Electronic Structure | Chemistry

📖 In-Depth Theory

Electron Shells

Electrons occupy SHELLS (energy levels) around the nucleus. They always fill from the INNERMOST (lowest energy) shell outward.

SHELL CAPACITIES (for first 20 elements):

1st shell: maximum 2 electrons.

2nd shell: maximum 8 electrons.

3rd shell: maximum 8 electrons.

Electrons are only added to the next shell when the current one is FULL.

Example — Sodium (Na, atomic number 11):

11 electrons to distribute.

1st shell: 2 (full) → 2nd shell: 8 (full) → 3rd shell: 1 remaining.

Electronic configuration written as: 2.8.1

Electronic Configurations of the First 20 Elements

H (1): 1

He (2): 2

Li (3): 2.1

Be (4): 2.2

B (5): 2.3

C (6): 2.4

N (7): 2.5

O (8): 2.6

F (9): 2.7

Ne (10): 2.8

Na (11): 2.8.1

Mg (12): 2.8.2

Al (13): 2.8.3

Si (14): 2.8.4

P (15): 2.8.5

S (16): 2.8.6

Cl (17): 2.8.7

Ar (18): 2.8.8

K (19): 2.8.8.1

Ca (20): 2.8.8.2

Note: the 4th shell starts filling at potassium (K) and calcium (Ca) because the 3rd shell fills to 8 before the 4th begins (for these lighter elements).

Electronic Structure and Chemical Properties

The NUMBER OF ELECTRONS IN THE OUTERMOST SHELL (valence electrons) determines an element's chemical properties.

Elements in the SAME GROUP of the periodic table have the SAME NUMBER of outer electrons:

Group 1: 1 outer electron (Li: 2.1, Na: 2.8.1, K: 2.8.8.1)

Group 7: 7 outer electrons (F: 2.7, Cl: 2.8.7)

Group 0: 8 outer electrons (full shell) — He: 2, Ne: 2.8, Ar: 2.8.8

This is why elements in the same group have similar chemical properties — they react in similar ways to achieve a full outer shell.

ATOMS REACT TO ACHIEVE A FULL OUTER SHELL:

METALS lose electrons → form positive ions (cations).

NON-METALS gain electrons (or share) → form negative ions (anions) or covalent bonds.

NOBLE GASES already have full outer shells → very unreactive.

⚠️ Common Mistake

Always fill shells in ORDER — 2 in the first, then 8 in the second, then continue. The 3rd shell fills to 8 before the 4th shell starts. Students sometimes try to put more than 8 in the 3rd shell for early elements — for the first 20, the 3rd shell only goes up to 8.

📌 Key Note

Shells fill innermost first: 2, 8, 8. Electronic configuration written as numbers separated by dots (e.g. 2.8.3 for Al). Same group = same outer electrons = similar chemistry. Full outer shell = stable (noble gas configuration).

🎯 Matching Activity — Match the Element to its Electronic Configuration

Match each element to its correct electronic configuration. — drag the symbols on the right to match the component names on the left.

Sodium (Na, 11)

Drop here

Chlorine (Cl, 17)

Drop here

Calcium (Ca, 20)

Drop here

Neon (Ne, 10)

Drop here

Magnesium (Mg, 12)

Drop here

2.8.1 — 11 electrons: 2 in shell 1, 8 in shell 2, 1 in shell 3

2.8.2 — 2 outer electrons, Group 2 element

2.8.8.2 — 20 electrons across four shells

2.8 — full outer shell, noble gas, very unreactive

2.8.7 — 17 electrons: 2 in shell 1, 8 in shell 2, 7 in shell 3

🎯 Test Yourself

Question 1 of 2

1. An element has the electronic configuration 2.8.6. Which group of the periodic table is it in?

2. Why do lithium (2.1) and sodium (2.8.1) have similar chemical properties?

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🧪 Electronic Structure

Electron Shells

Electronic Configurations of the First 20 Elements

Electronic Structure and Chemical Properties

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