Electrons occupy shells at set energy levels

Definition

Electron shell

A region around an atomic nucleus occupied by electrons with a particular energy.

The electrons in an atom are arranged in shells, also called energy levels, around the nucleus.
Each shell is a fixed energy level: the shell closest to the nucleus has the lowest energy.
Electrons fill the lowest energy shell first, then the next, and so on.
Each shell can hold only a limited number of electrons.
1. The first shell holds up to 2 electrons.
2. The second shell holds up to 8 electrons.
3. The third shell holds up to 8 electrons at this level.

Key Idea

Fill shells from the inside out: a shell only starts to fill once the ones below it are full.
The number of electrons equals the atomic number for a neutral atom.

Writing an electronic structure

The electronic structure is written as the number of electrons in each shell, separated by commas.
To work it out, place electrons into shells from the inside out until they are all used.
Oxygen has 8 electrons, so its structure is 2,6.
Sodium has 11 electrons, so its structure is 2,8,1.
Chlorine has 17 electrons, so its structure is 2,8,7.
Calcium has 20 electrons, so its structure is 2,8,8,2.
The structure links to the periodic table: the number of shells gives the period, and the outer electrons give the group.

Self review

How many electrons fill the first three shells?
- Answer: 2, then 8, then 8.
Write the electronic structure of sodium (11 electrons).
- Answer: 2,8,1.
Write the electronic structure of chlorine (17 electrons).
- Answer: 2,8,7.
An atom has the structure 2,8,2. What is its group and period?
- Answer: group 2, period 3.

Diagram of four electron shells with energy increasing away from the nucleus, plus a sodium atom containing 2, 8, and 1 electrons

An electron shell is a region around the nucleus occupied by electrons with a particular energy. Shells are also called energy levels because every shell corresponds to a set electron energy.

The first shell is closest to the nucleus and has the lowest energy. Shell energy increases with distance from the nucleus, so electrons fill shells from the inside out.

For the first 20 elements, the first shell holds up to 2 electrons, the second up to 8, and the third up to 8. A new shell starts to fill only after the lower-energy shells are full.

Which electron shell has the lowest energy?

Electrons occupy shells at set energy levels

Definition

Electron shell

A region around an atomic nucleus occupied by electrons with a particular energy.

The electrons in an atom are arranged in shells, also called energy levels, around the nucleus.
Each shell is a fixed energy level: the shell closest to the nucleus has the lowest energy.
Electrons fill the lowest energy shell first, then the next, and so on.
Each shell can hold only a limited number of electrons.
1. The first shell holds up to 2 electrons.
2. The second shell holds up to 8 electrons.
3. The third shell holds up to 8 electrons at this level.

Key Idea

Fill shells from the inside out: a shell only starts to fill once the ones below it are full.
The number of electrons equals the atomic number for a neutral atom.

Writing an electronic structure

The electronic structure is written as the number of electrons in each shell, separated by commas.
To work it out, place electrons into shells from the inside out until they are all used.
Oxygen has 8 electrons, so its structure is 2,6.
Sodium has 11 electrons, so its structure is 2,8,1.
Chlorine has 17 electrons, so its structure is 2,8,7.
Calcium has 20 electrons, so its structure is 2,8,8,2.
The structure links to the periodic table: the number of shells gives the period, and the outer electrons give the group.

Self review

How many electrons fill the first three shells?
- Answer: 2, then 8, then 8.
Write the electronic structure of sodium (11 electrons).
- Answer: 2,8,1.
Write the electronic structure of chlorine (17 electrons).
- Answer: 2,8,7.
An atom has the structure 2,8,2. What is its group and period?
- Answer: group 2, period 3.

Diagram of four electron shells with energy increasing away from the nucleus, plus a sodium atom containing 2, 8, and 1 electrons

An electron shell is a region around the nucleus occupied by electrons with a particular energy. Shells are also called energy levels because every shell corresponds to a set electron energy.

The first shell is closest to the nucleus and has the lowest energy. Shell energy increases with distance from the nucleus, so electrons fill shells from the inside out.

For the first 20 elements, the first shell holds up to 2 electrons, the second up to 8, and the third up to 8. A new shell starts to fill only after the lower-energy shells are full.

Which electron shell has the lowest energy?

1.1.7 Electronic structure

Electrons occupy shells at set energy levels

Writing an electronic structure

Review 1.1.7 Electronic structure by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

1.1.7 Electronic structure

Electrons occupy shells at set energy levels

Writing an electronic structure

Review 1.1.7 Electronic structure by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

1.1.7 Electronic structure

Electrons occupy shells at set energy levels

Writing an electronic structure

Review 1.1.7 Electronic structure by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

1.1.7 Electronic structure Revision Guide

1.1.7 Electronic structure

Electrons occupy shells at set energy levels

Writing an electronic structure

Review 1.1.7 Electronic structure by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

1.1.7 Electronic structure Revision Guide