Classification of Elements and Periodicity formulas for JEE Main & Advanced

7 formula entries for Classification of Elements and Periodicity, in the order the chapter builds them. Each shows the formula, when to use it and the first trap to avoid.

Effective Nuclear Charge (Zeff)

Z_{eff} = Z - σ

When to use. When comparing atomic radii, ionization energy, or electron gain enthalpy of elements.

Trap. Do not forget that σ depends on electron's group and orbital (Slater's rules).

Ionization Enthalpy (IE)

M (g) \to M^{+} (g) + e^{-} Δ H = {IE}_{1}

When to use. When comparing ease of electron removal or metallic character.

Trap. IE2 > IE1 always; do not compare IE1 of different elements without considering electronic configuration.

Electron Gain Enthalpy (ΔegH)

X (g) + e^{-} \to X^{-} (g) Δ H = Δ_{eg} H

When to use. When comparing tendency to form anions or oxidizing power.

Trap. Do not confuse with electronegativity; ΔegH is an experimental enthalpy, electronegativity is a relative scale.

Electronegativity (χ)

χ_{A} - χ_{B} = 0.102 \sqrt{Δ}

(Pauling), where Δ = bond energy difference

When to use. When predicting bond polarity, dipole moment, or acid-base strength.

Trap. Electronegativity is not a physical property; it is a relative scale.

Electronic Configuration and Position

s-block: group = ns electrons; p-block: group = 10 + (ns + np electrons); for d-block: group = (n-1)d + ns electrons (except anomalies).

When to use. When determining period and group from electronic configuration.

Trap. For d-block, group = sum of (n-1)d and ns electrons (e.g., Fe: 3d⁶4s² → group 8).

Pauling's Electronegativity Scale

χ_{A} - χ_{B} = 0.102 \sqrt{Δ}

When to use. When calculating electronegativity difference from bond energies.

Trap. Do not use this formula for ionic compounds; it is for covalent bonds.

Mulliken's Electronegativity

χ_{Mulliken} = \frac{I E + E A}{2}

When to use. When given IE and EA values to compute electronegativity.

Trap. IE and EA must be in same units (eV).

Values to remember

$Z$ = atomic number Effective Nuclear Charge (Zeff)
$σ$ = shielding constant (Slater's rules) Effective Nuclear Charge (Zeff)
$χ_{F} = 4.0$ Electronegativity (χ)
$χ_{O} = 3.5$ Electronegativity (χ)
$χ_{N} = 3.0$ Electronegativity (χ)
$χ_{C} l = 3.0$ Electronegativity (χ)

Read the full chapter: Classification of Elements and Periodicity for JEE, with what JEE asks and the common traps.