PERIODIC TABLE (CLASS XI ) CBSE NOTES

 

PERIODIC TABLE

1. Structure of the Periodic Table

• Arranged in order of increasing atomic number (Z).

• Periods: Horizontal rows (7 periods). Indicate number of shells.

• Groups: Vertical columns (18 groups). Elements in the same group have similar chemical properties due to the same number of valence electrons.

• Blocks: Based on the type of orbital being filled: s-block, p-block, d-block (transition metals), f-block (inner transition metals).

2. Classification of Elements

a) Metals

• Good conductors, malleable, ductile.

• Tend to lose electrons and form cations.

• Mostly on the left and centre of the table.

b) Non-metals

• Poor conductors, brittle.

• Tend to gain electrons and form anions.

c) Metalloids

• Show properties of both metals and non-metals.
  (B, Si, Ge, As, Sb, Te).

3. Important Groups

• Group 1 – Alkali Metals (Li, Na, K…): Highly reactive, soft, form +1 ions.

• Group 2 – Alkaline Earth Metals (Mg, Ca…): Less reactive than Group 1; form +2 ions.

• Group 17 – Halogens (F, Cl…): Highly reactive non-metals; form –1 ions.

• Group 18 – Noble Gases (He, Ne, Ar…): Very stable and unreactive.

4. Periodic Trends

a) Atomic Radius

• Decreases across a period (due to increased nuclear charge).

• Increases down a group (more electron shells).

b) Ionization Energy

• Energy required to remove an electron.

• Increases across a period.

• Decreases down a group.

c) Electronegativity

• Ability to attract electrons in a bond.

• Increases across a period.

• Decreases down a group.

• Highest: Fluorine.

d) Metallic Character

• Decreases across a period.

• Increases down a group.

5. Special Elements

• Hydrogen: Unique element; placed with Group 1 but not a metal.

• Transition Metals: d-block elements; show multiple oxidation states; form colored compounds.

• Lanthanides & Actinides: f-block; rare earth metals; actinides include radioactive elements.

6. Periodicity

• Repetition of chemical properties after regular intervals due to similar valence electron configurations.

7. Valency & Oxidation States

• Valency related to number of electrons in outer shell.

• Transition metals show variable oxidation states, important for catalysis.