Particles, Isotopes and Orbitals

Electrons occupy shells ( $n=1,2,3…$ ) divided into subshells (s, p, d), each made of orbitals holding at most two electrons of opposite spin. Subshells fill in order of energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Exceptions: Cr is [Ar] 3d⁵ 4s¹ and Cu is [Ar] 3d¹⁰ 4s¹. Transition-metal ions lose 4s electrons first. Relative atomic mass is the weighted mean from a mass spectrum: $A_r=\frac{\sum(m/z\times\text{abundance})}{\sum\text{abundance}}$ .

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH11AQA 7405 Paper 1≈ 45 minRevision 1 · updated 2026-10-30

Section 1 of 14

Learning objectives

Describe subatomic particles and their behaviour in electric fields.
Write electronic configurations using s, p and d notation, including ions.
Describe the shapes of s and p orbitals.
Calculate relative atomic mass from mass spectra.

Pause and think

Why does potassium put its 19th electron into 4s rather than 3d?

Section 2 of 14

Core idea

Start hereWhere electrons live

Electrons don't orbit like planets. They occupy fuzzy regions called orbitals, and the order those orbitals fill explains the whole layout of the Periodic Table.

The big idea

Shells contain subshells (s, p, d) made of orbitals, each holding two electrons. Subshells fill in order of energy: 4s before 3d in atoms, and 4s empties first in ions.

1. Building configurations

Order of filling

Drawing diagram…

Follow the arrows from lowest energy.

Worked exampleManganese and its ion

Write configurations for Mn ( $Z=25$ ) and Mn²⁺.

Goal: Both configurations.
Strategy: Fill in energy order; remove 4s first.

$\mathrm{Mn}: 1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^2$
25 electrons.
$\mathrm{Mn^{2+}}: [\mathrm{Ar}]\,3d^5$
Lose both 4s electrons.

AnswerMn [Ar] 3d⁵ 4s²; Mn²⁺ [Ar] 3d⁵.

✓ Check23 electrons in the ion ✓.

2. Mass spectra

Relative atomic mass from a spectrum

$\text{total mass}=\textstyle\sum(m/z)_i\,a_i$
Each peak: mass × relative abundance.
$\text{total particles}=\textstyle\sum a_i$
Abundances may not add to 100.
$A_r=\frac{\sum(m/z)_i a_i}{\sum a_i}$
Weighted mean.

Common misconceptionChromium and copper Cr [Ar] 3d⁵ 4s¹ and Cu [Ar] 3d¹⁰ 4s¹: one 4s electron moves into 3d to give a half-filled or filled subshell.

Pause and think

Why do electrons in a beam bend more than protons in the same electric field?

Summary

s 2, p 6, d 10.
4s in first, out first.
$A_r$ is the weighted mean.

Orbital: Region holding up to two electrons of opposite spin.
Subshell: Group of orbitals of the same type within a shell.
Isotopes: Atoms with the same proton number but different neutron numbers.

HistorySchrödinger. In 1926 Erwin Schrödinger's wave equation showed electrons are best described as clouds of probability, giving us the orbital shapes we draw today.

Fun factGold's colour. Gold's yellow colour comes from relativistic effects on its 6s and 5d orbitals; without Einstein's relativity, gold would look silvery.

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