Fundamental Particles and Atomic Notation — AQA A-level Chemistry

SPEC 3.1.1.1 - ATOMIC STRUCTURE

Fundamental Particles

Understanding the building blocks of all matter: protons, neutrons and electrons

What is an Atom?

Definition

An atom is the smallest particle of an element that can take part in a chemical reaction. All matter is made up of atoms.

Atoms are composed of three types of subatomic particle: protons, neutrons and electrons.

Key Concept

Our understanding of atomic structure has evolved over time through scientific discoveries: Dalton's solid spheres, Thomson's plum pudding model, Rutherford's nuclear model, and Bohr's electron shells.

The Three Fundamental Particles

Every atom contains three types of subatomic particle:

ParticleRelative MassRelative ChargeLocation

Proton1+1Nucleus

Neutron10Nucleus

Electron1/1836 (negligible)-1Shells around nucleus

Exam Tip

You must memorise these values. The relative mass of an electron is often approximated to zero or "negligible" in calculations.

Structure of the Atom

Structure of a Lithium atom (not to scale)

Key Facts

The nucleus is small, dense and positively charged

Electrons orbit the nucleus in shells (energy levels)

Most of the atom is empty space

The nucleus is approximately 10,000 times smaller than the atom

Key Relationships

Electrical Neutrality

In a neutral atom, the number of protons equals the number of electrons. The positive charges balance the negative charges exactly.

Mass Distribution

Virtually all the mass of an atom is concentrated in the nucleus, because protons and neutrons have a relative mass of 1, while electrons have negligible mass (1/1836).

Common Misconception

Electrons do NOT orbit the nucleus in fixed circular paths like planets around the Sun. They exist in regions of space called orbitals, where there is a high probability of finding them.

Worked Example01

A particle has a relative mass of 1 and is found in the nucleus of an atom, but carries no electrical charge. Identify this particle and state its role in the atom.

1

Identify the key information: relative mass = 1, location = nucleus, charge = 0

2

Compare with the data table: Only neutrons have relative mass 1, are in the nucleus, and have zero charge

3

The particle is a neutron

4

Role: Neutrons contribute to the mass of the atom and help hold the nucleus together (nuclear stability)

AnswerNeutron. Neutrons add mass to the atom and provide nuclear stability.

Worked Example02

Explain why atoms are electrically neutral, even though they contain charged particles.

1

Atoms contain protons (charge +1) and electrons (charge -1)

2

In a neutral atom, the number of protons equals the number of electrons

3

The total positive charge from protons is balanced by the total negative charge from electrons

4

Neutrons have no charge, so do not affect electrical neutrality

AnswerAtoms are electrically neutral because they contain equal numbers of protons (+1 charge each) and electrons (-1 charge each), so the charges cancel out.

Worked Example03

Explain why nearly all the mass of an atom is concentrated in the nucleus.

1

The nucleus contains protons and neutrons

2

Both protons and neutrons have a relative mass of 1

3

Electrons have a relative mass of only 1/1836 (approximately 0.0005)

4

The mass of electrons is negligible compared to protons and neutrons

AnswerNearly all the mass is in the nucleus because protons and neutrons (both with relative mass 1) are located there, while electrons (relative mass 1/1836) contribute negligible mass.

Evolution of Atomic Models

Our understanding of atomic structure has developed over time:

ScientistModelKey Features

Dalton (1803)Solid sphereAtoms are indivisible solid spheres

Thomson (1897)Plum puddingElectrons scattered throughout a positive sphere

Rutherford (1911)NuclearSmall dense nucleus, electrons orbit around it

Bohr (1913)PlanetaryElectrons in fixed circular energy levels

Bohr - Planetary

Comparing the Particles

Protons

Define the element (atomic number = number of protons)

Contribute to mass and positive charge of nucleus

All atoms of the same element have the same number of protons

Neutrons

Contribute to mass but not charge

Different numbers of neutrons create isotopes

Help stabilise the nucleus

Electrons

Determine chemical properties and bonding

Arranged in shells/energy levels around nucleus

Can be lost, gained or shared in reactions

Practice

Question 1:

ParticleRelative MassRelative Charge

Proton??

Neutron??

Electron??

Practice

Question 2: A student claims that "electrons contribute significantly to the mass of an atom." Explain whether this statement is correct, using data to support your answer.

The statement is incorrect.

Electrons have a relative mass of only 1/1836 compared to protons and neutrons (both relative mass 1).

This means an electron is approximately 1836 times lighter than a proton or neutron.

Therefore, electrons contribute negligible mass to an atom. Virtually all the mass comes from the protons and neutrons in the nucleus.

Practice

Question 3: State where in an atom you would find: (a) the majority of the mass, (b) all of the positive charge, (c) all of the negative charge. Explain each answer.

(a) Majority of the mass: The nucleus

Because protons (mass 1) and neutrons (mass 1) are both located in the nucleus, while electrons have negligible mass.

(b) All positive charge: The nucleus

Because protons (charge +1) are located in the nucleus. Neutrons have no charge.

(c) All negative charge: The electron shells

Because electrons (charge -1) are located in shells/energy levels surrounding the nucleus.

Summary

Key Facts to Remember

ParticleRelative MassRelative ChargeLocation

Proton1+1Nucleus

Neutron10Nucleus

Electron1/1836-1Shells

Exam Essentials

Atoms are neutral: number of protons = number of electrons

Mass is concentrated in the nucleus (protons + neutrons)

Electron mass is negligible (1/1836)

Most of the atom is empty space

Atomic models have evolved over time as evidence emerged