Electron Configuration and the Periodic Table

25min Part 1 / Ch1 / Lesson 3

Prerequisites: 1-1-2

Objectives

Write electron configurations for K, L, M shells
Explain the meaning of groups and periods in the periodic table
Understand the relationship between valence electrons and chemical properties

Electron Shells

Electron shell model: K shell (max 2) → L shell (max 8) → M shell (max 18)

Electrons fill shells from the nucleus outward: K shell (max 2) → L shell (max 8) → M shell (max 18). Maximum electrons per shell: $2n^2$ where $n$ is the shell number.

Shell	$n$	Max electrons $2n^2$
K	1	2
L	2	8
M	3	18

Electron configurations (Z = 1–20):

Element	Symbol	Configuration (K, L, M, N)	Valence e⁻
Hydrogen	H	(1)	1
Helium	He	(2)	0
Lithium	Li	(2, 1)	1
Carbon	C	(2, 4)	4
Oxygen	O	(2, 6)	6
Neon	Ne	(2, 8)	0
Sodium	Na	(2, 8, 1)	1
Calcium	Ca	(2, 8, 8, 2)	2

The Periodic Table and Valence Electrons

Period (row) = number of electron shells. Group (column) corresponds to the number of valence electrons. Elements in the same group have similar chemical properties.

Valence electrons: electrons in the outermost shell, responsible for chemical bonding.

Group 1 (H, Li, Na, K…): 1 valence e⁻ → easily lose 1 → form cations
Group 17 (F, Cl, Br, I…): 7 valence e⁻ → easily gain 1 → form anions
Group 18 (He, Ne, Ar…): 0 valence e⁻ (full shell) → stable → unreactive (noble gases)

Common Misconception

“M shell fills to 18 before N shell starts” → Not for the first 20 elements. Up to Ca (Z=20), the M shell fills to 8 before the N shell starts. The remaining M shell slots fill with transition metals.

Check Your Understanding

Q1 Maximum electrons in the K shell?

Q2 Electron configuration of oxygen (O, Z=8)?

Q3 Why do elements in the same group have similar properties?

Exercises

Q1. Write the electron configuration of chlorine Cl (Z=17) and state its number of valence electrons.

Solution

Configuration: (2, 8, 7)

K: 2, L: 8, M: 17 − 2 − 8 = 7

Valence electrons: 7. Group 17 elements all have 7 valence electrons and tend to gain 1 electron to form Cl⁻.

Q2. Explain why potassium (K, Z=19) has the configuration (2, 8, 8, 1) rather than (2, 8, 9).

Solution

Although M shell can hold up to 18 electrons, when it’s the outermost shell, 8 electrons form a stable octet. Adding a 9th electron to the M shell is less stable than starting the N shell. So K has (2, 8, 8, 1).

The 9th+ M shell electrons fill in with transition metals starting from Sc (Z=21).