Electronic structure — AQA GCSE Combined Science
Test yourself on Electronic structure with AQA GCSE practice questions.
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Electronic structure explained
Electrons in an atom fill the lowest available energy levels first, so the innermost shell fills before the next shell begins.
Read the full explanation
The first shell holds up to 2 electrons, the second up to 8, and the third up to 8 for the first 20 elements. Electronic structure can be written as numbers, such as sodium 2,8,1, or drawn as a diagram with a nucleus and shells carrying dots or crosses. Sodium has 11 electrons: 2 in the first shell, 8 in the second and 1 in the third. The outer-shell electron explains sodium's position in Group 1 and its tendency to form a 1⁺ ion.
showing two electrons in the lowest energy level, eight in the second energy level and one in the third energy level.
Electrons occupy energy levels (shells) around the nucleus, filling the lowest available level first. For the first 20 elements, the pattern is up to 2 electrons in the first level, 8 in the second, 8 in the third, and up to 2 in the fourth. Sodium (atomic number 11) therefore has 2, 8, 1. You show this as a dot-and-cross diagram with a small circle for the nucleus, labelled with the symbol, and arcs for each level carrying the correct number of electrons. The third level is the outer shell for sodium, so it has one outer electron, which explains its position in Group 1 and its tendency to form a 1⁺ ion.
Your focus
- Write the electronic structure of an atom using numbers or a shell diagram.
- Explain why electrons fill the lowest available energy levels first.
- Deduce an element's group and likely ion charge from its outer-shell electron count.
Show all 6 objectives
- Draw a labelled energy-level diagram for an atom with up to 20 electrons.
- State the electron arrangement of an element from its atomic number.
- Relate the number of outer-shell electrons to the element's group.
Electronic structure exam tips
Marking Points
- State that electrons occupy the lowest available energy levels, filling inner shells before outer shells.
- Apply shell capacities: 2 in the first shell, 8 in the second and 8 in the third for the first 20 elements.
- Represent electronic structure as numbers, such as 2,8,1 for sodium, or as a shell diagram.
- Relate the number of outer-shell electrons to the element's group and its ion charge, for example sodium forming Na⁺.
- Check that the total number of electrons equals the atomic number.
- State that electrons are arranged in energy levels (shells) around the nucleus, with the lowest energy level filled first.
- Give the correct occupancy for sodium as 2 electrons in the first level, 8 in the second and 1 in the third.
- Draw a diagram with a central nucleus labelled with the symbol and separate arcs or circles for each energy level.
- Place the correct number of electrons on each arc, using dots and/or crosses consistently.
- Identify the outer-shell electron and link it to sodium's Group 1 position.
Examiner Tips
- 💡Count the electrons first from the atomic number, then allocate them shell by shell from the inside out.
- 💡When drawing a shell diagram, place the nucleus label centrally and use consistent symbols for electrons.
- 💡Use the outer-shell number to predict the group and ion charge, which links structure to periodic table patterns.
- 💡Count the total electrons on your diagram and check it equals the atomic number (11 for sodium).
- 💡Remember the filling order for the first 20 elements is 2, 8, 8, 2.
Common Mistakes
- Filling the second shell with more than 8 electrons before starting the third. Correction: the second shell holds a maximum of 8 for these elements.
- Writing the electronic structure of sodium as 2,8,1 but drawing 11 electrons in the outer shell. Correction: the diagram must match the numerical structure.
- Confusing mass number with electron number. Correction: in a neutral atom, electrons equal the atomic number, not the mass number.
- Assuming the third shell can hold all remaining electrons for the first 20 elements; correction: the third shell holds up to 8 before the fourth shell begins filling.
- Putting 9 electrons in the second level for sodium; correction: the second level holds a maximum of 8.
- Drawing the nucleus without a label; correction: label the centre with the symbol Na.