The key idea
Electrons fill shells starting from the lowest energy level.The first shell holds 2 electrons, the second holds 8, and the third holds 8 at GCSE.The number of outer electrons determines reactivity.
Use the labels to explain the scientific relationship shown.
The bit that matters
Keep the idea tight, then use the worked example to practise the exact exam wording.
Electron shells and energy levels
Electrons occupy shells (energy levels) around the nucleus, starting with the lowest energy level closest to the nucleus.The first shell holds a maximum of 2 electrons, the second and third shells each hold a maximum of 8 at GCSE.Electrons fill the lowest available shell first before filling higher shells.
Writing electronic configurations
To write the electronic configuration, start with the atomic number (= number of electrons in a neutral atom).Fill the first shell with 2, then the second with up to 8, then continue.For example, magnesium (atomic number 12) has the configuration 2, 8, 2.Always check the total electrons equals the atomic number.
Electronic structure and the periodic table
The period number equals the number of electron shells an element has.The group number for main-group elements equals the number of electrons in the outer shell.This links directly to reactivity: atoms with 1 outer electron (Group 1) react by losing it; atoms with 7 outer electrons (Group 7) react by gaining one.
Ion formation from electronic structure
Metals tend to lose electrons to achieve a full outer shell, forming positive ions (cations).Non-metals tend to gain electrons, forming negative ions (anions).The charge on the ion equals the number of electrons lost or gained.For example, oxygen gains 2 electrons (2, 8, 6 → 2, 8) to form O²⁻.
Definitions to learn
Electron shell
An energy level around the nucleus of an atom in which electrons are found.
Electronic configuration
A notation showing the number of electrons in each shell of an atom, written as a series of numbers.
Outer shell
The highest occupied electron shell in an atom, which determines the atom's chemical behaviour.
Cation
A positively charged ion formed when an atom loses one or more electrons.
Anion
A negatively charged ion formed when an atom gains one or more electrons.
Write the electronic structure of a chlorine atom (atomic number 17).
First shell: fill with 2 electrons.
Second shell: fill with 8 electrons. Total so far = 10.
Third shell: remaining = 17 − 10 = 7 electrons.
Chlorine: 2, 8, 7.
Always write electronic structures as a series of numbers separated by commas, and check the total equals the atomic number.
Do not put more than 2 in the first shell or more than 8 in the second and third shells at GCSE level.
How to score full marks
- 1Write electronic configurations as numbers separated by commas (2, 8, 1) and always verify the total equals the atomic number.
- 2The group number = number of outer electrons — use this shortcut to check your configuration quickly.
- 3When explaining ion formation, state both what the atom does (loses/gains electrons) and why (to achieve a full outer shell).
Try these yourself
Open each answer only after you have explained the full chemical process.
1State the maximum number of electrons in the first electron shell.[1 mark]
- 1.Recall the shell capacity.
2Write the electronic structure of a sodium atom (atomic number 11).[1 mark]
- 1.Fill shells in order: 2, then 8, then remainder.
3A calcium atom has atomic number 20. Write its electronic structure.[1 mark]
- 1.Fill shells: 2, 8, 8, then remainder.
4Use electronic structure to explain why sodium and potassium have similar chemical properties.[2 marks]
- 1.Link outer electron number to reactivity.
5Explain why a fluorine atom tends to form a F⁻ ion.[3 marks]
- 1.Count outer electrons and determine what is needed for a full outer shell.
6Explain why noble gases are unreactive in terms of electronic structure.[2 marks]
- 1.Describe the outer shell.
7Describe how to find the number of electrons in the outer shell of any main-group element using its group number.[2 marks]
- 1.Relate group number to outer electrons.
8An ion has the electronic structure 2, 8. It was formed from a metal atom. Identify the original element and explain the charge of the ion.[3 marks]
- 1.Determine the original atom and what changed.