IB Diploma Chemistry SL · Structure 1 — models of the particulate nature of matter
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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