AQA A Level Chemistry 3.1.1 Atomic Structure Exam Questions

About AQA A Level Chemistry 3.1.1 Atomic Structure

Atomic structure is the first section of AQA A Level physical chemistry and underpins almost everything that follows. You need to know the fundamental particles, how isotopes are defined, how a time of flight (TOF) mass spectrometer works and how its data give relative atomic mass. You also need electron configurations up to krypton and the evidence for shells and sub-levels from ionisation energies. This content feeds directly into bonding, periodicity, transition metals and organic mass spectrometry. Examiners regularly ask for ionisation equations with state symbols, explanations of the dips in first ionisation energy across a period, and multi-step TOF calculations involving kinetic energy.

What you need to know

• Fundamental particles Protons (relative mass 1, charge +1), neutrons (relative mass 1, charge 0) and electrons (relative mass about 1/1840, charge -1). Mass number is protons plus neutrons; atomic number is the number of protons. Isotopes have the same number of protons but different numbers of neutrons, so they have the same chemical properties.
• Relative atomic mass Ar is the weighted mean mass of an atom of an element relative to 1/12 of the mass of an atom of carbon-12. Calculate it from a mass spectrum by multiplying each m/z by its abundance, adding, then dividing by the total abundance.
• TOF mass spectrometry Four stages: ionisation, acceleration, ion drift and detection. Electrospray ionisation adds a proton (M + H+ gives MH+); electron impact knocks out an electron, X(g) + e- → X+(g) + 2e-. All ions are accelerated to the same kinetic energy, so lighter ions travel faster and reach the detector first, where they gain an electron and create a current.
• TOF calculations Use KE = 1/2 mv^2 and v = d/t. Mass of one ion in kg = (molar mass in g mol-1 / 1000) / L, where L = 6.022 x 10^23 mol-1.
• Electron configuration Fill 1s, 2s, 2p, 3s, 3p, 4s then 3d. Chromium is [Ar]3d5 4s1 and copper is [Ar]3d10 4s1. When transition metals form ions, the 4s electrons are lost first, so Fe2+ is [Ar]3d6.
• First ionisation energy The energy needed to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions, e.g. Na(g) → Na+(g) + e-. It decreases down a group because of greater atomic radius and shielding, and generally increases across a period because nuclear charge rises with similar shielding.
• Dips and successive ionisation energies Aluminium is lower than magnesium because its outer electron is in a higher energy 3p sub-level. Sulfur is lower than phosphorus because of repulsion between the paired electrons in one 3p orbital. Large jumps in successive ionisation energies show where a new, inner shell is reached.

Common mistakes in 3.1.1 Atomic Structure

• Leaving out state symbols in ionisation equations. Every species must be gaseous, for example Mg+(g) → Mg2+(g) + e-.
• Removing 3d electrons before 4s electrons when forming transition metal ions. The 4s electrons are always lost first.
• Explaining the drop at aluminium only in terms of shielding. The key point is that the 3p electron is in a higher energy sub-level, further from the nucleus, so it needs less energy to remove.
• Stating that all ions are accelerated to the same speed in a TOF spectrometer. They are given the same kinetic energy, which is why lighter ions move faster.

Try three 3.1.1 Atomic Structure exam-style questions