AQA GCSE Chemistry Foundation (8462), Foundation tier · Atomic structure and the periodic table › Atoms, isotopes and electronic structure
Practise Electronic structure. 9 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How electrons are arranged in shells (energy levels), and how to write and draw the electronic structures of the first 20 elements. This is the basis for explaining the periodic table, bonding and reactivity, so it is used across both papers.
Key facts
Shells for the first 20 elements: up to 2, then 8, then 8, then the fourth shell
Innermost shells (lowest energy levels) fill first
Electrons in an atom = atomic number
Number of occupied shells = period number
Outer electrons = group number (Groups 1 to 7)
Examples: O 2,6; Na 2,8,1; Cl 2,8,7; K 2,8,8,1; Ca 2,8,8,2
Group 0: full outer shell (He 2; Ne 2,8; Ar 2,8,8)
Notes
Shells (energy levels)
Electrons are arranged in shells, also called energy levels, around the nucleus.
Electrons occupy the lowest available energy levels: the innermost shells fill first.
For the first 20 elements: the first shell holds up to 2 electrons, the second up to 8 and the third up to 8. Then the fourth shell starts.
Writing and drawing electronic structures
diagram
In an atom, the number of electrons = the atomic number.
Fill the shells in order and write the numbers separated by commas: carbon (6) is 2,4; sodium (11) is 2,8,1; chlorine (17) is 2,8,7; calcium (20) is 2,8,8,2.
In a diagram, draw the nucleus in the centre (often shown by its symbol), a circle for each shell and a dot or cross for each electron on the circles.
Inner shells fill first. Outer shell electrons are shown in red.
Check that the numbers add up to the atomic number.
Links to the periodic table
The number of occupied shells equals the period (row). Sodium, 2,8,1, has 3 occupied shells, so it is in period 3.
For Groups 1 to 7, the number of electrons in the outer shell equals the group number. Sodium has 1 outer electron, so it is in Group 1.
Group 0 elements have a full outer shell: 8 electrons, or 2 for helium.
Elements with the same number of outer electrons are in the same group and have similar chemical properties.
How to answer each type of question
Give the electronic structure of an atom
1 markGrade 4
Find the atomic number: this is the number of electrons.
Fill shells in order: 2, then 8, then 8, then the rest.
Check the numbers add up to the atomic number.
Example. (a) Give the electronic structure of an aluminium atom (atomic number 13). (b) Give the electronic structure of a potassium atom (atomic number 19).
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(a) 2,8,3 (1) (b) 2,8,8,1 (1)
Complete an electron shell diagram
1 markGrade 4
Count the electrons needed (the atomic number).
Put 2 on the inner circle, then fill the next circle.
Draw clear dots or crosses exactly on the circles.
Example. The diagram shows the nucleus of a nitrogen atom (atomic number 7) with two empty circles for its shells.
Complete the diagram to show the electronic structure of the nitrogen atom. Use a dot or cross for each electron.
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2 electrons on the inner shell and 5 electrons on the outer shell (1).
Use electronic structure to find group, period and element
2 to 3 marksGrade 5
Last number = number of outer electrons = group number.
How many numbers there are = number of shells = period.
Add them all up to get the atomic number, then look up the element.
Example. An element has the electronic structure 2,8,6. (a) Which group of the periodic table is it in? (b) Which period is it in? (c) Name the element.
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(a) Group 6, because it has 6 outer electrons (1) (b) Period 3, because it has 3 occupied shells (1) (c) 2 + 8 + 6 = 16, so sulfur (1)
Don’t lose marks
Writing potassium as 2,8,9. The third shell takes 8 at GCSE, then the fourth shell starts: 2,8,8,1.
Using the mass number instead of the atomic number to count electrons.
Putting more than 2 electrons in the first shell.
Drawing electrons in the nucleus, or leaving out the inner shells.
More tips
Memory tricks
Remember calcium, 2,8,8,2: it shows the whole pattern you need for the first 20 elements.
Last number = group; how many numbers = period.
Always check: the numbers add up to the atomic number.
Exam technique
Use the form the question asks for: numbers with commas (2,8,1) or a diagram. 'Draw' or 'complete the diagram' means a diagram.
When you explain trends or similar properties, always mention the electrons in the outer shell.
Use the periodic table on the data sheet to find atomic numbers.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Recall how many electrons each shell holdsFor the first 20 elements: 2 in the first shell, then up to 8 in the second and 8 in the third.
Grade 4
Write electronic structures of the first 20 elementsFill the innermost shells first, e.g. sodium (11 electrons) is 2,8,1.
Grade 4
Draw electron shell diagramsDraw circles for the shells around the nucleus and dots or crosses for the electrons.
Grade 5
Link electronic structure to group and periodOuter electrons = group number (Groups 1 to 7); number of occupied shells = period.
Grade 5
Identify an element from its electronic structureAdd up the electrons to get the atomic number, e.g. 2,8,5 is 15, which is phosphorus.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
An aluminium atom has 13 electrons. Give the electronic structure of an aluminium atom.
2,8,3
Which two of these elements have the same number of electrons in the outer energy level of their atoms?
magnesium and calcium
An atom of Q has three occupied energy levels. It has one electron in its outer energy level. Give the electronic structure and the atomic number of Q.
2,8,1; atomic number 11
The electronic structure of silicon is 2,8,4. Explain why silicon and germanium have similar chemical properties.
Both have 4 electrons in their outer energy level.
Give the electronic structure of an atom that has 17 electrons.
2,8,7
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
The electrons in an atom occupy the lowest available energy levels.
(a) An aluminium atom has 13 electrons. Give the electronic structure of an aluminium atom.[1]
(b) An atom of element Q has the electronic structure 2,8,6. How many electrons are in the outer energy level of an atom of Q?[1]
(c) What is the atomic number of element Q?[1]
(d) Potassium has the atomic number 19. Give the electronic structure of a potassium atom.[1]
(e) A lithium atom has 3 electrons. Explain why the electronic structure of lithium is 2,1.[1]
Show the answer and mark scheme
(a)Answer: 2,8,3
2,8,3
(b)Answer: 6
6
(c)Answer: 16
16
(d)Answer: 2,8,8,1
2,8,8,1
(e)Answer: The first energy level can only hold 2 electrons, so the third electron goes into the second energy level.
the first energy level / shell can hold only 2 electrons (so the third electron is in the second level)
Question 2Medium5 marks
The electronic structures of the first 20 elements follow a pattern.
(a) Give the electronic structure of an atom of each element. carbon (atomic number 6) magnesium (atomic number 12) argon (atomic number 18) calcium (atomic number 20)[4]
(b) Which two of these elements have the same number of electrons in the outer energy level of their atoms?[1]