Atomic Structure (A-Level) — A-Level Chemistry Revision

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What is Atomic Structure (A-Level)?

A-Level Chemistry delves deeper into atomic structure, building on GCSE concepts. It introduces the quantum mechanical model, where electrons exist in specific energy levels, sublevels (s, p, d, f), and orbitals, each holding up to two electrons with opposite spins. This model explains the shapes of these orbitals and the principles (Aufbau principle, Hund's rule, Pauli exclusion principle) governing electron configurations, which in turn determine an element's chemical properties and its position in the periodic table.

Board notes: AQA, Edexcel, and OCR all cover the fundamental principles of electron configuration, ionisation energies, and atomic structure. However, AQA often includes more questions on time-of-flight mass spectrometry. OCR may place a greater emphasis on interpreting successive ionisation energy graphs to determine an element's group. Edexcel tends to integrate these concepts with other topics like periodicity and bonding more frequently.

Worked example

Common mistakes

• 1Confusing electron shells with sub-shells and orbitals. Students often forget that shells are the main energy levels, which are further divided into sub-shells (s, p, d, f) containing specific numbers of orbitals.
• 2Incorrectly writing electron configurations, especially for transition metals like chromium and copper, which are exceptions to the Aufbau principle for stability reasons (e.g., Cr is [Ar] 4s1 3d5, not 4s2 3d4).
• 3Mixing up the definitions of ionisation energy. First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms, and subsequent ionisation energies involve removing electrons from positive ions.

Atomic Structure (A-Level) exam questions

Exam-style questions for Atomic Structure (A-Level) with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• Why does the 4s orbital fill before the 3d orbital?

  The 4s orbital is at a lower energy level than the 3d orbital, so electrons fill it first according to the Aufbau principle. However, once filled, the 3d orbitals become more stable and the 4s electrons are the first to be removed when forming a positive ion.

• What is the difference between an orbit and an orbital?

  In the outdated Bohr model, electrons were thought to travel in fixed circular paths called orbits. The modern quantum mechanical model describes orbitals as three-dimensional regions of space around the nucleus where there is a high probability of finding an electron.