Learning Objective

In this lesson we will learn how electrons are arranged around the nuclei of atoms.

Learning Outcomes

By the end of this lesson you will be able to:

  • Describe how electrons are arranged into electron shells, and explain how these shells are filled.

  • Describe how electron shells are labelled, including the valence shell.

  • Draw and write the electron configuration for the first 20 elements.

  • Describe the relationship between an element’s group number and the number of valence electrons in its atoms.

 

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Introduction

  • Electrons in an atom surround the nucleus in defined regions called electron shells.
  • There are ‘rules’ which determine how these shells are filled, and how many electrons they can contain.
  • The arrangement of electrons in shells around the nucleus of an atom is referred to as the electron configuration.

 

Electron Shell-filling Rules

  • The shells closer to the nucleus are smaller and can hold less electrons.
  • The inner shells begin to fill up with electrons before the outer shells.
  • Electrons in inner shells have less energy than electrons in outer shells.

 

First Shell

  • This first electron shell (closest to the nucleus) is the first shell to begin filling with electrons.
  • The first shell can hold a maximum of 2 electrons.
  • Once the first electron shell is full (contains 2 electrons), the second shell begins to fill.
  • Examples:
    Hydrogen atoms contain 1 electron:
    • 1 electron in the first shell.
    Helium atoms contain 2 electrons:
    • 2 electrons in the first shell.
    Lithium atoms contain 3 electrons:
    • 2 electrons in the first shell.
    • 1 electron in the third shell.

 

 first electron shell configuration hydrogen  first electron shell configuration helium  first electron shell configuration lithium

The first electron shell can hold a maximum of 2 electrons.

 

Second Shell

  • Once the first shell contains 2 electrons, the second shell begins to fill.
  • The second shell can hold a maximum of 8 electrons.
  • Once the second electron shell is full (contains 8 electrons), the third shell begins to fill.
  • Examples:
    Fluorine atoms contain 9 electrons:
    • 2 electrons in the first shell.
    • 7 electrons in the second shell.
    Neon atoms contain 10 electrons:
    • 2 electrons in the first shell.
    • 8 electrons in the second shell.
    Sodium atoms contain 11 electrons:
    • 2 electrons in the first shell.
    • 8 electrons in the second shell.
    • 1 electron in the third shell.

 

 second electron shell configuration fluorine  second electron shell configuration neon  second electron shell configuration sodium

The second electron shell can hold a maximum of 8 electrons.

 

Third Shell

  • Once the second shell contains 8 electrons, the third shell begins to fill.
  • The third shell can hold a maximum of 18 electrons.
  • However, when the third shell contains 8 electrons, the fourth shell begins to fill.
    That is, the fourth shell begins to fill before the third shell is full.
    For the first 20 elements, the most electrons the third shell will contain is 8.
  • Examples:
    Chlorine atoms contain 17 electrons:
    • 2 electrons in the first shell.
    • 8 electrons in the second shell.
    • 7 electrons in the third shell.
    Argon atoms contain 18 electrons:
    • 2 electrons in the first shell.
    • 8 electrons in the second shell.
    • 8 electrons in the third shell.
    Potassium atoms contain 19 electrons:
    • 2 electrons in the first shell.
    • 8 electrons in the second shell.
    • 8 electrons in the third shell.
    • 1 electron in the fourth shell.

 

 third electron shell configuration chlorine  third electron shell configuration argon  third electron shell configuration potassium

The third electron shell can hold a maximum of 18 electrons, but once it has 8, the fourth shell begins to fill.

 

Fourth Shell

  • Once the third shell contains 8 electrons, the fourth shell begins to fill.
  • The fourth shell can hold a maximum of 32 electrons.
    For the first 20 elements, the most electrons the fourth shell will contain is 2.
  • Similar to the third shell, when the fourth shell contains 8 electrons, the fifth shell begins to fill.
    That is, the fifth shell begins to fill before the fourth shell is full.

 

The Valence Shell and Valence Electrons

  • The outermost electron shell of an atom is known as the valence shell.
    Depending on the atom, the valence shell can be any shell number.
    For example, the valence shell for hydrogen is the first shell, the valence shell for sodium is the second shell, the valence shell for potassium is the third shell.
  • Electrons in the valence shell are known as valence electrons.
  • A valence shell can hold a maximum of 8 valence electrons.
    The only exception is when the valence shell is the first (only) shell; in this case the valence shell can hold a maximum of 2 electrons.
  • Having a maximum of 8 valence electrons explains why, from the third shell onwards, the next shell begins to fill before the current shell is full.
  • The number of valence electrons largely determines the chemical properties of an element.
  • For main group elements, all atoms within the same group have the same number of valence electrons.

 

Representing Electron Configuration

  • Instead of drawing the electron configuration of an atom each time, it can be written in an abbreviated form.
    This is simply the numbers of electrons in each shell, separated by a comma.
  • Example:
    A silicon atom has 14 electrons.
    There are 2 electrons in the first shell, 8 electrons in the second shell and 4 electrons in the third shell.
    The electron configuration of silicon can therefore can be written as 2,8,4.

 

Summary

  • Electrons in an atom are located in defined regions called electron shells, which surround the nucleus.
  • This arrangement of electrons is referred to as the electron configuration.
  • There are ‘rules’ which determine how electron shells are filled, and how many electrons they can contain.
    • Inner shells begin filling first; they are smaller and can hold less electrons.
    • A maximum of 2 electrons can occupy the first shell.
    • A maximum of 8 electrons can occupy the second shell.
    • A maximum of 18 electrons can occupy the third shell, but the fourth shell will begin to fill once the third shell contains 8 electrons.
    • A maximum of 8 electrons can occupy the valence shell (outermost shell) of any atom, unless the valence shell is the only shell, in which case there can be a maximum of 2 electrons.
  • The electron configuration of an atom can be written as the numbers of electrons in each shell, separated by a comma.

 

 electron configuration hydrogen  electron configuration helium  electron configuration lithium  electron configuration beryllium  electron configuration boron  electron configuration carbon  electron configuration nitrogen  electron configuration oxygen  electron configuration fluorine  electron configuration neon  electron configuration sodium  electron configuration magnesium  electron configuration aluminium  electron configuration silicon  electron configuration phosphorus  electron configuration sulfur  electron-configuration-chlorine  electron configuration argon  electron configuration potassium  electron configuration calcium

The electron configurations of the first 20 elements.

 

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