Class 10 Periodic Classification of Elements — Mendeleev to the Modern Table
"Periodic Classification of Elements" is a short chapter, but it is asked constantly in board exams because it is really a chapter about the history of an idea — why scientists needed to organise the elements, what earlier attempts got right and wrong, and how Mendeleev's table led to the modern one you use today. This guide walks through that history in the order NCERT presents it, with the classic worked examples examiners still ask.
Early attempts before Mendeleev
Dobereiner's law of triads (1817): when three elements with similar properties are arranged in increasing atomic mass, the atomic mass of the middle element is approximately the average of the other two.
Triad: Li (7), Na (23), K (39).
Average of Li and K = (7 + 39) ÷ 2 = 46 ÷ 2 = 23, which equals Na's actual
atomic mass. The triad law holds.
A second triad: Ca (40), Sr (88), Ba (137).
Average of Ca and Ba = (40 + 137) ÷ 2 = 177 ÷ 2 = 88.5 ≈ 88, close to Sr's
actual atomic mass. The law worked for a small number of triads, which is exactly why it was
abandoned — most elements could not be grouped this way.
Newlands' law of octaves (1866): when elements are arranged in increasing atomic mass, every eighth element has properties similar to the first, like the eighth note in a musical octave. Newlands listed Li, Be, B, C, N, O, F, then Na as the 8th element — and sodium does indeed resemble lithium (both are soft, reactive Group 1 metals). The law broke down once elements heavier than calcium were included, and Newlands also forced more than one element into a single slot to make the pattern fit, which is why it was not widely accepted.
Mendeleev's periodic table
Dmitri Mendeleev (1869) arranged the known elements in order of increasing atomic mass, in horizontal rows (periods) and vertical columns (groups), such that elements with similar chemical properties fell into the same group.
His boldest and most successful move was leaving gaps for elements not yet discovered, and predicting their properties from the pattern of the table. He named three such gaps eka-boron, eka-aluminium and eka-silicon — later confirmed to be scandium, gallium and germanium. For eka-silicon, Mendeleev predicted an atomic mass of about 72 and a density of about 5.5 g/cm³; germanium, discovered in 1886, was found to have an atomic mass close to 72.6 and a density close to 5.35 g/cm³ — a striking confirmation of the table's predictive power.
Limitations of Mendeleev's table
- Position of hydrogen could not be fixed. Hydrogen resembles both the alkali metals (forms H⁺) and the halogens (forms H⁻, exists as a diatomic gas), so it did not sit comfortably in one group.
- Isotopes had no separate place. Isotopes of the same element have different atomic masses but identical chemical properties, so arranging strictly by atomic mass could not accommodate them within a single slot.
- Some elements had to be placed out of atomic-mass order to keep similar properties together: cobalt (atomic mass 58.93) was placed before nickel (58.69); tellurium (127.6) was placed before iodine (126.9); argon (39.95) was placed before potassium (39.10). All three break the "increasing atomic mass" rule.
Worked example — using Mendeleev's method to predict a property
This is the reasoning examiners want, not just the germanium fact by rote. If an element sits directly below silicon in a Mendeleev-style table and its neighbouring known elements (gallium, arsenic, zinc, germanium's own group members) have properties changing smoothly across the row and down the column, the missing element's atomic mass and density can be estimated as roughly the average of its immediate neighbours — exactly the logic Mendeleev used for eka-silicon, eka-boron and eka-aluminium before those elements were ever isolated.
The modern periodic law
In 1913, Henry Moseley showed experimentally that an element's properties correlate with its atomic number (the number of protons), not its atomic mass. This became the modern periodic law:
Arranging elements by atomic number instantly resolves every anomaly listed above: argon (Z = 18) correctly precedes potassium (Z = 19); cobalt (Z = 27) correctly precedes nickel (Z = 28); and isotopes of the same element share one atomic number, so they now share one position in the table — the very problem Mendeleev's table could not solve.
Worked example — finding an element's position from atomic number
An element has atomic number 12. Find its period and group in the modern table.
Electronic configuration of Z = 12: 2, 8, 2 (three shells occupied) → this is
magnesium.
Number of shells = 3 → Period 3.
Number of valence electrons = 2 → Group 2.
An element has atomic number 17. Find its period and group.
Electronic configuration of Z = 17: 2, 8, 7 → three shells → Period 3; seven valence electrons → Group 17 (this is chlorine).
Common mistakes that cost marks
- Saying Mendeleev arranged elements by atomic number. He arranged them by atomic mass — atomic number was not even a known concept until Moseley's later work.
- Mixing up the three real anomalies. Co–Ni, Te–I and Ar–K are the standard textbook examples; inventing a different pair in an exam answer is a common and avoidable error.
- Forgetting that "period" = number of shells and "group" = valence electrons only works cleanly for the representative (s- and p-block) elements taught at this level.
- Confusing Newlands' octaves with Dobereiner's triads — one groups elements in threes by atomic mass averaging, the other groups them in eights by counting position in an ordered list. They are different ideas from different chemists.
Quick revision table
| Classification | Basis | Key weakness |
|---|---|---|
| Dobereiner's triads | Middle atomic mass = average of outer two | Worked for very few elements |
| Newlands' octaves | Every 8th element resembles the 1st | Broke down after calcium; forced fits |
| Mendeleev's table | Increasing atomic mass, grouped by properties | Hydrogen's position, isotopes, mass-order anomalies |
| Modern periodic table | Increasing atomic number | Resolves all of the above |
Check atomic masses as you revise. The free calculator suite's molar mass tool and interactive periodic table make it easy to verify the values used in this chapter's numericals.
Open the ABC Chemistry Calculator Suite →Moving into Class 11 chemistry soon, where this history sets up the modern periodic table in full detail? ABC Chemistry runs Class 11–12 chemistry coaching at the Gurugram centre and online classes across India — details at abcchemistry.in.