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Periodicity

Periodic trends in size, ionisation enthalpy, electron gain enthalpy and electronegativity, with their known exceptions. Short questions, decided by the exceptions.

Questions in the bank
117
q/paper in 2025–26
1.12
Calculation
13%
Notes pages
6

Strand: Structure and recall

When you’ll see it

Elements or ions to rank by radius, ionization enthalpy, electron gain enthalpy or electronegativity; an element to place from Z or a configuration; or oxides to class as acidic, basic, amphoteric or neutral.

How this chapter is tested

Almost every question is an order to check or a pair of statements to judge. Each trend takes a line to state: radius falls across a period and rises down a group; ionization enthalpy and electronegativity do the reverse. The marks sit on the exceptions, not on the arrows.

The exceptions are few and fixed: B below Be and O below N for ionization enthalpy, Ga level with Al and Pb above Sn, Cl more negative than F and O least negative in group 16, neon the most positive noble gas. The smooth order is always among the options, and it is always wrong where an exception applies.

Read every order twice: once for direction (signed values against magnitudes, for electron gain enthalpy) and once pair by pair. A two-statement question can pair a true trend with a false reason, so judge each part alone. The few numeric answers count isoelectronic species, or oxides of one kind, in a list.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • Position from Z

    Period = highest n; block = subshell of the last electron; an element above Z = 100 is named by digit roots (nil, un, bi, tri…) plus -ium; recover Z from an ion as electrons + charge.

  • Atomic and ionic radii

    A cation is smaller and an anion larger than its atom; isoelectronic ions shrink as Z rises: N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺; covalent radius is half the bond length.

  • Ionization enthalpy

    Period 2 runs Li < B < Be < C < O < N < F < Ne; group 13 gives B > Tl > Ga > Al > In and group 14 C > Si > Ge > Pb > Sn; a large jump after k electrons means k valence electrons.

  • Electron gain enthalpy

    Positive for the noble gases (neon most), Be and N; by magnitude Cl > F > Br > I and S > Se > Te > Po > O; electron affinity carries the opposite sign.

  • Electronegativity and metallic character

    F 4.0, O 3.5, Mg below Al; metallic character rises down a group and falls across a period; metalloids B, Si, Ge, As, Sb, Te; diagonal pairs Li–Mg, Be–Al, B–Si.

  • Nature of oxides

    Basic to amphoteric to acidic across a period; the neutral oxides are only CO, NO and N₂O; Al₂O₃, BeO, ZnO, SnO, PbO and the tin and lead dioxides are amphoteric; a higher oxidation state gives a more acidic oxide.

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • The smooth period order

    Li < Be < B < C < N < O < F misses both dips. The true order swaps B with Be and O with N.

  • Fluorine taken as most negative

    Chlorine has the most negative electron gain enthalpy; fluorine is second, because its small 2p shell repels the added electron. Oxygen is the least negative in group 16.

  • Signed values against magnitudes

    S > Se > Te > O is right by magnitude and wrong on signed values. When two options are exact reverses, decide which reading the question uses.

  • A blind fall down groups 13 and 14

    Ga's ionization enthalpy is just above Al's and Pb's above Sn's, from poor d and f shielding. Ga is also smaller than Al.

  • One oxide's nature carried to another

    CO, NO and N₂O are neutral, not acidic or amphoteric, while CO₂ and NO₂ are acidic. Sort each oxide of an element on its own.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

Periodicity notes

Drill every Periodicity question

117 questions from the bank, across 6 subtopics.

Drill the 117 questions

Drill one subtopic at a time

The 6 subtopics, in teaching order.

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