Structure of the Atom (Grade 10 NSC Matric Physical Sciences): Revision Notes

Structure of the Atom

Introduction to atomic structure

Understanding the structure of atoms helps us explain why materials have different properties and how they bond with each other. Scientists have developed detailed knowledge about what atoms look like inside, which forms the foundation of chemistry.

Atoms consist of a positively charged nucleus at the centre, surrounded by one or more negatively charged electrons that orbit around it. The nucleus can be broken down further into smaller building blocks, which we'll explore in detail.

Subatomic particles

Atoms are built from three fundamental particles, each with specific properties that determine how atoms behave. Let's examine each particle in detail.

The electron

Electrons are extremely tiny particles that carry a negative electric charge. Each electron has:

• Mass: $9.11 \times 10^{-31}$ kg
• Charge: -1 unit of electric charge ($-1.6 \times 10^{-19}$ C)

Electrons orbit around the nucleus and are responsible for chemical bonding between atoms. They are much lighter than the other atomic particles, being about 1800 times lighter than protons.

The proton

Protons are found inside the nucleus and carry a positive electric charge. Each proton has:

• Mass: $1.6726 \times 10^{-27}$ kg
• Charge: +1 unit of electric charge ($+1.6 \times 10^{-19}$ C)

The number of protons in an atom determines what element it is. For example, all carbon atoms have exactly 6 protons. Protons are much heavier than electrons - approximately 1800 times heavier.

The neutron

Neutrons are also located in the nucleus but carry no electric charge - they are electrically neutral. Each neutron has:

• Mass: $1.6749 \times 10^{-27}$ kg
• Charge: 0 (no charge)

Neutrons are slightly heavier than protons and help hold the nucleus together. Together, protons and neutrons are called nucleons.

Summary of particle properties

Particle	Mass (kg)	Units of charge	Charge (C)
Proton	$1.6726 \times 10^{-27}$	+1	$1.6 \times 10^{-19}$
Neutron	$1.6749 \times 10^{-27}$	0	0
Electron	$9.11 \times 10^{-31}$	-1	$-1.6 \times 10^{-19}$

Atomic number and mass number

The chemical properties of an element depend on the number of protons in its nucleus. This leads us to two crucial concepts that help identify and describe atoms.

Atomic number (Z)

The atomic number tells us what element we're dealing with. It represents the number of protons in the nucleus of an atom and is represented by the letter Z.

Atomic mass number (A)

The atomic mass number tells us about the total number of particles in the nucleus. It represents the total number of protons plus neutrons and is represented by the letter A.

Standard notation

Scientists use a standardised way to represent atoms that shows both the atomic number and mass number. This notation follows the format:

The standard notation shows:

• A (top): mass number (number of nucleons)
• Z (bottom): atomic number (number of protons)
• X: chemical symbol of the element

For example, an iron nucleus with 26 protons and 30 neutrons would be written as: $^{56}_{26}Fe$

Where $Z = 26$ and $A = 56$, giving us $N = A - Z = 56 - 26 = 30$ neutrons.

Nuclear notation format

Relative atomic mass

The relative atomic mass of an element takes into account that most elements exist as different isotopes (atoms with the same number of protons but different numbers of neutrons).

Ions and electrical neutrality

In a neutral atom, the number of electrons equals the number of protons, so the positive and negative charges balance out. However, atoms can gain or lose electrons to form ions - charged atoms.

When an atom gains electrons, it becomes negatively charged. When it loses electrons, it becomes positively charged. Importantly, gaining or losing electrons doesn't change what element the atom is - that's determined solely by the number of protons.