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1.8 Electronic Structures

GCSE Chemistry · AQA · Chapter 1: Atomic Structure

A free GCSE Chemistry lesson for AQA (8462) from OBA School: how electrons are arranged in shells, and how electronic structure links to the periodic table. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how electrons fill shells (energy levels), starting with the lowest
  • Write and draw the electronic structures of the first 20 elements
  • Link the number of outer electrons to the group, and the number of shells to the period
  • Write the electronic structures of simple ions

Key words

  • Shell (energy level): A region around the nucleus where electrons are found. Shells closer to the nucleus have lower energy.
  • Electronic structure: The arrangement of electrons in shells, written as numbers, e.g. 2,8,1.
  • Outer shell: The highest occupied shell, furthest from the nucleus.
  • Group: A column of the periodic table. Elements in a group have the same number of outer electrons.
  • Period: A row of the periodic table. Elements in a period have the same number of occupied shells.
  • Noble gas: An element in Group 0, with a full outer shell. Noble gases are very unreactive.

Electrons in shells

Electrons in an atom occupy shells, also called energy levels. The electrons fill the lowest energy level first, which is the shell closest to the nucleus. When a shell is full, the next electron goes into the next shell.

  • 1st; Maximum number of electrons (first 20 elements) 2
  • 2nd; Maximum number of electrons (first 20 elements) 8
  • 3rd; Maximum number of electrons (first 20 elements) 8 (then the 4th shell starts filling, for potassium and calcium)
  • 4th; Maximum number of electrons (first 20 elements) 2 (for calcium, element 20)

Writing electronic structures

To write an electronic structure, find the atomic number (the number of electrons in an atom), then fill the shells in order: 2, then 8, then 8, then the rest.

Worked example: sulfur and calcium

Sulfur, atomic number 16: 2 in the 1st shell, 8 in the 2nd, 16 − 2 − 8 = 6 in the 3rd → 2,8,6
Calcium, atomic number 20: 2, then 8, then 8, leaving 20 − 18 = 2 in the 4th shell → 2,8,8,2

Answer: S: 2,8,6 · Ca: 2,8,8,2

To draw a shell diagram, draw the nucleus with the symbol, a circle for each occupied shell, and the right number of dots or crosses on each circle. Spread the electrons out, and pair them up once there are more than four in a shell.

Electronic structure and the periodic table

  • The number of outer-shell electrons equals the group number for Groups 1 to 7. Chlorine (2,8,7) is in Group 7.
  • The number of occupied shells equals the period number. Chlorine has three shells, so it is in Period 3.
  • Group 0 elements (the noble gases) have a full outer shell: helium has 2 and the others have 8. This is why they are so unreactive.

Key fact: Elements in the same group have similar chemical properties because they have the same number of electrons in their outer shell. The outer electrons are the ones involved in reactions.

For example, each Group 1 metal reacts with chlorine in the same way, so the equations follow the same pattern:

lithium + chlorine → lithium chloride

sodium + chlorine → sodium chloride

potassium + chlorine → potassium chloride

Electronic structures of ions

When an atom forms an ion, it loses or gains electrons from its outer shell. Metals lose electrons and non-metals gain electrons, so that the ion has a full outer shell like a noble gas. (You will use this in Chapter 3.)

  • sodium, Na; Structure 2,8,1; Ion Na⁺; Structure 2,8
  • magnesium, Mg; Structure 2,8,2; Ion Mg²⁺; Structure 2,8
  • oxygen, O; Structure 2,6; Ion O²−; Structure 2,8
  • chlorine, Cl; Structure 2,8,7; Ion Cl−; Structure 2,8,8

Exam tip: For an ion, first work out the number of electrons (protons − charge), then fill the shells as usual. S²− has 16 + 2 = 18 electrons, so its structure is 2,8,8.

Common mistakes

  • Putting more than 2 electrons in the first shell.
  • Writing potassium as 2,8,9 or calcium as 2,8,10. For the first 20 elements the third shell holds 8, then the fourth shell starts.
  • Mixing up group and period: group = outer electrons (columns); period = number of shells (rows).
  • Counting all the electrons to find the group. Only the outer shell counts.
  • Forgetting to add or remove electrons when writing the structure of an ion.

Revision questions

Try these, then check your answers below each one.

Question 1. Write the electronic structure of sodium (atomic number 11), for example 2,4.

Answer: 2,8,1

Working: 11 electrons: 2 in the first shell, 8 in the second and 1 in the third: 2,8,1.

Question 2. What is the maximum number of electrons in the second shell? (Choose from: 2, 6, 8, 18.)

Answer: 8

Working: The second shell can hold up to 8 electrons.

Question 3. True or false? Elements in the same period have the same number of electrons in their outer shell.

Answer: False

Working: False. Elements in the same group have the same number of outer electrons. Elements in the same period have the same number of shells.

Related lessons

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