O Levels Chemistry Notes - Atomic Structure

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What Are the Different Parts of an Atom?

Subatomic particle




Relative mass




Relative charge



0 (neutral)



Electron shell




Key Points

Atomic Number

●      Total number of protons in the nucleus of an atom

Mass Number

●      Total number of protons and neutrons in the nucleus

Electron Arrangement

●      Electrons fill lower energy shells before higher energy shells

●      Each energy level has a maximum number of electrons.

Valence Shell and Electrons

●      The outermost electron shell; electrons in the valence shell are called valence electrons.

●      Used to determine whether an element is a metal or nonmetal.

●      Metals: 1, 2, 3 valence electrons.

●      Non-metals: 4, 5, 6, 7, 8 valence electrons.

Stable Electronic Configuration

●      When an atom has a full valence shell, it is stable and unreactive.

●      Noble gases have stable electronic configurations and are inert.

●      A maximum of 2 electrons at n=1 (duplet electronic configuration).

●      A maximum of 8 electrons at n=2 (octet electronic configuration).


●      A charged particle formed when an atom loses or gains electrons.

●      Proton and nucleon numbers remain unchanged.

●      Cation: Positively charged ion formed when an atom loses electrons (indicated by ‘+’).

●      Anion: Negatively charged ion formed when an atom gains electrons (indicated by ‘-‘).


●      Atoms of the same element with the same number of protons but different numbers of neutrons.

●      Have similar chemical properties but slightly different physical properties due to varying mass.

What are some common pitfalls?

1.  Confusion about Atomic Number and Mass Number
Pitfall: Students may mix up the concepts of atomic number (number of protons) and mass number (sum of protons and neutrons).

Elaboration: Clearly explain the definitions of atomic number and mass number. Use visual aids and examples to distinguish between the two. Emphasise that the atomic number uniquely identifies an element.

Atomic proton is also be known as proton number while atomic mass is also known as nucleon number.

Atomic number = number of protons

Mass number = number of protons + number of neutrons


2.  Misunderstanding Electron Configuration:
Pitfall: Students may struggle with writing electron configurations using the proper notation (e.g., 2,8,1 for sodium).

Elaboration: Electrons fill in the electron shell with the lowest energy completely first.

Example: Sodium has 11 electrons. Fill the first electron shell (n=1), which has a maximum electron count of 2. There are then 9 electrons left. Fill the next electron shell (n=2), which has a maximum electron count of 8. There are then 1 electron left. Fill the next electron shell (n=3). Hence the electronic configuration is 2,8,1.


3.  Difficulty in Determining Valence Electrons:
Pitfall: Students may find it challenging to identify the number of valence electrons in an atom.

Elaboration: 2 ways to determine the number of valence electrons:

Find the electronic configuration: the number of electrons in the outermost electron shell is the number of valence electrons
Look at the group number on the periodic table. Group number is the number of valence electrons. 


4.  Difficulty in understanding the periodic table:
Pitfall: Students may find it challenging to understand the periodic table.

Elaboration: Elements in the Periodic Table are arranged based on their proton number. They are also placed in groups and periods based on certain characteristics.

Elements in the same period has the same number of electron shells. Example: Elements in period 1 has 1 electron shell, periodic 2 has 2 electron shells.

Elements in the same group have the same number of valence electrons. Example: Elements in group 1 have 1 valence electron, elements in group 13 have 3 valence electrons.

Study Guide

Studying the topic of atomic structure effectively requires a combination of strategies that cater to the conceptual understanding and application of key concepts. Here are some specific studying tips for the topic of atomic structure:

1. Master the Basics First:

Begin by understanding the basic components of an atom, including protons, neutrons, electrons, and their respective properties (charge, mass, location).

2. Study the Periodic Table:

Familiarize yourself with the periodic table, paying special attention to group numbers, periods, and the arrangement of elements based on atomic number.

3. Electron Configurations:

Focus on writing electron configurations for different elements. Practice writing configurations for elements across the periodic table to become proficient.

4. Electronic Structure:

Learn how to draw electronic structures to represent the distribution of electrons in an atom. Practice drawing these diagrams for various elements.

Practice drawing ionic structures too! These are commonly drawn wrongly by many students.

5. Isotopes:

Learn how to identify isotopes and calculate the average atomic mass of an element considering isotopic abundances.

6. Practice Problem Solving:

Solve problems related to atomic structure, including determining the number of protons, neutrons, and electrons in various elements or ions.

7. Use Visual Aids:

Utilize diagrams, charts, and models to visualize atomic structure concepts. These visual aids can help you grasp abstract ideas more easily.

8. Real-World Applications:

Connect atomic structure to real-world applications, such as the use of radioactive isotopes in medicine, electron configuration in chemical reactions, and atomic models in technology.


By following this study plan, you can confidently approach questions related to the atomic structures in your ‘O’ levels chemistry exams. Understanding the core concepts will set a strong foundation for your success.

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