Chapter 3 - Classification of Elements and Periodicity in Properties

Classification of Elements and Periodicity in Properties explains how the modern periodic table organises elements and why their properties repeat in predictable patterns. The periodic table connects an element’s atomic number and electronic configuration with its position, valency and chemical behaviour.

Why are elements classified?

Studying every element independently would be inefficient. Classification groups elements with related configurations and properties, allowing students to compare elements and predict unfamiliar behaviour.

Development of periodic classification

Students study the main historical attempts:

• Döbereiner’s triads

• Newlands’ law of octaves

• Mendeleev’s periodic table

• Modern periodic law

Mendeleev arranged elements largely by atomic mass and properties. His table successfully predicted undiscovered elements but could not place isotopes correctly and contained some anomalous pairs.

The modern periodic law states that elemental properties are periodic functions of atomic number. This resolves many limitations of the earlier table.

Structure of the modern periodic table

Students learn about periods, groups and four blocks:

• s-block

• p-block

• d-block

• f-block

Electronic configuration helps identify an element’s period, block and often its group. The chapter also covers the systematic nomenclature of elements with atomic numbers above 100.

Periodic trends

Atomic and ionic radius

Atomic radius generally decreases across a period because effective nuclear charge increases. It increases down a group because additional shells are added.

Cations are generally smaller than their parent atoms. Anions are generally larger. Within an isoelectronic series, radius decreases as nuclear charge increases.

Ionisation enthalpy

Ionisation enthalpy is the energy required to remove an electron from an isolated gaseous atom. It generally increases across a period and decreases down a group.

Exceptions arise from subshell energy, penetration, shielding, electron pairing and the stability of half-filled or completely filled configurations.

Electron-gain enthalpy

Electron-gain enthalpy describes the enthalpy change when an isolated gaseous atom accepts an electron. Atomic size, nuclear attraction and electronic configuration affect its value.

Electronegativity

Electronegativity is the tendency of a bonded atom to attract the shared electron pair. It generally increases across a period and decreases down a group.

Valency and chemical character

Students connect position with valency, oxidation state, metallic character, non-metallic character, reactivity and the nature of oxides.

What can students study on Teachoo?

• Development of the periodic table

• Modern periodic law

• Electronic configuration and block identification

• Atomic and ionic radius comparisons

• Isoelectronic species

• Ionisation enthalpy orders

• Electron-gain enthalpy questions

• Electronegativity and valency

• NCERT reasoning questions

• MCQs, assertion-reason and case-based questions

Common mistakes

• Memorising arrows without understanding the cause

• Treating every trend as perfectly regular

• Confusing electron-gain enthalpy with electronegativity

• Ignoring electronic stability when explaining exceptions

• Comparing ions without checking whether they are isoelectronic

Best way to study this chapter

For every comparison, check four things: number of shells, nuclear charge, shielding and electronic configuration. Use the trend only after considering these factors.

Frequently asked questions

What is periodicity?

Periodicity is the repetition of similar properties at regular intervals when elements are arranged by increasing atomic number.

Why does atomic radius decrease across a period?

Nuclear charge increases while electrons enter the same main shell, so the electrons are attracted more strongly.

Why does atomic radius increase down a group?

Each step adds a new electron shell, increasing distance and shielding.

Why are periodic trends not perfectly regular?

Subshell arrangement, penetration, electron pairing and especially stable configurations produce exceptions.

What is an isoelectronic series?

It contains atoms or ions with the same number of electrons. The member with the greatest nuclear charge normally has the smallest radius.

How does electronic configuration locate an element?

The highest occupied principal shell gives the period, while the differentiating electron helps identify the block and group.

Learn Classification of Elements and Periodicity in Properties with Teachoo to understand periodic trends, solve comparison questions and master NCERT reasoning.