Lesson focus: Electron arrangements of the first 20 elements reveal periodA horizontal row of the periodic table; elements in the same period have the same number of occupied electron shells. number, outer-shell electron count and recurring chemical patterns.
Deduce first-20 electron configurations and relate them to position and ionAn atom (or group of atoms) that has an electric charge because it has gained or lost electrons. formation.
Build on earlier learning
Groups and periodsYear 8 used groups and periods. This lesson explains those patterns using electron arrangement.
You will be successful when you can…
- Write first-20 configurations.
- Relate shells to periods.
- Relate outer electrons to main groups.
- Predict simple ion charges.
Words that carry the science
- electron shell
- A simplified energy-level region occupied by electrons.
- electronic configuration
- The arrangement of electrons among shells or energy levels.
- period
- A horizontal row in the Periodic Table.
- group
- A vertical column whose main-group elements share outer-shell patterns.
- valence electron
- An electron in the outer occupied shell involved in bonding and reactions.
- ion
- A charged particle formed when electrons are lost or gained.
Nuclear charge determines electron count
01Nuclear charge determines electron count
A neutral atom has one electron per proton. For the first 20 elements, the school shell model fills 2 in the first shell, then 8, then 8 before starting the fourth.
Examples are sodium 2,8,1; chlorine 2,8,7; calcium 2,8,8,2. This is a useful introductory model, not a complete quantum description.
02Position follows occupied and outer shells
The number of occupied shells gives the periodA horizontal row of the periodic table; elements in the same period have the same number of occupied electron shells.. For main-group elements, the number of outer-shell electrons connects to groupA vertical column of the periodic table; elements in a group have the same number of outer-shell electrons, so they react in similar ways. position: Group 1 has one and Group 7 has seven; Group 0 has a full outer shell.
Helium is in Group 0 with two electrons because its first shell is full, despite not having eight.
03Atoms form ions through electron transfer
Metals such as sodium lose outer electrons to form positive ionAn atom (or group of atoms) that has an electric charge because it has gained or lost electrons.; non-metals such as chlorine gain electrons to form negative ions. The nucleus is unchanged.
Na forms Na⁺ with 2,8; Mg forms Mg²⁺ with 2,8; Cl forms Cl⁻ with 2,8,8. A full outer shell is a pattern, not a force or conscious goal.
Electron shells
Electrons fill the shells nearest the nucleus first: up to 2 in the first shell, then 8, then 8. The number of shells gives the period, and the outer electrons give the group.
Controls
Neutron numbers are for the most common isotope. Group 0 elements have full outer shells, so they are unreactive.
Place an element from its configuration
An atom has electronic configurationHow the electrons in an atom are arranged in shells, such as 2,8,1 for sodium. 2,8,3.
- Total electrons = 13, so a neutral atom has atomic number 13: aluminium.
- Three occupied shells place it in Period 3.
- Three outer electrons place this main-group element in Group 3 in the specification convention.
- It can lose three electrons to form Al³⁺ with configuration 2,8.
How to make the reasoning visible: Configuration predicts both table position and likely simple ion charge.
Read patterns in the first 20
Complete the table using atomic number and shell arrangement.
| Element | Configuration | Period | Likely ion |
|---|---|---|---|
| lithium, Z=3 | 2,1 | 2 | Li⁺ |
| oxygen, Z=8 | 2,6 | 2 | O²⁻ |
| potassium, Z=19 | 2,8,8,1 | 4 | K⁺ |
| calcium, Z=20 | 2,8,8,2 | 4 | Ca²⁺ |
Why are lithium and potassium in the same group?
What changes from Na to Na⁺?
Check your understanding
Answer from memory, then check against the suggested answer.
What determines period in the shell model?
What do Group 1 atoms share?
How does a cation form?
An element has configuration 2,8,7. Predict its period, group and ion.
Why is “atoms react to become stable” an incomplete explanation?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Sodium (atomic number 11) and chlorine (atomic number 17) react to form sodium chloride.
Explain how the electronic configurations relate to their positions in the Periodic Table.
(3)Explain how they form ions.
(3)Total for this question: 6 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., connect each configuration feature to one table position or ion outcome.