Periodic Classification of Elements — Class 10 Science (CBSE) explained

What you’ll learn

1. 1

   Lesson 1: Early Attempts: Finding Patterns

   5 min
   • Classification Makes Element Properties Easier to Study
   • A Triad Links Similar Properties with Atomic Mass
   • A Pattern Is Useful Only If It Classifies Enough Elements
   • Döbereiner’s Work Connects Pharmacy and ChemistryNot assessed
   • Newlands Finds Similarity at Every Eighth Element
   • The Musical Comparison Explains the NameNot assessed
   • A Fixed Octave Pattern Cannot Fit All Elements
2. 2

   Lesson 2: Mendeléev’s Periodic Table

   6 min
   • Mendeléev Uses Atomic Mass and Chemical Properties
   • Mendeléev’s Background and the Influence of His TableNot assessed
   • Groups and Periods Organise the Table
   • Chemical Similarity Can Take Priority over Mass Order
   • Gaps Become Testable Predictions
   • A New Group Accommodates Noble Gases
   • Hydrogen Resembles Two Different Families
   • Atomic Mass Creates Two Further Problems
3. 3

   Lesson 3: The Modern Periodic Table

   6 min
   • Atomic Number Provides the Modern Ordering Basis
   • Whole-Number Proton Counts Resolve Mass-Based Difficulties
   • Groups Connect Elements with Similar Outer Shells
   • A Period Shares the Same Number of Occupied Shells
   • Shell Capacity and the Outermost-Shell Limit Are Different
   • Valence Electrons Determine Combining Capacity
   • Atomic Radius Decreases across a Period
4. 4

   Lesson 4: Periodic Trends and Application

   7 min
   • New Shells Increase Atomic Size down a Group
   • Position Reveals Broad Property Regions
   • Metallic Character Follows the Ease of Losing Electrons
   • Electron Gain and Oxide Nature Complete the Trend Map
   • Use Configurations and Compounds as Evidence
   • A Position Diagram Supports Several Predictions
   • Compare Systems and Investigate Other Arrangements
   • Recap: Explain the Pattern, Then Practise

Key concepts

The triad criterion

Lithium comes first, sodium is in the middle, and potassium comes last.

Newlands’ repeating pattern

In 1866, John Newlands arranges the known elements in increasing order of atomic mass. His arrangement begins with hydrogen, the element with the lowest atomic mass, and ends with thorium, the fifty-sixth element.

Reading Mendeléev’s table

The vertical columns are groups, and the horizontal rows are periods.

Mass order and chemical evidence

Increasing atomic mass is an important basis of Mendeléev’s arrangement, but it is not followed without exception. Sometimes an element with a slightly greater atomic mass comes before an element with a slightly lower mass.

Periods and shell count

What stays the same is the number of occupied shells. Every element in this period has two.

Valency and chemical similarity

Valency is the combining capacity determined by the electrons in the outermost shell. In Activity six, consider magnesium and sulphur.

Metallic character

A trend arrow is useful, but the explanation must identify why the outermost electrons become easier or harder to remove.

Non-metallic character and oxide nature

Metal oxides are basic, while non-metal oxides are acidic in general.

Exam focus

School assessment only

For 2026-27, CBSE assesses this topic in school (periodic assessment or portfolio), not in the year-end board examination. It is still worth learning: it builds on and supports the board chapters.

Syllabus

Periodic Classification of Elements: formative assessment only this session; CBSE reading material provided.

Not assessed

These parts go beyond the 2026-27 syllabus and are taught for understanding only: Döbereiner’s Work Connects Pharmacy and Chemistry, The Musical Comparison Explains the Name and Mendeléev’s Background and the Influence of His Table.

Exam note

Formative-only: assessed in school (periodic assessment / portfolio), not in the year-end board examination.

Practice in the app

87 board-pattern questions with marking schemes: 26 multiple choice (1 mark), 14 assertion–reason (1 mark), 17 very short answer (2 marks), 16 short answer (3 marks), 4 long answer (5 marks) and 10 case-based (4 marks).

Sample questions with marking points

Board-pattern questions from the AI Lessons practice set for this chapter. The marking points show what an examiner looks for; full model answers are in the app.

1. Very short answer2 marks

   State the expected general nature of the oxides of sodium and sulphur. Relate each prediction to the classification of the element.

   Marking points

   • Sodium is a metal.0.5
   • Sodium oxide is expected to be basic.0.5
   • Sulphur is a non-metal.0.5
   • Sulphur oxides are expected to be acidic.0.5

2. Short answer3 marks

   Boron, carbon and nitrogen have atomic numbers 5, 6 and 7, respectively. Determine their valencies. Explain why valency does not keep increasing as atomic number increases through these three elements.

   Marking points

   • Determines boron's valency as 3.0.5
   • Determines carbon's valency as 4.0.5
   • Determines nitrogen's valency as 3.0.5
   • Identifies the increasing valence-electron counts as 3, 4 and 5, or gives the corresponding configurations.0.5
   • Explains that nitrogen's valency is 8 - 5 = 3, so valency decreases after carbon despite increasing valence-electron count.1

3. Long answer5 marks

   A student claims, 'Successful predictions prove that atomic mass provides a completely satisfactory basis for classifying every element.' Evaluate this claim using two achievements and three limitations of Mendeléev's classification.

   Marking points

   • Intentional gaps enabled successful predictions of undiscovered elements, supported by a correct example such as eka-aluminium and gallium.1
   • Noble gases could be accommodated in a new group without disturbing the existing order.1
   • Hydrogen resembles alkali metals in its electronic configuration or compound formulae.0.5
   • Hydrogen also resembles halogens in being diatomic, making a fixed, satisfactory position difficult.0.5
   • Isotopes have similar chemical properties but different atomic masses, creating conflicting placement requirements.1
   • Irregular atomic-mass increases prevent reliable prediction of the number of undiscovered elements between known elements.1

Frequently asked questions

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