Classification of Elements & Periodicity

Syllabus Unit: Inorganic Chemistry

Why it matters

Chapter overview

The modern periodic table arranges 118 elements by atomic number so that physical and chemical properties recur periodically — atomic size, ionisation enthalpy, electron gain enthalpy and electronegativity can all be predicted from an element's position. NEET repeatedly tests the exceptions to these trends (Mg > Al in IE1, Cl more negative than F in electron gain enthalpy) and the anomalous first member of every group, so this unit rewards knowing both the rule and where it breaks.

Chemistry Syllabus Inorganic Chemistry ~25% weightage

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1Periodic Trends (Size, Ionisation Enthalpy)

Atomic radius shrinks across a period and swells down a group; ionisation enthalpy runs almost exactly opposite — both governed by effective nuclear charge and shielding.

Learning Objectives

Explain how effective nuclear charge (Zeff) and shielding together set the trend in atomic radius across a period and down a group.
State the first ionisation enthalpy trend across a period and down a group, with the underlying reason.
Justify why successive ionisation enthalpies of the same element always increase (IE1 < IE2 < IE3...).
Identify and explain exceptions to the general IE trend, such as Mg > Al and P > S.

Key Terms

Atomic RadiusDistance from the nucleus to the outermost electron shell; measured as covalent, van der Waals or metallic radius depending on the type of bonding.

Effective Nuclear Charge (Zeff)The net positive charge felt by an outer electron after accounting for shielding by inner electrons; Zeff = Z − shielding constant.

Ionisation Enthalpy (IE)Minimum energy needed to remove the most loosely bound electron from an isolated gaseous atom to form a cation.

Successive Ionisation EnthalpyEnergy needed to remove each subsequent electron from an already-formed cation; rises steeply once a stable noble-gas core is reached.

Elements are arranged by increasing atomic number so that physical and chemical properties recur periodically — this is the modern periodic law. Two properties dominate NEET's periodicity questions: atomic radius and ionisation enthalpy, and both trace back to a single cause: effective nuclear charge (Zeff).

Atomic radius

Across a period (left to right): radius decreases — each added electron enters the same shell while nuclear charge rises, so Zeff pulls the electron cloud in tighter.
Down a group (top to bottom): radius increases — a new principal shell is added each period, and this outweighs the rising nuclear charge.
Cationic radius < parent atom; anionic radius > parent atom, because removing/adding an electron changes the effective charge felt per electron.

Ionisation enthalpy

Ionisation enthalpy is the energy needed to pull the outermost electron away from a gaseous atom (X(g) → X+(g) + e−). Because an electron held closer to and more strongly by the nucleus takes more energy to remove, IE broadly increases across a period and decreases down a group — the mirror image of atomic radius.

Common MistakeExceptions NEET tests directly: IE1(Mg) > IE1(Al) because Mg's 3s2 is a filled, extra-stable subshell while Al's outermost electron sits in a higher-energy, easier-to-remove 3p1 orbital. Similarly IE1(P) > IE1(S), because P's 3p3 is a stable half-filled configuration.

IE1 < IE2 < IE3 < ... always, for the same element — and jumps sharply once a noble-gas core is exposed (e.g. IE2 >> IE1 for Na, since Na+ has a stable neon core).