Mini-Lesson

The nuclear atom

This mini-lesson covers Structure 1 at SL: the particulate nature of matter, the nuclear atom, isotopes and relative atomic mass, and electron configurations.

Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.

Structure 1.1

The particulate nature of matter

All matter is made of particles — atoms, molecules or ions. Elements contain one kind of atom; compounds contain atoms of different elements chemically bonded in fixed ratios; mixtures are not chemically joined.

Solid — particles packed, vibrating in fixed positions.
Liquid — particles touching but able to move past each other.
Gas — particles far apart, moving randomly and quickly.

Kinetic theory: raising temperature increases the average kinetic energy of the particles.

Structure 1.2

Inside the nuclear atom

An atom has a tiny, dense nucleus of protons (+1) and neutrons (0), surrounded by electrons (−1) in energy levels.

Atomic number Z = number of protons (defines the element).
Mass number A = protons + neutrons.
Neutrons = A − Z.

Almost all of an atom's mass sits in the nucleus; the proton and neutron each have a relative mass of ~1, while the electron is ~1/1836.

Quick check

Sub-atomic particles

?Which sub-atomic particle carries a relative charge of −1?

Structure 1.2

Isotopes and relative atomic mass

Isotopes are atoms of the same element (same Z) with different numbers of neutrons (different A). They have identical chemical properties but slightly different masses.

The relative atomic mass (Ar) is the weighted mean of the isotope masses, based on their natural abundances — often measured by mass spectrometry.

Ar = Σ(isotope mass × abundance) ÷ 100weighted average across all isotopes

Calculate

Relative atomic mass

1Chlorine is 75% Cl-35 and 25% Cl-37. Calculate its relative atomic mass.

Ar = (35 × 75 + 37 × 25) ÷ 100.

Calculate

Counting neutrons

2How many neutrons are in an atom of K-39 (Z = 19)?

neutrons = mass number − atomic number = 39 − 19.

Calculate

Mass-spectrum average

3An element is 69.2% mass-63 and 30.8% mass-65. Calculate its relative atomic mass.

Ar = (63 × 69.2 + 65 × 30.8) ÷ 100. (It is copper.)

Structure 1.3

Electron configurations

Electrons occupy energy levels (shells) and sub-levels (s, p, d). Evidence comes from the emission spectrum of hydrogen: discrete coloured lines that converge at higher energy — proving electrons occupy fixed energy levels, not a continuum.

s holds 2 electrons, p holds 6, d holds 10.
Fill lowest energy first: 1s 2s 2p 3s 3p 4s 3d 4p …

Example — sodium (Z = 11): 1s² 2s² 2p⁶ 3s¹.

Quick check

Evidence for energy levels

?What does the line (rather than continuous) emission spectrum of hydrogen tell us?

Quick check

Reading a configuration

?An atom has the configuration 1s² 2s² 2p⁶ 3s² 3p⁴. How many electrons does it have in total?

Sort it

Where does it belong?

Tap a term, then tap the category it fits.

🟢 In the nucleus

🔵 Outside the nucleus

🟣 Isotopes have…

Match it

Match the particle to its property

Tap an item on the left, then its match on the right.

Particle

Property

Recap

The big ideas to know

Nuclear atom: protons + neutrons in the nucleus, electrons in energy levels

Notation: Z = protons, A = protons + neutrons, neutrons = A − Z

Isotopes: same Z, different neutrons; Ar = weighted mean of isotope masses

Electron config: emission spectra show discrete energy levels; fill s → p → d lowest first

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