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1.1.7 Electronic structure

1.1.7 Electronic structure

Electrons occupy shells at set energy levels

Definition

Electron shell

A region around an atomic nucleus occupied by electrons with a particular energy.

  1. The electrons in an atom are arranged in shells, also called energy levels, around the nucleus.
  2. Each shell is a fixed energy level: the shell closest to the nucleus has the lowest energy.
  3. Electrons fill the lowest energy shell first, then the next, and so on.
  4. Each shell can hold only a limited number of electrons.
    1. The first shell holds up to 2 electrons.
    2. The second shell holds up to 8 electrons.
    3. The third shell holds up to 8 electrons at this level.

A diagram of an atom showing the nucleus and three electron shells labeled n=1, n=2, and n=3. An arrow points outwards from the nucleus, indicating that energy increases with distance from the nucleus.

Key Idea
  • Fill shells from the inside out: a shell only starts to fill once the ones below it are full.
  • The number of electrons equals the atomic number for a neutral atom.

Writing an electronic structure

  1. The electronic structure is written as the number of electrons in each shell, separated by commas.
  2. To work it out, place electrons into shells from the inside out until they are all used.
  3. Oxygen has 8 electrons, so its structure is 2,6.
  4. Sodium has 11 electrons, so its structure is 2,8,1.
  5. Chlorine has 17 electrons, so its structure is 2,8,7.
  6. Calcium has 20 electrons, so its structure is 2,8,8,2.
  7. The structure links to the periodic table: the number of shells gives the period, and the outer electrons give the group.
Self review
  • How many electrons fill the first three shells?
  • Write the electronic structure of sodium (11 electrons).
  • Write the electronic structure of chlorine (17 electrons).
  • An atom has the structure 2,8,2. What is its group and period?
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Simplified shell model for the first 20 elements, with shells labelled n=1, n=2, and n=3

Electrons occupy orbitals in subshells. They fill the lowest-energy available subshells first. Shell number describes the main energy level and distance scale from the nucleus, but shell distance does not universally determine orbital energy because subshell energies can cross.

In the simplified shell model used for the first 20 elements, electron arrangements follow the pattern 2,8,8,2: the first shell holds up to 2 electrons, the second holds up to 8, and 8 electrons are placed in the third shell before the fourth shell begins to fill. Although the third shell can hold up to 18 electrons, the 4s4s4s subshell has lower energy than the remaining 3d3d3d subshells, so 4s4s4s fills first. This is why calcium is written as 2,8,8,2 rather than 2,8,10.

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How are electrons arranged in an atom?

1.1.7 Electronic structure Revision Guide

  1. GCSE
  2. /Chemistry
  3. /1.1.7 Electronic structure

Revision notes for AQA GCSE Chemistry 1.1.7 Electronic structure: explanations and worked examples.

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