# Classification of Elements and Periodicity

> Modern periodic table, atomic radius, ionisation enthalpy, electronegativity and their exceptions from NCERT — NEET-UG Chemistry notes.

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- Exam / course: NEET-UG · Subject: Chemistry
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- First published: 2026-10-02
- Last updated: 2026-10-02
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## Direct answer

Mendeleev's 1869 table arranged elements by increasing atomic weight, left deliberate gaps, and correctly predicted the properties of eka-aluminium (gallium) and eka-silicon (germanium). The modern periodic law, based on Moseley's atomic numbers, sorts elements into 18 groups and 7 periods. Across a period, atomic radius and metallic character decrease while ionisation enthalpy and electronegativity increase; down a group these trends reverse. The second period repeatedly breaks the pattern, and diagonal pairs — lithium with magnesium, beryllium with aluminium, boron with silicon — behave like neighbours rather than strangers.

## What you must remember

- Valence electrons equal the group number for groups 1–2 and the group number minus 10 for groups 13–18; the s, p, d and f blocks are named after the subshell being filled.
- Ionisation enthalpy rises across a period but drops at boron (from beryllium) and at oxygen (from nitrogen).
- Nitrogen's extra stability is the half-filled 2p3 arrangement with its three exchange pairs.
- Electron gain enthalpy of chlorine (−349 kJ/mol) is more negative than fluorine's (−333 kJ/mol) — NCERT's most quoted exception.
- Fluorine is the most electronegative element (4.0 on Pauling's scale); group 17 order is F > Cl > Br > I.
- Noble gases carry the highest ionisation enthalpies in their periods and show positive electron gain enthalpy.
- Noble-gas atomic radii are larger than the neighbouring halogen's because they are measured as van der Waals radii, not covalent radii.
- Across a period, oxides run from strongly basic (Na2O) through amphoteric (Al2O3) to acidic (SO3).

## Why the two famous exceptions occur

Reason through the boron anomaly first. Beryllium's outer electron sits in the filled 2s orbital, while boron's easiest-to-remove electron occupies the higher-energy 2p orbital, which is also better shielded — so boron's first ionisation enthalpy (801 kJ/mol) falls below beryllium's (899 kJ/mol) despite boron's greater nuclear charge. The nitrogen–oxygen case is different in kind: nitrogen's 2p3 is exactly half-filled, and the three possible spin-exchange pairs stabilise it, so nitrogen's first ionisation enthalpy (1402 kJ/mol) edges above oxygen's (1314 kJ/mol), where the fourth 2p electron enters an already-occupied orbital and suffers repulsion. The same two mechanisms — subshell energy gaps and half-filled stability — explain every similar dip in periods 3 and 4, which is why NEET can ask the question in any period and the reasoning still lands. For electron gain enthalpy, fluorine's 2p subshell is so compact that the incoming electron feels strong inter-electronic repulsion in the tiny 2p space, so chlorine's roomier 3p subshell actually accommodates the electron more comfortably.

## How NEET frames it

The examiner rarely asks "define ionisation enthalpy"; instead you get four elements and must order them. Expect a set such as B, Be, N, O, where blind application of "increases across the period" gives the wrong answer, or a comparison of group 2 with group 13 of the same period. A second framing is the radius question: van der Waals radii of noble gases exceed the covalent radii of adjacent halogens, so Ar appears "bigger" than Cl — a statement students must judge as correct, not as an error in the data. Assertion-reason pairs probe why fluorine's electron gain enthalpy is less negative than chlorine's, and match-the-following questions pair diagonal partners with shared properties, such as lithium and magnesium both forming nitrides directly. Keep the exceptions as visible as the trends; the exam lives in the exceptions.

## Frequently asked questions

### Why is the first ionisation enthalpy of boron less than that of beryllium?

Boron's outermost electron is in the higher-energy 2p orbital, which is farther out and better shielded than beryllium's 2s electron, so it is removed more easily.

### Why is chlorine's electron gain enthalpy more negative than fluorine's?

Fluorine's 2p subshell is very compact, and inter-electronic repulsion there reduces the energy released, whereas chlorine's larger 3p subshell accepts the electron with less repulsion.

### How does the atomic radius change down group 17?

It increases because each successive element adds a shell, and the shielding effect outweighs the rising nuclear charge.

### Which pairs of elements show a diagonal relationship?

Lithium–magnesium, beryllium–aluminium and boron–silicon, owing to similar charge density (ionic size to charge ratio) along a diagonal.

### Why are noble gas atomic radii larger than expected?

Their radii are van der Waals radii, measured between non-bonded atoms, which are inherently larger than the covalent radii quoted for bonded atoms of neighbouring elements.