Periodic position, outer electrons and ion predictions
Read periods and groups from atomic number and shell arrangement, then make limited, justified predictions.
Section 8.1 · Arrangement of elements
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Roman group labels follow this syllabus; modern tables may label the same main groups 1, 2 and 13–18. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Use the shell model for the first twenty elements.
- Distinguish atomic number, mass number and ionic charge.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which quantity identifies an element's position by atomic number?
What you will learn
- Describe arrangement by increasing atomic number, periods and groups.
- Use outer electrons and position to explain similarities and predict common ions and metallic character.
Use atomic number to place the element
The Periodic Table arranges elements in increasing proton number, also called atomic number. A period is a horizontal row and a group a vertical column. In the first-twenty-elements shell model, the number of occupied shells gives the period. Magnesium has atomic number 12 and arrangement 2,8,2, so it is in period 3; adding neutrons to make an isotope does not move it to another element's position.
For the main groups in this syllabus notation, similar outer-electron arrangements help explain similar chemical behaviour. Group I examples have one outer electron, Group II two and Group VII seven. Across a period, character generally changes from metallic on the left to non-metallic on the right; period 3 progresses from sodium, magnesium and aluminium towards non-metallic elements, with silicon showing an intermediate metalloid character. This trend is not a statement that all neighbours have identical properties.
Predict common ions without turning the pattern into a universal rule
Group I metals commonly lose one electron to form +1 ions; Group II metals lose two to form +2 ions; aluminium in Group III forms Al³⁺. Group VI examples such as oxygen can gain two to form −2 ions and Group VII halogens gain one to form −1 ions. Losing or gaining electrons changes charge but not proton number. A magnesium ion has fewer electrons than its atom, not fewer protons.
Use the outer-electron pattern and group information together to justify a prediction. Do not assign every Group IV element a simple +4 or −4 ion, or apply one main-group charge to every transition element. Group VIII noble gases have full outer shells and do not normally form the simple ions of these examples; helium's full first shell contains two electrons. Some modern tables call the halogens Group 17 and noble gases Group 18, so match the table convention rather than counting seventeen outer electrons.
Pause and explain
Why do sodium and potassium show similar chemical behaviour?
Worked example
An atom has atomic number 17 and shell arrangement 2,8,7. State its period, syllabus group and common ion, with reasons.
Show the worked solution
- Three occupied shells place it in period 3.
- Seven outer electrons place chlorine in syllabus Group VII, corresponding to modern Group 17.
- Gaining one electron completes the outer shell and gives Cl⁻; proton number stays 17.
Answer Period 3, Group VII; Cl⁻ forms by gaining one electron.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Use 2,8,2 to give magnesium's period, group and ion electron arrangement.
Give me a hint
Count occupied shells first, then outer electrons.
Compare my reasoning
- Three shells mean period 3; two outer electrons mean syllabus Group II.
- Magnesium loses two electrons to form Mg²⁺.
- The ion has arrangement 2,8; its twelve protons remain unchanged.
Period 3, Group II; Mg²⁺ has arrangement 2,8.
Look for these in your work
- Period and group use different counts.
- Electrons change while proton identity is conserved.
Compare common ion charges for atoms with arrangements 2,8,1 and 2,8,7.
Give me a hint
Complete the outer shell by the appropriate loss or gain.
Compare my reasoning
- The Group I atom loses one electron and forms +1.
- The Group VII atom gains one electron and forms −1.
- Both reach 2,8 or 2,8,8 respectively; equal charge magnitude does not mean equal electron transfer direction.
2,8,1 gives +1 by loss; 2,8,7 gives −1 by gain.
Look for these in your work
- Each charge sign follows the transfer direction.
- The original atom arrangements are kept distinct.
An isotope of magnesium has more neutrons than another magnesium isotope. A student moves it to a new group and predicts a different simple-ion charge. Correct the reasoning.
Give me a hint
Group behaviour follows proton identity and outer electrons, not changing isotope neutrons.
Compare my reasoning
- Both isotopes have magnesium's atomic number 12.
- Their neutral atoms have the same 2,8,2 electron arrangement in this model.
- Both remain in period 3, Group II and commonly form Mg²⁺; extra neutrons change isotope mass instead.
No group or common-charge change: both are magnesium in period 3, Group II, forming Mg²⁺.
Look for these in your work
- Isotope mass is separated from periodic identity.
- The ion prediction uses the unchanged outer-electron arrangement.
Common mistakes
- Ordering the table by isotope mass number rather than proton number.
- Swapping horizontal periods and vertical groups.
- Removing protons when forming a positive ion.
- Using modern Group 17 as a count of seventeen outer electrons.
What can you explain now?
An atom has arrangement 2,8,2. State its period and common simple-ion charge.
Compare with the explanation
Period 3; +2 ion.
Three occupied shells give period 3, and losing the two outer electrons gives the common Group II ion.
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Make it stick
Explain a main-group position from shell arrangement, then predict a common ion with its transfer direction. Give a limitation of a universal group-charge rule.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.