Electronic Configuration

What is Electronic Configuration?

Electronic configuration refers to the arrangement of electrons in the orbitals of an atom. It determines chemical behavior, reactivity, bonding and periodic-table position. Electrons fill orbitals according to quantum rules and energy order.

Rules for Writing Electronic Configuration

• Aufbau Principle: Fill lowest energy orbitals first (1s → 2s → 2p → 3s …).
• Pauli Exclusion Principle: Max two electrons per orbital with opposite spins.
• Hund’s Rule: Single-occupy degenerate orbitals before pairing.

Aufbau Diagram — order of orbital filling

Notation & Examples

Use subshell labels with superscripts to show electron counts. Examples: Hydrogen: 1s1, Oxygen: 1s2 2s2 2p4, Sodium: 1s2 2s2 2p6 3s1.

Electronic Configuration of First 20 Elements

Element	Symbol	Z	Electronic Configuration
Hydrogen	H	1	1s1
Helium	He	2	1s2
Lithium	Li	3	1s2 2s1
Beryllium	Be	4	1s2 2s2
Boron	B	5	1s2 2s2 2p1
Carbon	C	6	1s2 2s2 2p2
Nitrogen	N	7	1s2 2s2 2p3
Oxygen	O	8	1s2 2s2 2p4
Fluorine	F	9	1s2 2s2 2p5
Neon	Ne	10	1s2 2s2 2p6
Sodium	Na	11	1s2 2s2 2p6 3s1
Magnesium	Mg	12	1s2 2s2 2p6 3s2
Aluminium	Al	13	1s2 2s2 2p6 3s2 3p1
Silicon	Si	14	1s2 2s2 2p6 3s2 3p2
Phosphorus	P	15	1s2 2s2 2p6 3s2 3p3
Sulfur	S	16	1s2 2s2 2p6 3s2 3p4
Chlorine	Cl	17	1s2 2s2 2p6 3s2 3p5
Argon	Ar	18	1s2 2s2 2p6 3s2 3p6
Potassium	K	19	1s2 … 4s1
Calcium	Ca	20	1s2 … 4s2

Significance

• Explains chemical bonding and valency
• Determines position in the periodic table
• Predicts physical and chemical properties
• Identifies reactivity and inert behavior

(MCQ)

Q1: What is the electronic configuration of Oxygen?
A) 1s² 2s² 2p⁶
B) 1s² 2s² 2p⁴
C) 1s² 2s¹ 2p⁵
D) 1s² 2s² 2p⁵
Answer & Explanation

Correct: B — Oxygen has 8 electrons; the filling order gives 1s² 2s² 2p⁴. (2 in 1s, 2 in 2s, remaining 4 in 2p)

Q2: Which rule explains why electrons occupy separate orbitals before pairing?
A) Aufbau Principle
B) Pauli Exclusion Principle
C) Hund’s Rule
D) Dalton’s Law
Answer & Explanation

Correct: C — Hund’s Rule: electrons maximize multiplicity by occupying degenerate orbitals singly before pairing.

Q3: What is the electronic configuration of Sodium (Na)?
A) 1s² 2s² 2p⁶
B) 1s² 2s² 2p⁶ 3s¹
C) 1s² 2s² 2p⁴ 3s²
D) 1s² 2s² 3p⁵
Answer & Explanation

Correct: B — Sodium has 11 electrons; core 1s²2s²2p⁶ then 3s¹.

Further reading: Atomic Structure •

Wikipedia — Electronic configuration.