# Main energy levels

> How many electrons fit on each level


## Objectives

- Name the main energy levels K, L, M, N and their numbers (n = 1 to 4).
- Use $2n^2$ to find the maximum number of electrons in a main energy level.
- Arrange the electrons of atoms up to Z = 20 and find their valence electrons.

## 1. Main energy levels: from zero




## 2. How many fit?

How many electrons can the third level (M) hold at most?

- 9
- 18 ✓
- 16
- 8

The M level (n = 3) holds at most 18 electrons: by the rule $2n^2$, $2 \times 3^2 = 18$.

[ar-EG] مستوى الـ M (n = 3) بيشيل لحد 18 electrons بالكتير: بالقاعدة $2n^2$، $2 \times 3^2 = 18$.
## 3. Levels and the 2n² rule

Main energy levels Imaginary regions around the nucleus in which the electrons move according to their energy. K, L, M, N (n = 1 to 4). The farther from the nucleus, the higher the energy. $N_{\max}$ = maximum number of electrons in level n (K to N): K 2, L 8, M 18, N 32


## 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) K L M N Valence electrons The electrons in the outermost energy level of the atom (for the main-group atoms in this lesson, Z = 1 to 20). Look at Z = 19 and 20 M can hold 18, but in potassium and calcium it has only 8. Their 19th and 20th electrons go to N. Sublevels explain why (next lessons).


## 5. Use the rule

An atom has the arrangement 2, 8, 7. Which statement is right?

- Its M level is full
- Its atomic number is 17, and it has 7 valence electrons ✓
- It has 17 valence electrons
- Its atomic number is 7

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.

[ar-EG] الـ atomic number هو 17 والذرة فيها 7 valence electrons: 2 + 8 + 7 = 17 electrons في المجموع، والمستوى الخارجي (M) فيه 7. والـ M يقدر يشيل لحد 18، فهو مش مليان.
## 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 $2n^2$ electrons: K 2, L 8, M 18, N 32. Valence electrons are in the outermost level: calcium (2, 8, 8, 2) has 2. Main energy levels K to N Maximum number of electrons in a main energy level = $2n^2$ (K to N). You need the level arrangement for the [Ar] core in Ch.1. Can you explain why the L level holds 8 electrons but the M level holds 18?



Chapter: Transition Elements (https://yaani.ai/ar/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/ar/lesson/chem-s3-en.lesson-atom.energy-levels