Using electronic configuration to find the group and period
The number of shells gives the period. The valence electrons give the group: the same number for groups 1 and 2, and ten more for groups 13 to 18.
How to place an element from its atomic number
This works for the main group elements and noble gases, which is all you need up to Z = 20.
- 1Write the electronic configuration from Z: fill K with 2, L with 8, then M, with at most 8 in the outer shell.
- 2Count the shells. That number is the period.
- 3Count the valence electrons, the last number in the configuration.
- 4If there are 1 or 2, that is the group: group 1 or group 2.
- 5★If there are 3 to 8, add 10: 3 gives group 13, 7 gives group 17, 8 gives group 18.
- 6Helium (2) is the exception: its only shell is full, so it goes in group 18.
★ key step
Examples of placing elements
| Element (Z) | Configuration | Period (shells) | Group (from valence electrons) | Nature |
|---|---|---|---|---|
| Lithium (3) | 2, 1 | 2 | 1 valence electron: group 1 | Metal |
| Nitrogen (7) | 2, 5 | 2 | 5: group 15 | Non-metal |
| Neon (10) | 2, 8 | 2 | 8: group 18 | Noble gas |
| Aluminium (13) | 2, 8, 3 | 3 | 3: group 13 | Metal |
| Sulphur (16) | 2, 8, 6 | 3 | 6: group 16 | Non-metal |
| Calcium (20) | 2, 8, 8, 2 | 4 | 2: group 2 | Metal |
Worked example: from position to configuration and formulas
Element E is in period 3 and group 13. (a) Write its configuration. (b) Is it a metal or a non-metal? (c) Write the formulas of its compounds with fluorine (2, 7) and with sulphur (2, 8, 6).
- (a) Period 3 means three shells. Group 13 means 13 − 10 = 3 valence electrons. So E is 2, 8, 3: aluminium, Z = 13.
- (b) It loses its 3 valence electrons easily, so it is a metal with valency 3.
- (c) Fluorine (2, 7) has valency 8 − 7 = 1. Cross over the valencies, E 3 and F 1: the formula is EF₃, that is AlF₃.
- Sulphur (2, 8, 6) has valency 8 − 6 = 2. Cross over E 3 and S 2: the formula is E₂S₃, that is Al₂S₃.
2, 8, 3; a metal with valency 3; AlF₃ and Al₂S₃
Quick checks that save marks
- The configuration must add up to Z: 2 + 8 + 3 = 13 for aluminium.
- The period comes from the number of shells, not from the electrons in the last shell.
- The same last number means the same group: 2, 7 (F) and 2, 8, 7 (Cl) are both in group 17.
- The same number of shells means the same period: 2, 8, 1 (Na) and 2, 8, 7 (Cl) are both in period 3.
- 1 to 3 valence electrons usually mean a metal (hydrogen and boron are exceptions). 5 to 7 usually mean a non-metal.
Calcium (2, 8, 8, 2) sits in group 2 with magnesium, so the two form similar compounds. Calcium carbonate is the marble of the Taj Mahal; magnesium carbonate is the white chalk gymnasts put on their hands.
1 marks: “In which period is the element with atomic number 19 placed?” Period 4, because 2, 8, 8, 1 has four shells.
2 marks: “An element has the configuration 2, 8, 5. Give its group and period, and say whether it is a metal or a non-metal.” Three shells, so period 3. Five valence electrons, so group 10 + 5 = 15. It is phosphorus, a non-metal.
5 marks: “Elements A, B and C have atomic numbers 9, 12 and 17. (a) Write their configurations. (b) Which two are in the same group, and which two in the same period? (c) Write the formula of the compound of B and C.” (a) A 2, 7; B 2, 8, 2; C 2, 8, 7. (b) A and C are in group 17 (7 valence electrons each); B and C are in period 3 (three shells each). (c) B has valency 2 and C valency 1, so the formula is BC₂ (magnesium chloride, MgCl₂).
Shells set the row. The last number sets the column: as it is for 1 and 2, add 10 after that.