Periodic Classification of Elements

An element X has atomic number 12. (a) Write its electronic configuration (b) To which group and period does it belong? (c) Is it a metal or non-metal? [CBSE 2025]

How does atomic size vary: (a) Down a group (b) Across a period? Give reason for your answer. [CBSE 2024]

What is meant by valency of an element? How does valency vary in a group and in a period? [CBSE 2023]

State Mendeleev's periodic law. What were the two major shortcomings of Mendeleev's periodic table? [CBSE 2022]

Why do elements in the same group show similar chemical properties? Explain with the example of group 1 elements. [CBSE 2021]

Properties of elements are periodic function of their atomic number

Arranged in increasing order of atomic number

Groups: 18 vertical columns (1 to 18) Elements in same group have same valence electrons

Periods: 7 horizontal rows Period number = Number of shells

Group 1: Alkali metals (Li, Na, K, Rb, Cs, Fr) Group 2: Alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra) Group 17: Halogens (F, Cl, Br, I, At) Group 18: Noble gases (He, Ne, Ar, Kr, Xe, Rn)

Increases down a group Decreases across a period (left to right)

Non-metallic character: Opposite trend

Increases down a group (more shells) Decreases across a period (nuclear charge increases)

Steps to find group and period:

1. Write electronic configuration
2. Period = Number of shells
3. Group = Valence electrons (for elements with ≤2 shells)
4. Group = 10 + valence electrons (for d-block)

Example: Sodium (11) Config: 2, 8, 1 Period: 3 (3 shells) Group: 1 (1 valence electron)

Down a group:

• Atomic size increases (more shells)
• Metallic character increases
• Ionization energy decreases
• Electronegativity decreases

Across a period (left to right):

• Atomic size decreases (nuclear charge)
• Non-metallic character increases
• Ionization energy increases
• Electronegativity increases

1. Systematic study of elements
2. Predict properties of elements
3. Identify trends and patterns
4. Correlation between elements
5. Discovery of new elements

Modern table overcomes Mendeleev's limitations:

• Based on atomic number (not atomic mass)
• Positions of isotopes resolved
• No anomalous pairs