The group number of a main-group element equals its number of outer electrons, and its period number equals the number of occupied shells. This turns an electron arrangement such as 2,8,5 into an address in the table.
It appears in short questions (“state the group of the element with arrangement 2,8,7”) and underpins every trend explanation in periodic patterns.
How does the electron arrangement give the address?
Electrons fill shells from the nucleus outwards: up to 2 in the first, up to 8 in the second, and up to 8 in the third (for the first twenty elements). You write the result as numbers separated by commas.
Read the arrangement like this:
- The last number is the outer electrons, so it is the group number for Groups I to VII.
- The count of numbers is the number of shells, so it is the period.
A full outer shell is Group VIII. Helium (2) is placed there even though its full shell holds two electrons.
Worked example
Find the group and period of the element with atomic number 15, then name it.
Step 1, electrons: an atom is neutral, so 15 protons means 15 electrons.
Step 2, fill the shells: 2 in shell one leaves 13. 8 in shell two leaves 5. The last 5 go in shell three. Arrangement: 2,8,5.
Step 3, read it: the last number is 5, so Group V. There are three numbers, so period 3.
Step 4, name it: period 3, Group V is phosphorus, P.
Now work backwards.
An element is in Group II, period 4. The last number is 2 and there are four numbers: 2,8,8,2.
Add them: 2 + 8 + 8 + 2 = 20. Atomic number 20 is calcium.
What mistake goes wrong here?
Students often take the group from the total number of electrons or the first number in the arrangement.
Mistaken answer: “Chlorine has 17 electrons, so it is in Group 17, and since the arrangement is 2,8,7 it is in period 2.”
Two errors: the group comes from the last number (7), not the total, and the period comes from the count of shells (three), not the first number.
The correction: write the arrangement, circle the last number for the group, count the numbers for the period. Chlorine, 2,8,7, is Group VII, period 3. Some textbooks label the same column Group 17, so use the numbering your course uses, and read the Cambridge syllabus for the form it prefers.
Check yourself
1. An atom has arrangement 2,8,6. Give its group and period.
Show answer
The last number is 6, so Group VI. There are three numbers, so period 3. (Its atomic number is 2 + 8 + 6 = 16, which is sulfur.)
2. Write the arrangement of the element in Group VII, period 2, and give its atomic number.
Show answer
Two shells and 7 outer electrons: 2,7. Atomic number 2 + 7 = 9 (fluorine).
3. Magnesium has atomic number 12. Explain in one sentence why it reacts like calcium.
Show answer
Magnesium is 2,8,2 and calcium is 2,8,8,2, so both have two outer electrons and both lose them to form a 2+ ion.
Where this leads next
Group and period are the tools for the next lesson, comparing reactivity patterns using evidence. You can also practise balancing the simple equations that follow with the equation balance reasoning trainer, and test the whole module with the periodic patterns practice set.
Some students can fill shells correctly but lose the link to reactions under exam pressure. A teacher in online one-to-one Chemistry tuition can watch that happen and fix it at the source.