OxfordAQA International A Level Chemistry 3.1.1 Atomic Structure Exam Questions

About OxfordAQA International A Level Chemistry 3.1.1 Atomic Structure

Atomic structure is the first section of physical chemistry in the OxfordAQA International AS and A Level Chemistry course and is examined in Unit 1. You need the properties of protons, neutrons and electrons, 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 Z = 36 and the trends in ionisation energy across Period 3 and down Group 2. This content underpins bonding, periodicity, Group 2 and transition metals. Examiners often ask for ionisation equations with state symbols and explanations of the dips in first ionisation energy across a period.

What you need to know

• Fundamental particles Protons have relative mass 1 and charge +1, neutrons relative mass 1 and charge 0, and electrons relative mass about 1/1840 and charge -1. Models of the atom have developed over time as new evidence appeared.
• Isotopes Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties because they have the same electron configuration.
• TOF mass spectrometry The four stages are ionisation, acceleration, ion drift and detection. All ions are given the same kinetic energy, so lighter ions travel faster and reach the detector first, where they gain electrons and produce a current.
• Relative atomic mass Ar = sum of (isotopic mass x abundance) / total abundance. The specification limits this to spectra of monatomic ions.
• Electron configuration Sub-shells fill in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Chromium is [Ar] 3d5 4s1 and copper is [Ar] 3d10 4s1, and transition metals lose their 4s electrons first when forming ions.
• Ionisation energies First ionisation energy: X(g) → X+(g) + e-. It decreases down Group 2 because outer electrons are further from the nucleus and more shielded. Across Period 3 it generally increases, with dips at aluminium (3p electron in a higher energy sub-shell) and sulfur (paired electrons in a 3p orbital repel).

Common mistakes in 3.1.1 Atomic Structure

• Leaving out state symbols in ionisation equations. Every species must be gaseous, for example Mg(g) → Mg+(g) + e-.
• Saying all ions in a TOF spectrometer have the same speed. They have the same kinetic energy, so lighter ions move faster.
• Removing 3d electrons before 4s electrons for transition metal ions. Fe2+ is [Ar] 3d6.
• Explaining the general rise across a period by more shells. The number of shells stays the same; nuclear charge increases with similar shielding.

Try three 3.1.1 Atomic Structure exam-style questions