What Is Atomic Structure?

Atomic structure explains how atoms are built and how their particles are arranged.

All matter is made of tiny particles called atoms. Atoms are extremely small and form the building blocks of elements and compounds.

Scientists discovered that atoms are made of three main subatomic particles:

• Protons
• Neutrons
• Electrons

Understanding atomic structure helps students explain chemical reactions, bonding, and the periodic table.

Structure of an Atom

An atom has two main regions:

1. Nucleus

The nucleus is the central part of the atom and contains:

• Protons
• Neutrons

The nucleus holds almost all of the mass of the atom.

2. Electron Shells

Electrons move around the nucleus in energy levels called shells.

These shells represent different distances from the nucleus where electrons can exist.

Protons

Protons are positively charged particles found inside the nucleus.

Properties of protons:

• Charge = +1
• Relative mass = 1
• Located in the nucleus

The number of protons in an atom determines the element.

For example:

Carbon has 6 protons, so its atomic number is 6.

Neutrons

Neutrons are neutral particles located in the nucleus.

Properties of neutrons:

• Charge = 0
• Relative mass = 1
• Located in the nucleus

Neutrons help make the nucleus stable.

Different numbers of neutrons can lead to isotopes of the same element.

Electrons

Electrons are negatively charged particles that move around the nucleus.

Properties of electrons:

• Charge = –1
• Relative mass = 1/1836 (very small)
• Located in electron shells

Electrons are responsible for chemical bonding and reactions.

Atomic Number

The atomic number is the number of protons in the nucleus of an atom.

Example:

Hydrogen → Atomic number = 1
Carbon → Atomic number = 6
Oxygen → Atomic number = 8

The atomic number identifies the type of element.

Mass Number

The mass number is the total number of protons and neutrons in an atom.

Mass number = protons + neutrons

Example:

Carbon atom:

Protons = 6
Neutrons = 6

Mass number = 12

Example of Atomic Structure

Carbon atom:

Protons = 6
Neutrons = 6
Electrons = 6

Electron arrangement:

2,4

This means:

• 2 electrons in the first shell
• 4 electrons in the second shell

Why Atomic Structure Is Important

Understanding atomic structure helps explain:

• The periodic table
• Chemical bonding
• Formation of ions
• Chemical reactions

It is one of the most important topics in O Level Chemistry.

Exam Tip (5070)

Students are often asked to:

• Define atomic number
• Define mass number
• Identify protons, neutrons and electrons

Example exam question:

State the number of protons, neutrons and electrons in a carbon-12 atom.

Answer:

Protons = 6
Neutrons = 6
Electrons = 6

Practice Question

An atom has:

Atomic number = 11
Mass number = 23

How many neutrons does the atom have?

Answer

Neutrons = Mass number – Atomic number

23 – 11 = 12 neutrons

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