Physical Science / Unit 3: Atoms and the Periodic Table
3.5 Groups and Periods
Notes
Elements in a group have similar properties because they have the same number of valence electrons.
- Alkali metals (Group 1): very reactive, soft, 1 valence electron.
- Alkaline earth metals (Group 2): reactive, 2 valence electrons.
- Halogens (Group 17): very reactive nonmetals, 7 valence electrons.
- Noble gases (Group 18): unreactive, full outer shell (8, or 2 for helium).
- Transition metals are in Groups 3-12.
- Across a period, properties change from metal to nonmetal.
Practice
Which group contains the least reactive elements?
Their outer energy level is full, so they rarely react.
Why do lithium, sodium and potassium react in similar ways?
Same group means same number of valence electrons.
Fluorine is in Group 17. How many valence electrons does a fluorine atom have?
Group 17 elements have 7 valence electrons.
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