What went wrong when the king rode under the arch?

It was too low and knocked off his crown

How does the chain of blame unfold in the poem?

Each accused person blames another, ending with the king's own changes

On what basis is the modern periodic table arranged?

Elements are arranged in order of increasing atomic number in seven periods and eighteen groups.

Who showed atomic number to be more fundamental than atomic mass?

Henry Moseley, from the regularity of characteristic X-ray spectra (a plot of \(\sqrt{\nu}\) versus \(Z\) is a straight line).

What is a Döbereiner triad?

A set of three similar elements in which the atomic weight of the middle element is roughly the average of the other two, e.g. Li, Na, K.

Why does atomic radius decrease across a period?

Because effective nuclear charge increases while electrons are added to the same valence shell, pulling them in more strongly.

Why is a cation smaller than its parent atom?

It has the same nuclear charge but fewer electrons, so the remaining electrons are held more tightly.

Why is the first ionisation enthalpy of oxygen lower than that of nitrogen?

Nitrogen has a stable half-filled \(2p^{3}\) configuration, whereas the electron removed from oxygen comes from a paired orbital with extra repulsion.

What are metalloids? Give examples.

Elements bordering the diagonal line that show both metallic and non-metallic character, such as Si, Ge, As, Sb and Te.

How does the nature of oxides change across a period?

Oxides change from basic on the left through amphoteric in the middle to acidic on the right.

How are elements with \(Z>100\) named?

By IUPAC nomenclature using numerical roots for the digits of the atomic number with the suffix -ium.

How does electronegativity vary in the table?

It increases across a period and decreases down a group, fluorine being the most electronegative element.

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Chapter 3: Classification of Elements and Periodicity in Properties — Class 11 Chemistry

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Chapter 3: Classification of Elements and Periodicity in Properties

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The modern periodic law states that properties of elements are periodic functions of their:

Summary

This unit traces the development of the periodic law and the modern periodic table. Early attempts — Döbereiner's triads, Newlands' law of octaves and de Chancourtois' telluric helix — preceded the independent work of Mendeleev and Lothar Meyer, who arranged elements by increasing atomic weight so that similar properties recurred periodically. Mendeleev left gaps and successfully predicted eka-aluminium (gallium) and eka-silicon (germanium). Moseley later showed that atomic number, not atomic weight, is the fundamental property, giving the modern periodic law: the properties of elements are periodic functions of their atomic numbers. The long-form table has seven periods and eighteen groups, and elements are classified by the orbital being filled into s-, p-, d- and f-blocks; the f-block lanthanoids and actinoids are inner-transition elements. Over three-quarters of the elements are metals, with metalloids (Si, Ge, As, Sb, Te) bordering the diagonal zig-zag line, and superheavy elements (\(Z>100\)) named by IUPAC numerical roots. Periodic trends arise from the interplay of nuclear charge and shielding: atomic and ionic radii decrease across a period and increase down a group, while ionisation enthalpy and electronegativity increase across and decrease down. Chemical reactivity is greatest at the two extremes of a period and least in the centre, and the nature of oxides changes from basic through amphoteric to acidic across a period.

Genesis of periodic classificationModern periodic law and the long-form tables-, p-, d- and f-block elementsNomenclature of elements with Z greater than 100Atomic and ionic radii trendsIonisation enthalpy and electron gain enthalpyElectronegativityPeriodicity of valence and chemical reactivity

Key terms

Modern periodic law

The physical and chemical properties of elements are periodic functions of their atomic numbers.

Ionisation enthalpy

The energy required to remove an electron from an isolated gaseous atom in its ground state; always positive.

Electron gain enthalpy

The enthalpy change when an electron is added to a neutral gaseous atom to form an anion; may be exothermic or endothermic.

Electronegativity

A qualitative measure of the tendency of an atom in a molecule to attract the shared bonding electrons to itself.

Isoelectronic species

Atoms and ions that have the same number of electrons, such as \(O^{2-}\), \(F^{-}\), \(Na^{+}\) and \(Mg^{2+}\).

Diagonal relationship

The similarity between the first member of a group and the second member of the next group, e.g. Li with Mg and Be with Al.

Important questions

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Modern periodic law

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The physical and chemical properties of elements are periodic functions of their atomic numbers.

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Practice quiz · Classification of Elements and Periodicity in Properties

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