UNIT 2 CHEMISTRY · PEARSON AAQ · A1.1.3
Group, Period and Blocks
FREE CHEMISTRY REVISION · TOPIC 3
A position that explains the electrons.
Connect groups, periods and blocks to electron arrangements, then apply the same ideas to atoms and ions.
New Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · A1.1.3
What you will learn
- Identify groups, periods and the s, p and d blocks.
- Use a neutral atom’s electron arrangement to identify its periodic-table position.
- Use group and period to predict simple electron arrangements and common ions.
- Reconstruct a neutral atom from an ion’s electron count and charge.
Mapped to Pearson AAQ Unit 2 A1.1.3, Issue 7 (May 2026), printed page 36. Supplied configurations are used here to interpret position. Detailed orbital-filling rules, exceptions and electron-in-box diagrams belong to A1.2.
1. Read groups and periods
The periodic table is arranged by increasing atomic number. A group is a vertical column; a period is a horizontal row. Position is assigned to the element, using its neutral ground-state atom, even when the element forms ions.
Group: outer electrons
For main-group elements, the number of outer-shell electrons follows a clear pattern: Groups 1 and 2 have one and two; Groups 13–18 have three to eight. Similar outer arrangements help explain similar chemical behaviour.
Period: highest occupied shell
The period is the highest occupied principal energy level, n, in the neutral ground-state atom. For simple shell arrangements, count the occupied shells: 2,8,7 has three, so chlorine is in Period 3.
Numbering: this page uses modern Groups 1–18. Older school tables call Groups 13–17 Groups 3–7 (or III–VII), and Group 18 Group 0. Read the headings on the table provided. Modern Group 3 is in the d block; it is not the same as older main-group “Group 3”.
Helium is an exception: its arrangement is 2 (1s²), so its outer shell is full with only two electrons. It is in Group 18, Period 1, not Group 2.
2. Locate the s, p and d blocks
A block identifies the type of subshell being filled across that part of the table. In supplied configurations, a superscript counts electrons in a subshell: 3p⁵ means five electrons in 3p. [Ne] and [Ar] stand for the electron configurations of neon and argon.
| Block | Location | Pattern to recognise |
|---|---|---|
| s | Groups 1–2, plus helium | Outer ns¹ or ns²; helium is 1s². |
| p | Groups 13–18, excluding helium | Outer ns²np¹ to ns²np⁶. |
| d | Groups 3–12, central region | The (n − 1)d subshell is filled across the series; the first series is in Period 4. |
Do not identify every block just from the outermost shell. Scandium has supplied configuration [Ar] 3d¹4s²: it is a d-block element in Period 4, although its highest occupied shell contains 4s electrons. Group 12 belongs to the d block; “d-block element” and the strict chemical definition of “transition element” are not identical.
3. Work from arrangement to position
Worked example 1 · Aluminium: 2,8,3
- Add the electrons: 2 + 8 + 3 = 13. The atom is neutral, so Z = 13: aluminium.
- Three occupied shells give Period 3.
- Three outer electrons give Group 13 (older Group 3).
- The outer configuration is 3s²3p¹, so aluminium is in the p block.
Worked example 2 · Calcium: [Ar] 4s²
- [Ar] represents 18 electrons; add two: 20 electrons, so the neutral atom is calcium.
- The highest occupied shell is n = 4: Period 4.
- The two outer 4s electrons give Group 2, s block.
- Its shell arrangement is 2,8,8,2.
Worked example 3 · Scandium: [Ar] 3d¹4s²
- Count 18 + 1 + 2 = 21 electrons: neutral scandium.
- The highest occupied n is 4, so it is in Period 4, not Period 3.
- It is at the start of the first d-block series: d block, Group 3.
Use the table or supplied configuration for d-block examples. The main-group rule “group number equals outer-shell electrons” does not apply to modern Groups 3–12.
For a neutral main-group atom: identify the occupied shells → count outer electrons → assign the modern group → identify s or p block.
4. Work from position to atoms and ions
For the first 20 elements, simple shell arrangements are useful. Do not extend a “2,8,8” rule indiscriminately through the d block: shells can contain more than eight electrons, and subshells matter.
Worked example 4 · Period 3, Group 2
The element is magnesium. A neutral atom has three occupied shells and two outer electrons: 2,8,2, or 12 electrons. Its common Mg²⁺ ion forms by losing the two outer electrons:
Mg: 2,8,2 → Mg²⁺: 2,8 + 2e⁻
Mg²⁺ has 10 electrons. Magnesium remains in Group 2, Period 3 and the s block. The ion has only two occupied shells, but that does not reassign the element’s period.
Worked example 5 · Period 3, Group 17
The element is chlorine: three occupied shells and seven outer electrons, 2,8,7. Its common Cl⁻ ion gains one electron:
Cl: 2,8,7 + e⁻ → Cl⁻: 2,8,8
Cl⁻ has 18 electrons. Chlorine remains a Group 17, Period 3, p-block element. A full outer shell in an ion does not make that element a noble gas.
Useful common monatomic charges: Groups 1 and 2 often form 1+ and 2+ ions; Groups 16 and 17 often form 2− and 1− ions. Aluminium commonly forms Al³⁺. These are patterns for suitable elements, not a rule that every main-group element forms a simple ion. Many p-block elements form covalent bonds, and d-block ions often have variable charges.
5. Reconstruct an element from its ion
Electron count: n+ ion → Z − n; n− ion → Z + n.
To recover Z: add n to a positive ion’s electron count; subtract n from a negative ion’s electron count.
Worked example 6 · X²⁺ has 18 electrons
- The atom lost two electrons, so the neutral atom has 18 + 2 = 20 electrons.
- Z = 20 identifies calcium, with neutral arrangement 2,8,8,2.
- Therefore X is in Group 2, Period 4, s block. Ca²⁺ has arrangement 2,8,8.
Worked example 7 · Y⁻ has 18 electrons
- The ion gained one electron, so the neutral atom has 18 − 1 = 17 electrons.
- Z = 17 identifies chlorine: 2,8,7.
- Therefore Y is in Group 17, Period 3, p block.
Ca²⁺, Cl⁻ and Ar all have 18 electrons: they are isoelectronic. Their proton numbers differ, so they remain different elements in different positions. An ion’s electron arrangement alone does not identify its element unless you also know its charge or proton number.
6. Common misconceptions and exam tips
- “Group 17 means 17 outer electrons.” It means seven for a main-group neutral atom. Modern group numbering includes the d block.
- “A 2+ ion has gained two electrons.” It has lost two. Check charge using protons − electrons.
- “Mg²⁺ is in Period 2.” Use neutral magnesium to assign its period: Period 3.
- “Helium is p block because it is in Group 18.” Its 1s² configuration places it in the s block.
- “3d means Period 3.” The first d-block series is Period 4; check the highest occupied n in the neutral atom.
- “Isoelectronic ions are the same element.” Equal electron counts do not imply equal proton counts.
Exam method: state whether the arrangement describes an atom or ion. Recover the neutral electron count when needed, identify Z, then give the group numbering used, period and block. Keep detailed orbital-filling and electron-in-box work for A1.2.
7. Check your understanding
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Your revision summary
- Groups are columns; periods are rows.
- Use the neutral ground-state atom to assign position.
- Main-group outer electrons follow Groups 1, 2 and 13–18; helium is the two-electron exception in Group 18.
- s block: left two columns plus helium; p block: right six columns except helium; d block: centre.
- Ions gain or lose electrons, while the element’s proton number and periodic-table position stay the same.
Without notes, explain why Ca²⁺ and Cl⁻ can have the same electron arrangement while calcium and chlorine occupy different groups and periods.