instance, is likely to sit near the metal–non-metal boundary in a period, in a similar position to silicon.

🧑‍🏫 Teacher's explanation

Think of identifying an unknown element like being a detective at a crime scene. You don't have a name, but you have 'fingerprints': how much energy it takes to pull away its electrons (ionization energy), how big it is (atomic radius), and how it reacts with others (chemical properties).

Ch 3 · Chemical periodicity of other elements · AS & A Level Chemistry 9701

All chapters in The Periodic Table: chemical periodicity