📊 Classification of Elements and Periodicity

1. Modern Periodic Law

📊 Modern Periodic Law (Moseley)

The physical and chemical properties of the elements are periodic functions of their atomic numbers.

The long form of the periodic table consists of 18 vertical columns (groups) and 7 horizontal rows (periods).

2. Electronic Configuration & Types of Elements

s-block elements: Groups 1 & 2. Highly reactive metals.
p-block elements: Groups 13 to 18. Contains metals, metalloids, and non-metals.
d-block elements: Groups 3 to 12. Transition elements. They mostly form colored ions and exhibit variable valency.
f-block elements: Lanthanoids and Actinoids. Inner transition elements.

3. Periodic Trends in Physical Properties

📏 Atomic Radius

Decreases across a period (due to increasing effective nuclear charge). Increases down a group (due to addition of new shells).

⚡ Ionization Enthalpy (IE)

The energy required to remove an electron from an isolated gaseous atom. Increases across a period. Decreases down a group.

🧲 Electron Gain Enthalpy (ΔegH)

Enthalpy change when an electron is added to an isolated gaseous atom. Becomes more negative across a period. Becomes less negative down a group.

🔗 Electronegativity

The ability of an atom in a chemical compound to attract shared electrons to itself. Fluorine is the most electronegative element. Increases across a period, decreases down a group.

4. Periodic Trends in Chemical Properties

Valence: Electrons in the outermost shell determine the valence. Across a period, valence with respect to Hydrogen increases from 1 to 4, then falls to 1.

Anomalous Properties of Second Period Elements

The first element of each group (Li, Be, B, C, N, O, F) differs from the rest of their group due to small size, large charge/radius ratio, high electronegativity, and absence of d-orbitals.