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28 Dec 2025

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2 pages

Understanding Electron Configuration

Elyssa Bugden

@elyssabugden_eedz

Ever wondered how electrons actually behave around atoms and why... Show more

# KEY POINTS

The quantum number
is just another name
for the shell number

Every orbital can hold
up to two electrons if
they have opposite

Electron Shells and Quantum Numbers

Think of electrons as tenants living in a massive apartment building around the nucleus. The principal quantum number (n) is basically the floor number - higher floors mean more energy and further from the nucleus.

Each floor (shell) has different types of flats called subshells: s, p, d, and f. The s subshell is like a studio flat (holds 2 electrons), p is like a 3-bedroom house (holds 6 electrons), d is more spacious (holds 10), and f is massive (holds 14). Shell 1 only has 1s, shell 2 has 2s and 2p, shell 3 adds 3d, and so on.

Here's where chemistry gets cheeky - 4s actually fills before 3d because it has lower energy, even though logic suggests otherwise. It's like having a ground floor flat that's somehow cheaper than a basement one. The filling order goes: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p...

Key Rule: Electrons are antisocial - they'll spread out in separate orbitals before pairing up, and when they do pair up, they spin in opposite directions to minimise repulsion.

Electron Configuration and the Periodic Table

The periodic table is basically a map of electron configuration. Each block (s, p, d, f) corresponds to which subshell is being filled. S-block contains alkali and alkaline earth metals, d-block has transition metals, p-block contains non-metals and metalloids, and f-block includes lanthanides.

Writing electron configuration is like giving someone directions. Instead of writing the full address every time, you can use shorthand notation - just write the nearest noble gas in square brackets, then add the extra bits. Sodium becomes $Ne$ 3s¹ instead of 1s² 2s² 2p⁶ 3s¹.

For ions, always add or remove electrons from the highest energy level first. This means 4s electrons come and go before 3d electrons, even though 3d is technically in shell 3. It's all about energy levels, not shell numbers.

Pro Tip: To find an element's electron configuration quickly, locate it on the periodic table and trace backwards through the blocks - the position tells you exactly which subshells are filled.

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Biology

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290

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28 Dec 2025

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2 pages

Understanding Electron Configuration

Elyssa Bugden

@elyssabugden_eedz

Ever wondered how electrons actually behave around atoms and why the periodic table looks the way it does? Understanding electron configuration is like learning the address system for electrons - it tells you exactly where they live and how they... Show more

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Electron Shells and Quantum Numbers

Key Rule: Electrons are antisocial - they'll spread out in separate orbitals before pairing up, and when they do pair up, they spin in opposite directions to minimise repulsion.

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Electron Configuration and the Periodic Table

Pro Tip: To find an element's electron configuration quickly, locate it on the periodic table and trace backwards through the blocks - the position tells you exactly which subshells are filled.