Electronic Configurations of Elements and the Periodic Table

NCERT Reference: Chapter 3 – Classification of Elements and Periodicity in Properties, Pages 87–88

Quick Notes

Elements in the same period have valence electrons in the same principal energy level.
Elements in the same group have similar outer electron configurations.
Periods start with filling a new shell (n).
s- and p-block elements show gradual changes in configuration across a period.
d-block elements involve filling of (n−1)d subshells.
Groupwise similarities arise from identical valence shell configurations.

Full Notes

The arrangement of elements in the periodic table is governed by their electronic configurations. These configurations determine how elements behave chemically and why they fall into patterns of groups and periods.

Electronic Configurations in Periods

Each period in the periodic table corresponds to the filling of a new principal energy level (n).

NCERT 11 Chemistry diagram showing principal quantum numbers n increasing by period with shells filling across a row.

As you move across a period (from left to right):

Electrons are added one by one to the same outer shell.
The number of valence electrons increases, but the principal quantum number (n) remains constant.

Let’s break this down by period:

1st Period (n = 1):
Contains 2 elements: Hydrogen (1s¹) and Helium (1s²)
Filling of the 1s orbital

2nd Period (n = 2):
Starts at Lithium (Z = 3): 1s² 2s¹
Ends at Neon (Z = 10): 1s² 2s² 2p⁶
s- and p-subshells of n = 2 get filled

3rd Period (n = 3):
Starts at Sodium (Z = 11): [Ne] 3s¹
Ends at Argon (Z = 18): [Ne] 3s² 3p⁶

4th Period (n = 4):
Begins at Potassium (Z = 19): [Ar] 4s¹
Introduces d-block (transition elements) starting at Scandium (Z = 21)
Ends at Krypton (Z = 36)

Key Point:
The general trend is that across a period, the number of electrons in the outermost shell increases, but they are still in the same principal energy level.

Groupwise Electronic Configurations

Groups (vertical columns) show similar electronic configurations in the valence shell, which leads to similar chemical properties.

NCERT 11 Chemistry chart illustrating groups with shared valence-shell configurations across the periodic table.

Group 1 (Alkali metals): All have ns¹ configuration
- Li: [He] 2s¹
- Na: [Ne] 3s¹
- K: [Ar] 4s¹
Group 2 (Alkaline earth metals): All have ns² configuration
Group 17 (Halogens): All have ns² np⁵
Highly reactive, need one more electron to complete octet
Group 18 (Noble gases): All have ns² np⁶
Stable, inert due to completely filled shells (except He: 1s²)
d-block (Transition elements): General configuration: (n−1)d^1–10 ns^1–2
Example: Fe = [Ar] 3d⁶ 4s²
f-block (Inner transition elements): General configuration: (n−2)f^1–14 (n−1)d^0–1 ns²
These elements fill the f-orbitals of inner shells.

Summary

Periods correspond to filling the same principal energy level.
Groups share similar valence shell configurations.
s and p blocks change gradually across periods.
d block fills the (n−1)d subshells.
Electronic configuration underpins periodic trends and chemical behaviour.