Main energy levels
How many electrons fit on each level
1. Main energy levels: from zero
2. How many fit?
Calcium's 20 electrons are arranged in 4 energy levels around the nucleus. How many electrons fit in each level?
The first level (K) holds 2. The second (L) holds 8. Predict:
Pick the right answer
How many electrons can the third level (M) hold at most?
- A. 8
- B. 18
- C. 16
- D. 9
الشرح
The M level (n = 3) holds at most 18 electrons: by the rule 2n², 2 × 3² = 18.
3. Levels and the 2n² rule
Main energy levels
Imaginary regions around the nucleus in which the electrons move according to their energy.
N_ = 2n²
4. Arrange the electrons
Choose an atomic number from 1 to 20. In a neutral atom, the number of electrons = Z. See how they are arranged.
Atomic number (Z) 11
K 2
L 8
M 1
N 0
Valence electrons
The electrons in the outermost energy level of the atom (for the main-group atoms in this lesson, Z = 1 to 20).
5. Use the rule
Fill in the blanks
Aluminum (Z = 13): K _____, L _____, M _____. It has _____ valence electrons.
الشرح
2 + 8 + 3 = 13. The outermost level (M) has 3 valence electrons.
Pick the right answer
An atom has the arrangement 2, 8, 7. Which statement is right?
- A. Its atomic number is 17, and it has 7 valence electrons
- B. Its atomic number is 7
- C. It has 17 valence electrons
- D. Its M level is full
الشرح
The atomic number is 17 and the atom has 7 valence electrons: 2 + 8 + 7 = 17 electrons in all, and the outermost level (M) holds 7. M can hold up to 18, so it is not full.
6. Summary
- Main energy levels: K, L, M, N (n = 1 to 4).
- Back to the hook: from K to N, level n holds at most 2n² electrons: K 2, L 8, M 18, N 32.
- Valence electrons are in the outermost level: calcium (2, 8, 8, 2) has 2.
| Level | n | Maximum electrons (2n²) |
|---|---|---|
| K | 1 | 2 |
| L | 2 | 8 |
| M | 3 | 18 |
| N | 4 | 32 |
Can you explain why the L level holds 8 electrons but the M level holds 18?