Is it correct to say: Confusing Mendeleev's law (atomic mass) with modern law (atomic number)?

Mendeleev → atomic MASS. Moseley → atomic NUMBER (Z). The switch resolved the Ar-K and Te-I anomalies.

Is it correct to say: Saying the modern table has 8 groups?

Modern (long) form has 18 groups and 7 periods. Mendeleev's original had 8 groups + subgroups (A, B).

Is it correct to say: Placing hydrogen anywhere and everywhere?

Modern table places H at the top of group 1 due to 1s¹ configuration, but its true position remains debated (also fits group 17 in some contexts).

Is it correct to say: Saying atomic size increases across a period?

Across a period atomic size DECREASES — electrons enter the same shell but nuclear charge rises. Down a group it INCREASES.

Is it correct to say: Confusing valency with valence electrons?

Valence electrons = electrons in outermost shell. Valency = combining capacity, which is (valence e⁻) for groups 1-4 and (8 − valence e⁻) for groups 13-17.

Is it correct to say: Placing all noble gases with valency 8 or 1?

Noble gases (group 18) have valency ZERO — outer shell already stable.

Periodic Classification of Elements

Döbereiner's triads, Newlands' octaves, Mendeleev's and modern periodic tables, periodic trends — NCERT Class 10 Chemistry Ch 5

📐 7 formulas✏️ 3 examples🎯 6 practice⚖️ 4-5 marks🏫 CBSE📚 Class 10✓ 2025–26 syllabus

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Board Exam Tips

→Merits and demerits of Mendeleev's periodic table — a repeated 3-mark question.
→Modern periodic law statement + who gave it (Moseley). One-line but almost always asked.
→Trends: atomic size, metallic character, valency across a period and down a group — draw arrows on a mini table.
→Learn the 4 blocks (s, p, d, f) and how many periods/groups in the modern table (7 periods, 18 groups).
→Explain why atomic size decreases across a period but increases down a group — 2 marks safe answer.

📐 Formulas(7)

Döbereiner's Triad★ Board fav

Atomic mass of middle \approx \frac{m _{1} + m _{3}}{2}

Newlands' Octaves

Property (n) \approx Property (n + 8)

Mendeleev's Periodic Law★ Board fav

Properties = f (Atomic Mass)

Modern Periodic Law (Moseley)★ Board fav

Properties = f (Atomic Number, Z)

Valency Trend

Valency_{group} = group number (for groups 1-2) or 8 - group number (13-17)

Atomic Size Trend (Period)

r_{Li} > r_{Be} > \dots > r_{F}

Atomic Size Trend (Group)

r_{Li} < r_{Na} < r_{K} < r_{Rb} < r_{Cs}

✏️ Solved Examples

1Solved Exampleeasy3 steps

Predict the atomic mass of potassium given the atomic masses of lithium (7) and sodium (23) in the Li-Na-K triad.

Döbereiner: middle mass = (first + third)/2

23 = \frac{7 + m _{K}}{2}

2Solved Exampleboard4 steps

State the modern periodic law. List any three achievements and any two limitations of Mendeleev's periodic table.

Modern periodic law: The physical and chemical properties of elements are a periodic function of their atomic numbers (Moseley).

3Solved ExampleHOTS4 steps

An element X has atomic number 17. (a) State its group and period. (b) Predict its valency. (c) Compare its atomic size with an element of atomic number 9. (d) Write the formula of its compound with sodium.

Z = 17 ⇒ electronic configuration 2,8,7 ⇒ 3 shells ⇒ period 3; 7 valence e⁻ ⇒ group 17 (halogens).

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Confusing Mendeleev's law (atomic mass) with modern law (atomic number)
✓Mendeleev → atomic MASS. Moseley → atomic NUMBER (Z). The switch resolved the Ar-K and Te-I anomalies.
2
Saying the modern table has 8 groups
✓Modern (long) form has 18 groups and 7 periods. Mendeleev's original had 8 groups + subgroups (A, B).
3
Placing hydrogen anywhere and everywhere
✓Modern table places H at the top of group 1 due to 1s¹ configuration, but its true position remains debated (also fits group 17 in some contexts).
4
Saying atomic size increases across a period
✓Across a period atomic size DECREASES — electrons enter the same shell but nuclear charge rises. Down a group it INCREASES.
5
Confusing valency with valence electrons
✓Valence electrons = electrons in outermost shell. Valency = combining capacity, which is (valence e⁻) for groups 1-4 and (8 − valence e⁻) for groups 13-17.
6
Placing all noble gases with valency 8 or 1
✓Noble gases (group 18) have valency ZERO — outer shell already stable.

🎯 Practice Yourself

🎯Practice Yourself6 questions

Q1
State one advantage and one drawback of Newlands' law of octaves.
Q2
Give one reason why cobalt (Co, 58.9) is placed before nickel (Ni, 58.7) in the modern table.
Q3
Identify the group and period of the element with electronic configuration 2,8,8,2.
Q4
State the trend in metallic character across a period and down a group.
Q5
Element A (Z = 11) and element B (Z = 17) combine to form a compound. Write its formula and predict the type of bonding.
Q6
Why is the atomic size of a Cl⁻ ion greater than that of a Cl atom?

📝 Notes

Periodic Classification of Elements

The Periodic Table is chemistry's master map — it arranges 118 elements so that similar elements sit together, and their properties become predictable.

From Döbereiner to Moseley — the timeline

| Year | Contributor | Idea | |---|---|---| | 1817 | Döbereiner | Triads (Li-Na-K, Ca-Sr-Ba, Cl-Br-I) | | 1866 | Newlands | Law of Octaves — repeats every 8th element | | 1869 | Mendeleev | Periodic table by atomic mass, blank spaces | | 1913 | Moseley | Periodic law by atomic number Z |

Structure of the modern periodic table

7 periods (rows) — period number = number of shells being filled.
18 groups (columns) — grouped by valence-shell electron count.
Blocks: s (groups 1-2 + He), p (13-18), d (3-12), f (lanthanides + actinides).
Metals on the LEFT / bottom; non-metals TOP-RIGHT; metalloids along the staircase (B, Si, Ge, As, Sb, Te).

Trends to memorise

| Property | Across a period → | Down a group ↓ | |---|---|---| | Atomic size | Decreases | Increases | | Metallic character | Decreases | Increases | | Non-metallic character | Increases | Decreases | | Electronegativity | Increases | Decreases | | Ionisation enthalpy | Increases | Decreases | | Valency | 1→4→0 (peak middle) | Same |

Position of hydrogen

Debated placement:

With alkali metals (group 1): both have 1 valence electron and lose it to form +1 ion.
With halogens (group 17): H₂ is a diatomic gas; needs 1 electron to reach noble-gas config.

Modern tables show H in group 1 by convention.