Introduction
In this article, you will learn how electrons are arranged in an atom and how to write the electronic configuration of the first 20 elements.
By the end of this lesson, you will be able to:
- Understand how electrons are arranged in atoms
- Write the electronic configuration of atoms
What is Electronic Configuration?
The way electrons are arranged in an atom is called its electronic configuration.
Electrons move around the nucleus in electron shells.
- The shell furthest from the nucleus is called the outermost shell or valence shell.
- The electrons in this shell are called valence electrons. These are the electrons involved in chemical bonding.
Energy Levels of Electrons
Electrons are negatively charged, while the nucleus is positively charged (due to protons). This means electrons are attracted to the nucleus.
- Electrons closer to the nucleus experience stronger attraction → lower energy level.
- Electrons further from the nucleus experience weaker attraction → higher energy level.
Analogy: Strict Mother and Wild Kids
Imagine the nucleus is a very strict mother, and the electrons are her children.
- Children closest to their mother (inner shells) are under strong control. They behave well and have lower energy.
- Children further away (outer shells) feel less control. They become wilder and have higher energy.
In the same way, electrons have higher energy levels the further they are from the nucleus.
Rules for Writing Electronic Configuration
(For the first 20 elements)
- Electrons are arranged in shells around the nucleus.
- The first shell can hold a maximum of 2 electrons.
- The second and third shells can hold a maximum of 8 electrons each.
- Electrons always fill the shells starting from the one closest to the nucleus (lowest energy level).
Examples of Electronic Configuration
| Element | Proton Number | Electronic Configuration | Occupied Shells | Valence Electrons |
|---|---|---|---|---|
| Hydrogen | 1 | 1 | 1 | 1 |
| Helium | 2 | 2 | 1 | 2 |
| Lithium | 3 | 2,1 | 2 | 1 |
| Boron | 5 | 2,3 | 2 | 3 |
| Potassium | 19 | 2,8,8,1 | 4 | 1 |
Watch video to learn how to write Electronic Configuration for first 20 elements in The Periodic Table.
Practice Questions
Try these questions. Check your solutions and learn explanations in this video.
Question 1 An atom has a proton number of 11. (a) How many electrons does it have? (b) Write its electronic configuration. (c) How many valence electrons does it have?
Question 2 An atom has a proton number of 17. (a) Write its electronic configuration. (b) How many valence electrons does it have?
Question 3 Which element has the electronic configuration 2,8,8,1?
Question 4 An atom has the electronic configuration 2,8,4. (a) What is its proton number? (b) Which group in the Periodic Table does this element belong to?
Summary
- The way electrons are arranged in an atom is called its electronic configuration.
- The outermost shell is also called the valence shell, and the electrons in this shell are called valence electrons. These electrons are involved in chemical bonding.
- Rules for writing electronic configuration:
- Electrons are arranged in shells around the nucleus.
- The first shell can hold a maximum of 2 electrons.
- The second and third shells can hold a maximum of 8 electrons each (for elements with proton numbers 1 to 20).
- Electrons fill the shells starting from the one closest to the nucleus (lowest energy level).
