Electron configuration is the arrangement of electrons in electron shells of an atom.

What are electron shells?

Electrons orbit the nucleus in different energy levels in an atom. The energy levels that the electrons move around in are called electron shells.

Each electron shell can contain different number of electrons.

• The first shell can hold up to 2 electrons.
• The second shell can hold up to 8 electrons.
• The third shell can hold up to 8 electrons.

Rules for Determining Electron Configuration

1. Electrons always occupy the electron shell that is closest to the nucleus first.
2. When an electron shell contains the maximum number of electrons that it can hold, the next electron will be placed in the next electron shell.

For IGCSE Chemistry, you should be able to determine the electron configuration of the first 20 elements! 

👇 Let's take a look at some examples!! 👇 

Electron Configuration of Hydrogen (H)

• The atomic number of hydrogen is 1, so a hydrogen atom contains 1 electron.
• The electron is placed in the first electron shell.
• Electron configuration: (1)

Electron Configuration of Lithium (Li) 

• The atomic number of lithium is 3, so a lithium atom contains 3 electrons.
• First, 2 electrons are placed in the first electron shell, which can hold up to 2 electrons.
• There is 1 electron left, which is placed in the second electron shell.
• Electron configuration: (2, 1)

Electron Configuration of Chlorine (Cl)

• The atomic number of chlorine is 17, so a chlorine atom contains 17 electrons.
• First, 2 electrons are placed in the first electron shell.
• There are 15 electrons left. 8 electrons are placed in the second electron shell.
• Now, there are 7 electrons left. These 7 electrons are placed in the third electron shell.
• Electron configuration: (2, 8, 7)

Electron Configuration of Calcium (Ca)

• The atomic number of calcium is 20, so a calcium atom contains 20 electrons.
• First, 2 electrons are placed in the first electron shell.
• There are 18 electrons left. 8 electrons are placed in the second shell.
• There are 10 electrons left. 8 electrons are placed in the third shell.
• There are 2 electrons left. 2 electrons are placed in the fourth shell.
• Electron configuration: (2, 8, 8, 2)

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