Maharashtra State Board · Class 10 · Chemistry · Chapter 2

Periodic Classification of Elements — Formula Sheet

Board Formulas

10 formulas

1.Dobereiner's Law of Triads
$Atomic mass of middle \approx \frac{m _{1} + m _{3}}{2}$
$m_{1}, m_{3}$ : Atomic masses of first and third elements
Triad = three elements with similar chemistry. Example: Li(7), Na(23), K(39) — (7+39)/2 = 23.
2.Newlands' Law of Octaves
$Property of n^{th} element \approx (n + 8)^{th}$
Every 8th element resembled the first, like musical octaves. Failed beyond Ca.
3.Mendeleev's Periodic Law★
$Properties = f (atomic mass)$
Properties of elements are a periodic function of their atomic mass.
4.Modern Periodic Law (Moseley, 1913)★
$Properties = f (atomic number Z)$
Properties are a periodic function of Z. Resolves Ar-K and Co-Ni anomalies.
5.Maximum Electrons in Shell (Bohr-Bury)
$Max = 2 n^{2}$
n = 1 → 2 e-, n = 2 → 8 e-, n = 3 → 18 e-, n = 4 → 32 e-.
6.Valency Rule★
$Valency = ve if ve \leq 4; Valency = 8 - ve if ve > 4$
Group 18 valency = 0 (already stable octet). ve = valence electrons.
7.Atomic Radius Trend★
$Across period ↓; Down group ↑$
Effective nuclear charge pulls e- inward across a period; adding a shell increases size down a group.
8.Electronegativity Trend
$Across period ↑; Down group ↓$
F is most electronegative (3.98 Pauling). Metallic character shows the opposite trend.
9.Ionisation Energy Trend
$Across period ↑; Down group ↓$
Energy to remove outermost e-. Highest for noble gases, lowest for alkali metals.
10.Atomic Number and Mass Number
$A = Z + N$
A = mass number, Z = number of protons (= atomic number), N = number of neutrons.