Periodic Table — Part 1: The modern periodic law and atomic size: visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

Why a table?

Part 1 — 118 elements, but only a few patterns. The periodic table puts elements with similar properties in the same column, so learning one element tells you a lot about its neighbours.

In this lesson you will

State the modern periodic law
Find the group and period of an element (atomic number 1–20) from its electronic configuration
Explain trends using nuclear charge and the number of shells
Compare the atomic sizes of two elements and give a reason

Table of four families of elements with members up to calcium and their shared behaviour
Family	Members (up to calcium)	Shared behaviour
Alkali metals (group 1)	Li, Na, K	Soft, very reactive metals
Alkaline earth metals (group 2)	Be, Mg, Ca	Reactive metals, form M²⁺ ions
Halogens (group 17)	F, Cl	Reactive non-metals, form X⁻ ions
Noble gases (group 18)	He, Ne, Ar	Very unreactive gases

Concept

The modern periodic law

Modern periodic law: the properties of elements are a periodic function of their atomic numbers. Mendeleev arranged elements by atomic mass, which put some pairs in the wrong order (argon, 39.9, comes before potassium, 39.1). Arranging by atomic number (the number of protons) removes these problems, because the atomic number fixes the number of electrons — and the valence electrons decide chemical properties.

Table comparing atomic mass and atomic number as the basis of the periodic table
Basis	Problem or advantage
Atomic mass (Mendeleev)	Ar (39.9) would come after K (39.1); isotopes would need separate places
Atomic number (modern)	Ar (18) before K (19) naturally; isotopes share one place
Why it works	Atomic number → electronic configuration → valence electrons → properties

Check your understanding

Quick check: the periodic law

Decide why atomic number, not atomic mass, is the basis of the modern table.

Question: Multiple choice

Multiple choice1 mark

In the modern periodic table, argon (atomic mass 39.9) is placed before potassium (atomic mass 39.1). This is because elements are arranged in order of their:

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