3.2 Subatomic Particles

This topic introduces the three fundamental particles that make up an atom — protons, neutrons, and electrons — and explains their charges, masses, locations, and behaviour in an electric field.

Definition

Subatomic particles are the fundamental particles that make up atoms.

Simple explanation: An atom is not the smallest thing — it is made of even smaller particles called subatomic particles.

Main Idea

The three main subatomic particles are:

• Proton
• Neutron
• Electron

Protons and neutrons are found in the nucleus of an atom, whereas electrons orbit around the nucleus in energy levels or shells. They play crucial roles in determining the properties and behaviour of atoms and molecules.

Core Concept

Detailed Explanation

Proton

• Relative charge: +1
• Relative mass: Approximately 1 atomic mass unit (amu) or 1.6726 × 10⁻²⁷ kg

Simple explanation: The proton is a positively charged particle found in the nucleus. Its mass is about 1 amu.

Neutron

• Relative charge: 0 (neutral)
• Relative mass: Approximately 1 atomic mass unit (amu) or 1.6749 × 10⁻²⁷ kg

Simple explanation: The neutron has no charge. It is found in the nucleus along with protons. Its mass is almost the same as a proton.

Electron

• Relative charge: −1
• **Relative mass:** Approximately 11836 amu or 9.11 × 10⁻³¹ kg

Simple explanation: The electron is a negatively charged particle that moves around the nucleus in shells. It is much lighter than protons and neutrons.

Relationships Between Subatomic Particles

The book explains three key relationships:

1. Mass Relationship

Protons and neutrons have roughly the same mass, around 1 amu. This mass contributes significantly to the total mass of the atom.

Electrons have much less mass, so their contribution to the total mass of an atom is usually negligible.

Simple explanation: Almost all the mass of an atom is in its nucleus (protons + neutrons). Electrons hardly add any mass.

2. Charge Relationship

The interaction between the negatively charged electrons and positively charged protons in the nucleus is what holds the atoms together.

Simple explanation: The attraction between opposite charges (protons positive, electrons negative) keeps the atom together.

3. Nuclear Force

• neutron–neutron
• proton–proton
• neutron–proton

Simple explanation: The nucleus should fall apart because protons repel each other. The strong nuclear force overcomes this repulsion and holds the nucleus together.

The Behaviour of Protons, Neutrons, and Electrons in an Electric Field

What happens when a beam of these particles passes between two electrically charged plates?

Protons are positively charged and are deflected on a curved path toward the negative plate.

Electrons are negatively charged and are deflected on a curved path toward the positive plate.

Neutrons have no charge, so they go straight ahead.

Important: If the electrons and protons are traveling at the same speed, the electrons being lighter are deflected far more strongly than the heavier protons.

• Positive particles → attracted to negative plate.
• Negative particles → attracted to positive plate.
• Neutral particles → no deflection.
• Lighter particles → deflected more.

Charge Neutrality

Atoms are electrically neutral because the number of protons (positively charged) in the nucleus is equal to the number of electrons (negatively charged) in the electron cloud. The charges balance each other so there is no net charge on the atoms.

Simple explanation: In a neutral atom, positive charge = negative charge. So the atom has no overall charge.

Intermediate Level

Why the Atom Is Neutral

In a neutral atom: - Number of protons = Number of electrons

The positive charge from protons exactly balances the negative charge from electrons. So: - Net charge = (+ number of protons) + (− number of electrons) = 0

Why Electrons Are Deflected More Than Protons

In an electric field:

• The force on a charged particle depends on its charge.
• The acceleration depends on both charge and mass (F = ma).
• Electrons and protons have the same magnitude of charge (1 unit each).
• But electrons are about 1836 times lighter than protons.
• So electrons accelerate much more → they are deflected much more.

Mass of the Atom

• Protons and neutrons have mass ~1 amu each.
• Electrons have mass ~11836 amu (negligible).

Example: A carbon atom has 6 protons + 6 neutrons in the nucleus (mass ≈ 12 amu), and 6 electrons (total mass ≈ 61836 ≈ 0.003 amu — negligible).

Important Points

• - Three subatomic particles: proton, neutron, electron.
• - Proton: +1 charge; ~1 amu; in nucleus.
• - Neutron: 0 charge; ~1 amu; in nucleus.
• - **Electron:** −1 charge; ~11836 amu; in shells around nucleus.
• - Protons and neutrons have roughly the same mass; electrons are much lighter.
• - The attraction between protons and electrons holds the atom together.
• - Strong nuclear force holds protons and neutrons together in the nucleus.
• - In an electric field:
• - Protons → deflected toward negative plate.
• - Electrons → deflected toward positive plate.
• - Neutrons → no deflection (go straight).
• - Electrons deflected more than protons (because they are lighter).
• - Atoms are electrically neutral because number of protons = number of electrons.

Remember

• - Proton = positive, heavy, in nucleus.
• - Neutron = neutral, heavy, in nucleus.
• - Electron = negative, very light, in shells.
• - Strong nuclear force holds nucleus together.
• - Electric field: protons → negative plate; electrons → positive plate; neutrons → straight.
• - Electrons deflect more because they are lighter.
• - Atom is neutral because protons = electrons.

Common Mistakes

• 1. Thinking electrons are in the nucleus. Electrons are in shells around the nucleus.
• 2. Thinking neutrons have a charge. Neutrons are neutral.
• 3. Thinking electrons and protons have the same mass. Electrons are ~1836 times lighter.
• 4. Thinking the atom has a net charge. A neutral atom has equal protons and electrons, so net charge is zero.
• 5. Forgetting the strong nuclear force. Without it, the nucleus would break apart.
• 6. Thinking all particles are deflected equally in an electric field. Electrons deflect more because they are lighter.

Quick Revision

• Subatomic particles: proton, neutron, electron.
• Proton: +1 charge; ~1 amu (1.6726 × 10⁻²⁷ kg); in nucleus.
• Neutron: 0 charge; ~1 amu (1.6749 × 10⁻²⁷ kg); in nucleus.
• **Electron:** −1 charge; ~11836 amu (9.11 × 10⁻³¹ kg); in shells.
• Protons and neutrons are held in the nucleus by the strong nuclear force.
• In an electric field: protons → negative plate; electrons → positive plate; neutrons → straight. Electrons deflect more because they are lighter.
• Atoms are electrically neutral because number of protons = number of electrons.

60-Second Revision

• Proton: +1, heavy, nucleus.
• Neutron: 0, heavy, nucleus.
• Electron: −1, very light, shells.
• Strong nuclear force holds nucleus together.
• Electric field: protons → negative; electrons → positive; neutrons → straight.
• Atom is neutral: protons = electrons.
• Warning: electrons are not in the nucleus; they orbit around it.