CBSE · Class 10 · Chemistry · Chapter 5

Periodic Classification of Elements — Formula Sheet

Board Formulas

7 formulas

1.Döbereiner's Triad★
$Atomic mass of middle \approx \frac{m _{1} + m _{3}}{2}$
Groups of 3 elements with similar chemical properties, in which the middle element's atomic mass is the arithmetic mean of the other two. Example: Li (7), Na (23), K (39).
2.Newlands' Octaves
$Property (n) \approx Property (n + 8)$
Every 8th element (when arranged by atomic mass) had similar properties — like musical octaves. Worked only up to calcium.
3.Mendeleev's Periodic Law★
$Properties = f (Atomic Mass)$
Properties of elements are a periodic function of their atomic masses. Left gaps for undiscovered elements (eka-aluminium = Ga, eka-silicon = Ge).
4.Modern Periodic Law (Moseley)★
$Properties = f (Atomic Number, Z)$
Properties of elements are a periodic function of their ATOMIC NUMBERS (not masses). Solved most anomalies of Mendeleev's table.
5.Valency Trend
$Valency_{group} = group number (for groups 1-2) or 8 - group number (13-17)$
Group 1 → valency 1; group 2 → 2; group 17 → 1; group 16 → 2. All noble gases → 0.
6.Atomic Size Trend (Period)
$r_{Li} > r_{Be} > \dots > r_{F}$
DECREASES across a period (nuclear charge increases, electrons in same shell).
7.Atomic Size Trend (Group)
$r_{Li} < r_{Na} < r_{K} < r_{Rb} < r_{Cs}$
INCREASES down a group (a new shell is added at every step).