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:
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Describe how electrons are arranged into electron shells, and explain how these shells are filled.
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Describe how electron shells are labelled, including the valence shell.
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Draw and write the electron configuration for the first 20 elements.
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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.
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.
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.
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.
The electron configurations of the first 20 elements.
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