# Atomic structure

> CIE A-Level Chemistry · CIE 9701 (2022-2025)
> Source: https://www.owlsprep.com/study/cie-9701-u2-overview/
> Weight: n/a

This unit introduces the fundamental structure of atoms, the building blocks of all matter. You will explore the nucleus, isotopes, electron arrangement, and how experimental evidence supports modern atomic models.

**Prerequisites:** Basic atomic structure from IGCSE/GCSE Chemistry

## Learning objectives

- Describe the fundamental structure of the atom including the nucleus and electron cloud
- Calculate relative atomic mass from isotopic abundance data
- Differentiate atomic orbitals and write electron configurations for atoms and ions
- Interpret trends in ionisation energies to confirm electronic structure models

## Unit at a Glance

This unit builds from the core of the atom outwards: starting with the massive central nucleus before moving to the arrangement of electrons, which controls all chemical bonding and reactivity. The concepts you learn here underpin every other topic in A-level chemistry, from periodic trends to organic reaction mechanisms.

You will move beyond simple introductory atomic models to learn the quantum orbital model, and use experimental evidence (mass spectrometry and ionisation energy measurements) to confirm this model's structure.

This unit is split into 3 linked sub-topics:
- [Atomic nucleus and isotopes](https://www.owlsprep.com/study/cie-9701-u2-atomic-nucleus-and-isotopes/) — Learn nuclear structure, proton/neutron/nucleon notation, and calculate relative atomic mass from isotopic abundance data.
- [Electron orbitals and configuration](https://www.owlsprep.com/study/cie-9701-u2-electron-orbitals-and-configuration/) — Understand s, p, d orbitals, energy levels, and apply the Aufbau principle to write electron configurations for atoms and ions.
- [Ionisation energies](https://www.owlsprep.com/study/cie-9701-u2-ionisation-energies/) — Define ionisation energies, explain periodic and group trends, and use successive ionisation data to deduce electron structure.

## Common pitfalls

- **Wrong:** Confusing nucleon (mass) number with proton (atomic) number in isotopic notation
  - Why it fails: Proton number defines the element, while nucleon number varies between isotopes
  - Correct: Always write the mass (nucleon) number at the top left and proton number at the bottom left of the element symbol
- **Wrong:** Writing 4s before 3d in standard electron configurations for transition metals
  - Why it fails: While 4s fills first, it is ordered by principal quantum number in standard notation
  - Correct: Write 3d before 4s in final electron configurations for all period 4 elements
- **Wrong:** Ignoring large jumps in successive ionisation energy data
  - Why it fails: Large jumps indicate the removal of an electron from a new inner shell closer to the nucleus
  - Correct: Use the position of the large jump to identify which group the element belongs to

## Cheatsheet

| Concept / Formula | Key Description |
| --- | --- |
| Relative atomic mass: $A_r = \frac{\sum (\text{isotope mass} \times \text{abundance})}{\sum \text{abundance}}$ | Calculates average atomic mass of an element from its isotopes |
| $n$ = principal quantum number | Defines the main energy level (shell) for electrons |
| Maximum electrons per shell = $2n^2$, 2 electrons per orbital | Rule for the maximum electron capacity of any electron shell or single atomic orbital |
| First ionisation energy definition | Energy required to remove 1 mole of electrons from 1 mole of gaseous atoms |
| Large jump in successive ionisation energies | Indicates an electron is being removed from a new inner electron shell |

## What's next

Begin this unit by learning the foundational structure of the nucleus and isotopes, which sets up all subsequent concepts in this unit. Once you complete all three sub-topics here, you will move on to the next unit on chemical bonding, which builds directly on the electron configuration knowledge you gain here.

- [Atomic nucleus and isotopes](https://www.owlsprep.com/study/cie-9701-u2-atomic-nucleus-and-isotopes/)
- [Electron orbitals and configuration](https://www.owlsprep.com/study/cie-9701-u2-electron-orbitals-and-configuration/)
- [Ionisation energies](https://www.owlsprep.com/study/cie-9701-u2-ionisation-energies/)

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