Physical Science / Unit 3: Atoms and the Periodic Table
3.6 Electrons and Energy Levels
Notes
Electrons occupy energy levels (shells) around the nucleus.
- The first level holds up to 2 electrons, the second holds up to 8, and the third holds up to 8 in the elements you will see first.
- Valence electrons are those in the outermost level. They determine how an atom bonds.
- Example: Oxygen (8 electrons) has 2 in level 1 and 6 in level 2, so 6 valence electrons.
- The period number equals the number of occupied energy levels for the main-group elements.
Practice
How many electrons can the first energy level hold?
The first level holds a maximum of 2.
A neutral magnesium atom has 12 electrons. How many valence electrons does it have?
Arrangement is 2, 8, 2, so 2 valence electrons.
An atom of an element in Period 3 has electrons in how many energy levels?
The period number equals the number of occupied levels.
Ready for more? Take the Unit 3 test.