The number of shells gives the period, and the number of outer electrons gives the group. The two rules are separate, and keeping them apart solves most placement questions.
This lesson is part of the periodic table section. It feeds directly into explaining trends using atomic structure.
What do the rules say?
For the first twenty elements, write the electron arrangement by filling shells 2, 8, 8, 2 in order. Then apply two rules.
- Period = number of shells containing electrons.
- Group = 1 or 2 if the outer shell has 1 or 2 electrons. For 3 to 8 outer electrons, group = 10 + that number.
Helium, with 2, is placed in group 18 because its shell is full.
Worked example: two elements
Element X has proton number 13. The electron arrangement is 2.8.3. That is three shells, so period 3, and three outer electrons, so group 10 + 3 = group 13.
Element Y has proton number 4. The arrangement is 2.2. That is two shells, so period 2, and two outer electrons, so group 2.
Working from the proton number each time means you do not need to memorise positions. Y is beryllium and X is aluminium, but you can name the position without knowing that.
The mistake that costs marks
The common slip is to read the group from the number of shells, or to read the period from the outer electrons. Another is to add 10 only half the time.
| Element | Wrong reading | Correct reading |
|---|---|---|
| 2.8.3 | Group 3, period 3 | Group 13, period 3 |
| 2.8.1 | Group 3 (number of shells) | Group 1, period 3 |
| 2.2 | Period 2 group 10 + 2 = 12 | Group 2, period 2 |
Only the groups with 3 to 8 outer electrons need the extra 10. For groups 1 and 2 the outer electrons equal the group directly. You can test more elements with the atomic structure and periodic position practice tool.
Check yourself
An atom has proton number 19. Write its electron arrangement and state its group and period.
Answer
There are 19 electrons. Fill 2, then 8, then 8, leaving 1: 2.8.8.1.
Four shells means period 4. One outer electron means group 1. This element is potassium.
What to study next
The next lesson uses these positions to explain reactivity, in explaining trends using atomic structure. You can also jump to the periodic table practice set.
For a teacher to check your placement on unseen elements, see online one-to-one Chemistry tuition.