Shells Define the Period
Match electron shells to the correct period row
What this lesson covers
Why it matters
Imagine an atom as a high-rise building. The period number tells you exactly how many floors are occupied by electrons.
Predict first
Why is Carbon placed in Period 2, not Period 1 or 3?
The period number equals the number of occupied electron shells. Carbon (2,4) has electrons in K and L shells, so it sits in Period 2.
- It has 6 protons in its nucleus
- Its electrons occupy 2 energy shells — correct
- It is a non-metal
- It has 4 valence electrons
What you do
Build magnesium — 12 protons, 12 neutrons and 12 electrons (mass number 24) — one particle at a time. Watch the electrons fill K (2), then L (8), then M (2): three occupied shells put Mg in Period 3.
Check yourself
An element has the electronic configuration 2, 8, 1. Which period does it belong to?
The configuration 2, 8, 1 shows electrons in 3 shells (K, L, M). Therefore, it is in Period 3.
Why do elements in the same period have the same number of electron shells?
By definition, the period number indicates the number of occupied energy levels (shells) in the atom.
Fluorine (F) has atomic number 9. Its configuration is 2, 7. Which statement is true?
Fluorine has electrons in K (2) and L (7) shells. Two occupied shells mean Period 2.
- Period 1
- Period 2
- Period 3 — correct
- Period 11
- They have the same atomic mass
- They have the same number of valence electrons
- They are all metals
- The period number is defined by the number of occupied shells — correct
- It is in Period 7 because it has 7 valence electrons
- It is in Period 9 because its atomic number is 9
- It is in Period 2 because it has 2 occupied shells — correct
- It is in Period 1 because it is a gas