Electronic structure — AQA GCSE Chemistry
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Electronic structure explained
Electronic structure covers 3 specification points, including The electrons in an atom occupy the lowest… and The electronic structure of an atom can be….
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Open a point below for its explanation and revision guidance.
The electrons in an atom occupy the lowest available energy levels (innermost available shells).
Electrons in an atom are arranged in energy levels, often called shells. The lowest available energy level is the one closest to the nucleus and has the lowest energy. Electrons fill this level first because the atom is most stable when its electrons occupy the lowest available energy states. Only when the lowest level is full do electrons occupy the next level outwards, and so on. For example, hydrogen has one electron in the first level, while lithium has two electrons filling the first level and one electron in the second level. This filling order explains why the outer electron of an alkali metal is in a higher energy level and is relatively easily lost.
The electronic structure of an atom can be represented by numbers or by a diagram.
The electronic structure of an atom shows how its electrons are arranged in energy levels or shells. This arrangement can be written as a series of numbers, where each number gives the number of electrons in successive energy levels starting from the innermost. For example, the numbers 2,8,1 show two electrons in the first level, eight in the second and one in the third. Alternatively, a diagram can show a central nucleus with circles or arcs representing energy levels and dots or crosses representing electrons. Both representations carry the same information, so a question may ask for either form. The number representation is compact, while the diagram makes the relative positions of the shells clear.
For example, the electronic structure of sodium is 2,8,1 or showing two electrons in the lowest energy level, eight in the second energy level and one in the third energy level.
Sodium has atomic number 11, so a neutral sodium atom contains 11 electrons. These electrons fill the lowest available energy levels first. The first energy level holds a maximum of two electrons, so it is filled with two. The second energy level holds up to eight electrons in this context, so it is filled with eight. The remaining one electron occupies the third energy level. The electronic structure is therefore written as 2,8,1, meaning two electrons in the lowest energy level, eight in the second energy level and one in the third energy level. A diagram would show a nucleus with two electrons on the first shell, eight on the second and one on the third. The single outer electron explains why sodium is in Group 1 and forms a 1⁺ ion.
Your focus
- Describe that electrons occupy energy levels or shells around the nucleus.
- State that electrons fill the lowest available energy level first.
- Apply the filling order to write or draw the electronic structure of a simple atom.
Show all 9 objectives
- Write the electronic structure of an atom using numbers.
- Draw a diagram showing electrons in energy levels around a nucleus.
- Check that a representation contains the correct total number of electrons.
- Write the electronic structure of sodium as 2,8,1.
- Explain the meaning of each number in the structure 2,8,1.
- Relate the outer electron of sodium to its position in Group 1.
Electronic structure exam tips
Marking Points
- Electrons occupy energy levels, also called shells, around the nucleus.
- The lowest available energy level is the innermost shell, closest to the nucleus.
- Electrons fill the lowest available energy level first before occupying higher levels.
- A level must be full before electrons occupy the next available level outwards.
- The arrangement of electrons in energy levels determines an atom's electronic structure.
- A number representation lists the number of electrons in each energy level in order from the innermost level outwards.
- A diagram representation shows a nucleus with energy levels drawn as circles or arcs around it.
- Electrons in a diagram are shown as dots or crosses placed on the appropriate energy level.
- The total number of electrons in either representation equals the number of protons in the atom.
- Both representations must show the same distribution of electrons between energy levels.
- Sodium has 11 electrons because its atomic number is 11.
- The first energy level contains two electrons and is filled first.
- The second energy level contains eight electrons.
- The third energy level contains the remaining one electron.
- The electronic structure of sodium is written as 2,8,1 or shown as 2, 8 and 1 electrons in successive shells.
Examiner Tips
- 💡Use the phrase lowest available energy level when explaining why the first shell fills first.
- 💡Link the filling order to the position of the element in the periodic table, since the number of outer electrons matches the group for main-group elements.
- 💡When drawing a shell diagram, place the nucleus at the centre and add electrons to the innermost shell before any outer shell.
- 💡Check that the numbers or diagram add up to the atomic number of the element given.
- 💡If asked for a diagram, keep the shells evenly spaced and place electrons clearly on the correct shell.
- 💡If asked for numbers, separate them with commas and do not include spaces that could be misread.
- 💡Use the atomic number to work out the total number of electrons before arranging them.
- 💡For main-group elements, the number of electrons in the outer shell equals the group number.
- 💡When drawing sodium, show one electron alone in the third shell to make the Group 1 behaviour clear.
Common Mistakes
- Thinking electrons fill the outermost shell first: correct this by stating that the innermost, lowest-energy shell fills first.
- Assuming all shells hold the same number of electrons: correct this by noting that the first shell holds a maximum of two electrons and later shells hold more.
- Writing electron arrangements with electrons in a higher level while a lower level is not full: correct this by filling each lower level to its capacity before moving outwards.
- Writing the numbers in the wrong order, such as listing outer-shell electrons first: correct this by always starting with the innermost energy level.
- Drawing a diagram with the wrong number of electrons in a shell: correct this by counting the electrons and checking the total equals the atomic number.
- Using a diagram without a nucleus or without clear energy levels: correct this by including a labelled nucleus and distinct shells.
- Writing sodium as 8,2,1: correct this by filling the innermost level first, giving 2,8,1.
- Placing more than two electrons in the first energy level: correct this by limiting the first level to two electrons.
- Forgetting that sodium has 11 electrons: correct this by using the atomic number to determine the total number of electrons.