In this lesson we will learn how electrons are arranged around the nuclei of atoms.
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.
(Image: nikolaybarbov, Adobe Stock)
- 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.