ScienceCBSE Class 10 · Metals and Non-metals

Periodic Classification of Elements Class 10: Notes for School Assessment

Updated 14 October 2026 · Fact-checked

Assessed in school only, not in the board exam.

Periodic Classification of Elements traces how chemists sorted the elements: Döbereiner's triads, Newlands' law of octaves, Mendeléev's table based on atomic mass and the modern table based on atomic number, with trends in valency, atomic size and metallic character. It is taught from the CBSE Reading Material and assessed in school, not in the board exam.

Understand Periodic Classification of Elements

This topic is assessed in school only, not in the board exam. It is taught from the CBSE Reading Material for 2026-27, so prepare it for your school test or project.

As more elements were discovered, chemists needed a way to arrange them so that similar elements sat together. The first try simply split elements into metals and non-metals. In 1817 Döbereiner noticed groups of three similar elements, called triads, in which the middle element's atomic mass is roughly the average of the other two. Lithium (6.9), sodium (23.0) and potassium (39.0) form one: (6.9 + 39.0) ÷ 2 ≈ 23. But he could find only three such triads, so the idea was of limited use.

In 1866 Newlands lined up the known elements by increasing atomic mass and found that every eighth element resembled the first, like the notes of music: his law of octaves. It worked only up to calcium, assumed no new elements would be found, and forced unlike elements into the same column.

Mendeléev made the real breakthrough. He studied 63 elements, especially the formulae of their oxides and hydrides, and arranged them by increasing atomic mass and similar properties. His periodic law said properties are a periodic function of atomic mass. His boldest move was to leave gaps for undiscovered elements and predict their properties; gallium, scandium and germanium later fitted the gaps. His table also took in the noble gases as a new group without disturbing the rest. Its weaknesses: no fixed place for hydrogen, no answer for isotopes, and atomic masses that do not rise in a regular way.

In 1913 Moseley showed that atomic number is the more basic property. The modern periodic law says properties are a periodic function of atomic number. The modern table has 18 groups and 7 periods: elements in a group share the same number of valence electrons, and elements in a period have the same number of occupied shells. Across a period atomic size falls and metallic character decreases; down a group atomic size and metallic character increase.

Periodic Classification of Elements: formulas and key facts

Döbereiner's triad rule: $atomic mass of middle element \approx \frac{mass of first + mass of third}{2}$; Check with Li, Na, K: (6.9 + 39.0) ÷ 2 = 22.95, close to 23.0 for sodium.
Mendeléev's periodic law: $Properties of elements = periodic function of atomic mass$; Mendeléev's basis: atomic mass plus similar chemical properties (formulae of oxides and hydrides).
Modern periodic law: $Properties of elements = periodic function of atomic number$; Moseley, 1913. Atomic number removed the cobalt-nickel inversion and settled isotopes.
Maximum electrons in a shell: $2 n^{2}$; K shell 2, L shell 8; the outermost shell holds at most 8, which is why periods 2 and 3 have 8 elements each.
Trend across a period: $atomic size ↓, metallic character ↓$; Nuclear charge rises and pulls the electrons closer, so atoms hold on to electrons more tightly.
Trend down a group: $atomic size ↑, metallic character ↑$; A new shell is added at each step, so the outer electrons are farther from the nucleus and easier to lose.

Periodic Classification of Elements in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Mendeléev's gaps and predictions

Suppose your class is arranging a shelf of 20 labelled spice jars by strength of flavour, and one slot between two similar spices is clearly missing a jar that the shop has not stocked yet. Your friend wants to close the gap so the shelf looks neat.

Using what Mendeléev did with the gaps in his table, explain why it is wiser to leave the slot empty, and what you can say about the missing jar before it arrives.

Try it first, then show the solutionSolution

Mendeléev left gaps where no known element fitted the pattern instead of forcing elements into them.
He predicted the properties of the missing elements from their neighbours, naming them eka-boron, eka-aluminium and eka-silicon.
When scandium, gallium and germanium were discovered, their properties matched his predictions, which convinced chemists his table was right.
So an empty slot keeps the pattern correct and lets you predict the missing item from the jars on either side.

Answer: Leave the slot empty. Mendeléev kept gaps where the pattern demanded an element nobody had found yet, and predicted its properties from its neighbours. When gallium, scandium and germanium were later discovered and matched his predictions, the gaps became the strongest proof of his table. In the same way, the empty slot keeps your order right and tells you roughly what the missing spice should be like.

Memory hook: An honest gap beats a forced fit: Mendeléev's gaps proved his table.

Tricks to remember Periodic Classification of Elements

Short memory aids. Say each one aloud once, then use it on a question the same day.

Three steps before the modern table: D-N-M
Döbereiner (1817, threes), Newlands (1866, eights), Mendeléev (1872 table, gaps and predictions).
Use it when: Writing the history in the right order.
Eka means one place below
Eka-boron = scandium, eka-aluminium = gallium, eka-silicon = germanium.
Use it when: Matching Mendeléev's predicted elements to the real ones.
Across shrinks, down grows
Across a period atoms shrink and become less metallic; down a group they grow and become more metallic.
Use it when: Any trend question on size or metallic character.

Exam tips

Your school test may ask you to compare the three early classifications: learn one strength and one limitation of each.
Practise reading the trend data: given atomic radii of a period or group, arrange them and state the trend with its reason.
Write electronic configurations for the first 20 elements; most valency and position questions depend on them.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What did Döbereiner notice about the atomic masses in a triad?Show answer
The middle element's atomic mass was roughly the average of the other two.
2.Up to which element did Newlands' law of octaves work?Show answer
Up to calcium; it worked only for the lighter elements.
3.Which compounds did Mendeléev use to classify elements?Show answer
The oxides and hydrides formed by each element.
4.State the modern periodic law.Show answer
The properties of elements are a periodic function of their atomic number.
5.What do elements in the same group have in common?Show answer
The same number of valence electrons (the same outer-shell configuration).
6.Why does atomic size increase down a group?Show answer
A new shell is added at each step, so the outer electrons are farther from the nucleus.

Periodic Classification of Elements: frequently asked questions

Is the periodic table in the Class 10 board exam?

No. For 2026-27, Periodic Classification of Elements is taught from the CBSE Reading Material and assessed in school only, through internal or formative assessment. It is not set in the board theory paper.

Why could Mendeléev not place hydrogen correctly?

Hydrogen behaves like the alkali metals in forming compounds with halogens, oxygen and sulphur, but like the halogens it forms diatomic molecules and covalent compounds. It fits both places, so no fixed position could be given.

How did the modern table fix the cobalt-nickel problem?

Mendeléev had to put cobalt (58.9) before nickel (58.7) against the atomic-mass order. Arranged by atomic number, cobalt comes before nickel naturally, so the inversion disappears.