AQA A-Level Chemistry: Atomic Structure (7405)

This guide covers Section 3.1.1 Atomic Structure, the first topic within Physical chemistry (3.1) in AQA A-level Chemistry (7405), for teaching from September 2015. The specification is organised into three broad strands – Physical chemistry, Inorganic chemistry and Organic chemistry – each containing multiple named topics; 3.1.1 is the very first of these.

Where this fits in 7405

Sections 3.1.1-3.1.7 of Physical chemistry, 3.2.1-3.2.3 of Inorganic chemistry and 3.3.1-3.3.6 of Organic chemistry are designed for the first year of study and are also the AS-level content, so AS and A-level students can be taught together throughout. Atomic structure underpins how electron configuration is used to explain periodicity (3.2.1) and the shapes of molecules covered later in Bonding (3.1.3).

Syllabus coverage

AQA A-LEVEL CHEMISTRY (7405) — SECTION 3.1.1 ATOMIC STRUCTURE

• 3.1.1.1 Fundamental particles — the relative charge and mass of protons, neutrons and electrons, and how an atom is built from a nucleus surrounded by electrons
• 3.1.1.2 Mass number and isotopes — determining numbers of fundamental particles from mass number, atomic number and charge, and using a time-of-flight mass spectrometer to determine relative atomic mass from isotopic abundance
• 3.1.1.3 Electron configuration — electron configurations of atoms and ions up to Z = 36 in terms of shells and sub-shells, and ionisation energies

How to approach it

Because mass spectrometry (3.1.1.2) combines a practical technique with a calculation skill, practise both explaining how a time-of-flight mass spectrometer works and calculating relative atomic mass from isotopic abundance data, since exam questions frequently test both together. Electron configuration (3.1.1.3) is tested with a specific notation (1s², 2s², 2p⁶ and so on) that needs practising until it is automatic, since errors here compound into later topics on bonding and periodicity. Because this topic carries mathematical skills weighting (AQA specifies that 20% of the overall A-level assesses mathematical skills), practising calculation-based questions on isotopic abundance and relative atomic mass is time well spent even within this conceptually-focused opening topic.

Official syllabus

AQA A-level Chemistry (7405) specification, for teaching from September 2015 — aqa.org.uk.

Sub-shells and orbitals

At A-Level the shell model is refined into sub-shells (s, p, d, f) made of orbitals, each holding a maximum of two electrons with opposite spins. An s sub-shell has one orbital (2 electrons), p has three (6), d has five (10).

Electrons fill in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. The 4s sub-shell fills before 3d because it is slightly lower in energy — but it is also emptied first when the atom ionises, which is a frequent source of error.

Chromium and copper are the standard exceptions: chromium is 1s2 2s2 2p6 3s2 3p6 3d5 4s1 and copper is 3d10 4s1, because a half-filled or full d sub-shell is more stable.

Ionisation energy

The first ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms, to form one mole of gaseous 1+ ions:

X(g) -> X+(g) + e-

Three factors control its size: nuclear charge, atomic radius, and shielding by inner shells. Ionisation energy increases across a period because nuclear charge rises while shielding stays roughly constant, and decreases down a group because the outer electron is further out and better shielded.

Reading ionisation energy data

Successive ionisation energies reveal electronic structure. A large jump occurs when an electron is removed from a shell closer to the nucleus — the position of that jump tells you the group. An element whose fourth ionisation energy is far greater than its third has three outer electrons, so it is in Group 3.

Two dips interrupt the general rise across Period 3. Aluminium is lower than magnesium because its outer electron is in a 3p orbital, higher in energy and more shielded than 3s. Sulfur is lower than phosphorus because in sulfur two electrons share one 3p orbital and repel each other, making one easier to remove.

Mass spectrometry

Time-of-flight mass spectrometry gives isotopic masses and abundances directly. Ions of the same charge are accelerated to the same kinetic energy, so lighter ions travel faster and arrive sooner. Relative atomic mass is then the weighted mean of the peaks.

Worked example

Successive ionisation energies of an element (kJ mol-1) are 590, 1150, 4940, 6480.

590 -> 1150   roughly double, same shell
1150 -> 4940  more than four times, LARGE JUMP

The jump occurs between the second and third electrons, so the element has two outer-shell electrons and is in Group 2.

Common mistakes

Writing the 3d sub-shell before 4s when giving configurations of transition metals, then removing 3d electrons first on ionisation — 4s is always lost first. Omitting the state symbols (g) in an ionisation energy equation, which loses the mark. Explaining the aluminium dip using shielding by inner shells rather than the 3p/3s energy difference. Saying “the nucleus is bigger” when the correct phrase is “greater nuclear charge”.

Quick revision checklist

• Write full electron configurations, including the Cr and Cu exceptions.
• Define first ionisation energy with a correctly stated equation and state symbols.
• Explain trends in ionisation energy across a period and down a group using all three factors.
• Account for the Al and S dips in Period 3.
• Deduce the group of an element from successive ionisation energy data.
• Explain how time-of-flight mass spectrometry separates isotopes and calculate Ar from a spectrum.