This mini-lesson covers the whole of AQA 3.1.1 Atomic structure: fundamental particles and isotopes, time-of-flight mass spectrometry (ionisation, acceleration, ion drift, detection), electron configuration in s, p and d sub-shells, and ionisation energies and the trends they reveal.
Work through each screen, answer the questions as you go (some are wordy, most are calculations) and collect ⭐ stars. Everything here is A-level standard — the maths is done properly, not skipped. Press Start when you're ready.
Atomic structure · particles
Fundamental particles and isotopes
An atom is a nucleus of protons and neutrons surrounded by electrons. Relative to a proton:
Proton — relative mass 1, relative charge +1
Neutron — relative mass 1, relative charge 0
Electron — relative mass 1/1836, relative charge −1
Atomic (proton) number Z = number of protons — it fixes the element. Mass number A = protons + neutrons.
Isotopes are atoms of the same element with different numbers of neutrons. Because chemistry is controlled by the electrons, isotopes of an element react identically; only their mass (and so density, rate of diffusion, mass-spectrum position) differs.
Definitions to learn word-for-word:Relative atomic mass is the average mass of an atom of an element, on a scale where an atom of carbon-12 is exactly 12. Relative molecular mass is the same idea for a molecule.
Atomic structure · TOF mass spectrometry
Time-of-flight (TOF) mass spectrometry
A TOF mass spectrometer has four stages. AQA expects you to be able to describe all of them.
1 Ionisation.Electron impact: a hot filament fires high-energy electrons at the sample, knocking one electron off each particle: X(g) → X⁺(g) + e⁻. Used for elements and low-Mr substances; it can cause fragmentation. Electrospray: the sample is dissolved in a volatile solvent and pushed through a fine needle at high voltage, so each molecule gains a proton: X(g) + H⁺ → XH⁺(g). Used for large, fragile molecules (Mr of the ion is then Mr + 1).
2 Acceleration. A negatively charged plate accelerates the positive ions so they all have the same kinetic energy. Lighter ions therefore end up moving faster.
3 Ion drift. Ions enter a field-free flight tube of fixed length d and separate by mass.
4 Detection. Ions hit the detector, gain electrons and generate a current. The size of the current is proportional to the abundance of that isotope.
Ek = ½mv² and t = d √(m / 2Ek)same kinetic energy for every ion → time of flight depends only on mass. Flight time is proportional to √m.
Why the whole instrument is a vacuum: air particles would collide with the ions, deflecting them and stopping them reaching the detector.
Quick check
Quick check
?A protein of Mr 12 000 is analysed. Which ionisation method is used, and what is the m/z of the molecular ion?
Calculate
Your turn
1A sample of chlorine contains 75.0% ³⁵Cl and 25.0% ³⁷Cl. Calculate the relative atomic mass of this sample.
Hint: A_r = (75.0 × 35 + 25.0 × 37) ÷ 100.
Calculate
Your turn
2A copper sample is 69.2% ⁶³Cu and 30.8% ⁶⁵Cu. Calculate its relative atomic mass to 3 significant figures.
3An ion of mass 1.66 × 10⁻²⁵ kg is accelerated to a kinetic energy of 4.20 × 10⁻¹⁶ J and drifts down a 0.850 m flight tube. Calculate its time of flight in microseconds (µs).
µs
Hint: v = √(2E_k/m) = √(8.40 × 10⁻¹⁶ ÷ 1.66 × 10⁻²⁵) = 7.11 × 10⁴ m s⁻¹; t = d ÷ v = 0.850 ÷ 7.11 × 10⁴ = 1.19 × 10⁻⁵ s.
Atomic structure · electron configuration
Electrons in sub-shells: s, p and d
Electrons occupy energy levels divided into sub-shells. Each orbital holds a maximum of 2 electrons of opposite spin.
s — 1 orbital → holds 2 electrons
p — 3 orbitals → holds 6 electrons
d — 5 orbitals → holds 10 electrons
1s 2s 2p 3s 3p 4s 3d 4p4s is filled before 3d because it is lower in energy — but 4s is emptied FIRST when an ion forms
Examples: Fe is 1s²2s²2p⁶3s²3p⁶4s²3d⁶ (or [Ar] 4s²3d⁶). Fe²⁺ is [Ar] 3d⁶ — the two 4s electrons are lost, not 3d. Fe³⁺ is [Ar] 3d⁵.
The two exceptions you must know:Cr is [Ar] 4s¹3d⁵ and Cu is [Ar] 4s¹3d¹⁰ — a half-full or full d sub-shell is extra stable.
Quick check
Quick check
?What is the electron configuration of the Fe²⁺ ion?
Sort it
Which block does the element sit in?
Tap an element, then tap the block its highest-energy electron is in.
🟩 s-block
🟪 p-block
🟦 d-block
Atomic structure · ionisation energy
First and successive ionisation energies
The first ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms, forming one mole of gaseous 1+ ions.
Mg(g) → Mg⁺(g) + e⁻state symbols matter — everything must be gaseous, and it is per mole
Three factors control its size:
Nuclear charge — more protons pull the electron more strongly, so IE rises.
Atomic radius — the further out the electron, the weaker the attraction, so IE falls.
Shielding — inner shells repel the outer electron, so IE falls.
Down a group IE falls: radius and shielding both increase and outweigh the extra nuclear charge. Across a period IE generally rises (more protons, similar shielding), with two dips:
Al < Mg — Al's outer electron is in a 3p orbital, higher in energy and better shielded than Mg's 3s.
S < P — in S two electrons are paired in one 3p orbital, and their mutual repulsion makes one easier to remove.
Successive ionisation energies always increase (you are pulling an electron off an increasingly positive ion). A large jump shows a new, closer, less-shielded shell has been reached — which reveals the group of the element.
Calculate
Your turn
4An element has successive ionisation energies (kJ mol⁻¹) of 738, 1451, 7733, 10 543. The huge jump comes after the 2nd electron. Which group of the Periodic Table is the element in?
Hint: A big jump after removing n electrons means the element has n electrons in its outer shell — count how many come off cheaply.
Match it
Match the TOF stage to what happens
Tap a stage on the left, then its description on the right.
TOF stage
What happens
Quick check
Quick check
?Why is the first ionisation energy of sulfur lower than that of phosphorus, even though sulfur has one more proton?
Quick check
Quick check
?In a TOF instrument two ions have the same kinetic energy. Ion X has four times the mass of ion Y. How do their flight times compare?
Atomic structure · exam traps
The four things examiners catch you on
State symbols in ionisation-energy equations. Everything must be gaseous: Mg(g) → Mg⁺(g) + e⁻. Dropping the (g) loses the mark.
4s fills first but empties first. Fe is [Ar]4s²3d⁶, but Fe²⁺ is [Ar]3d⁶ — not [Ar]4s²3d⁴.
Electrospray adds a proton, so the ion is XH⁺ and m/z = Mr+ 1. Electron impact removes one, so m/z = Mr.
Flight time goes as √m, not m. Four times the mass means twice the time.
The Ar definition, word for word: "the average mass of an atom of an element, relative to one twelfth of the mass of an atom of carbon-12." Vague versions score zero.
Quick check
Quick check
?Which equation represents the second ionisation energy of magnesium?
Calculate
Your turn
5Magnesium is 79.0% ²⁴Mg, 10.0% ²⁵Mg and 11.0% ²⁶Mg. Calculate its relative atomic mass to 1 decimal place.