Electron shells

What this chapter teaches

Electrons fill shells from the inside. For the first 20 elements the shells fill to 2, 8 and 8, then shell 4 starts. The group number is the number of outer electrons, and the period number is the number of shells.

Three questions to ask

There is no one right wording. Listen for the idea.

1. What is the electron arrangement of magnesium, which has 12 electrons?
2. How can you tell the group of an element from its electron arrangement?
3. Why is the period not the same as the group?

After the chapter, the learner can

• Write electron arrangements for the first 20 elements
• Link an element's group and period to its electrons