This mini-lesson walks you through the whole of Eduqas Topic 2 — Particles and atomic structure: the structure of the atom, the sub-atomic particles, atomic and mass number, isotopes and relative atomic mass, the development of the atomic model, and electronic structure.
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.
The structure of the atom
What an atom is made of
An atom has a central nucleus of protons and neutrons, surrounded by electrons arranged in shells (energy levels):
The nucleus holds the protons and neutrons; the electrons orbit in shells around it.
Mostly empty space: the nucleus is around 10,000 times smaller than the whole atom — if an atom were a sports stadium, the nucleus would be a pea on the centre spot. Almost the entire atom is empty space between the nucleus and the electrons.
Sub-atomic particles
Charges and masses
You must know the relative charge and relative mass of each of the three sub-atomic particles, and where it is found:
Protons and neutrons each have a relative mass of 1; an electron's mass is so small it is taken as negligible.
Atoms are neutral: an atom has equal numbers of protons and electrons, so the +1 and −1 charges cancel and the overall charge is zero.
Quick check
Spot the particle
?Which sub-atomic particle has a relative charge of 0 and a relative mass of 1?
Atomic number & mass number
The two numbers on every element
Each element in the periodic table is written with two numbers:
Atomic number (proton number) = the number of protons. It identifies the element and is the smaller number.
Mass number = the total number of protons + neutrons in the nucleus. It is the larger number.
neutrons = mass number − atomic number(because mass number counts protons AND neutrons, subtract the protons to leave the neutrons)
Calculate
Your turn — counting neutrons
1A sodium atom has a mass number of 23 and an atomic number of 11. How many neutrons does it contain?
neutrons
Hint: neutrons = mass number − atomic number = 23 − 11.
Isotopes
Same element, different neutrons
Isotopes are atoms of the same element (so the same number of protons) but with different numbers of neutrons — and therefore different mass numbers.
Carbon-12 and carbon-14 both have 6 protons (both are carbon) — they differ only in their neutrons.
Watch out: isotopes differ in their neutrons, not their protons. Change the number of protons and you change the element entirely. Because isotopes have the same electron arrangement, they have the same chemical reactions.
Quick check
Are they isotopes?
?Atom X has 17 protons and 18 neutrons. Atom Y has 17 protons and 20 neutrons. What is the relationship between X and Y?
Higher tier only
Relative atomic mass, Ar
Because an element is a mixture of isotopes, its relative atomic mass (Ar) is the weighted mean mass of its atoms — each isotope's mass counted in proportion to how common it is:
Ar = Σ(isotope mass × % abundance) ÷ 100add up (mass × abundance) for every isotope, then divide by 100
Worked example — chlorine
Chlorine is 75% chlorine-35 and 25% chlorine-37.
Ar = (35 × 75 + 37 × 25) ÷ 100
= (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5
Watch out: Ar is a weighted mean, not a simple average. It lands closer to the mass of the more abundant isotope — that's why chlorine's Ar (35.5) is nearer 35 than 37.
Higher tier · Calculate
Your turn — relative atomic mass
2Boron exists as two isotopes: 20% boron-10 and 80% boron-11. Calculate the relative atomic mass (Ar) of boron.
(Ar)
Hint: Ar = (10 × 20 + 11 × 80) ÷ 100.
Development of the atomic model
How the model changed over time
The model of the atom developed as new experimental evidence appeared. You should know this order:
1. Dalton — atoms are tiny solid spheres that cannot be divided.
2. Thomson — discovered the electron and proposed the plum-pudding model: a ball of positive charge with electrons dotted in it.
3. Rutherford — the alpha-scattering experiment showed a tiny, dense, positive nucleus (the nuclear model); most of the atom is empty space.
4. Bohr — electrons orbit the nucleus in fixed shells (energy levels) at set distances.
5. Chadwick — discovered the neutron in the nucleus.
Alpha-scattering: most particles pass straight through (empty space); a few deflect and very few bounce back (a tiny dense positive nucleus).Order it
Build the model in order
Tap each scientist in the order their model came, earliest first.
Electronic structure
How electrons fill the shells
Electrons occupy shells around the nucleus, filling the lowest (innermost) shell first. For the first 20 elements:
1st shell holds up to 2 electrons
2nd shell holds up to 8 electrons
3rd shell holds up to 8 electrons (for these elements)
Sodium has 11 electrons, written as 2, 8, 1.Calculate
Your turn — electron configuration
3A chlorine atom has 17 electrons. When written as a configuration (filling 2, then 8, then the rest), how many electrons are in its outer shell?
electrons
Hint: 17 electrons fill as 2, 8, then what's left → 2 + 8 = 10, so 17 − 10 = ? in the outer shell.
Linking to the periodic table
Configuration tells you the place
The electron configuration links directly to an element's position in the periodic table:
Group number = number of electrons in the outer shell (for groups 1–7).
Period number = number of occupied shells.
Sodium (2, 8, 1) sits in Group 1, Period 3.
Watch out: read the group from the outer-shell electrons and the period from the number of shells — don't mix them up. (Group 0 / noble gases have a full outer shell.)
Match it
Configuration → element
Tap a configuration on the left, then its element on the right.
Recap
The key facts to know
Atom: central nucleus (protons + neutrons) + electrons in shells; mostly empty space
Particles: proton +1 / mass 1 · neutron 0 / mass 1 · electron −1 / mass ≈ 0
Atomic number = protons · Mass number = protons + neutrons