Chemistry - The Periodic Table (Modern Periodic Law, Groups and Periods)

The Modern Periodic Law states that the physical and chemical properties of elements are periodic functions of their atomic numbers ($Z$).

The Modern Periodic Table is organized into $18$ vertical columns called Groups and $7$ horizontal rows called Periods.

The Period Number of an element corresponds to the total number of electron shells (orbits) present in its atom. For example, elements in Period $3$ have $3$ shells ($K, L, M$).

The Group Number is determined by the number of valence electrons. Elements in the same group have the same number of valence electrons and exhibit similar chemical properties.

Group 1 elements (except Hydrogen) are known as Alkali Metals (valence electrons = $1$).

Group 2 elements are known as Alkaline Earth Metals (valence electrons = $2$).

Groups 3 to 12 are known as Transition Elements.

Group 17 elements are known as Halogens (valence electrons = $7$).

Group 18 elements are known as Noble Gases or Inert Gases, characterized by a stable octet ($ns^2 np^6$) configuration, except for Helium ($1s^2$).

Valency is the combining capacity of an atom. For groups $1, 2, 13, 14$, valency equals the number of valence electrons. For groups $15, 16, 17$, valency is calculated as $8 - \text{valence electrons}$.