The Nucleus

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Notes

Composition of the Nucleus

The **nucleus** is composed of **protons** (positive charge) and **neutrons** (no charge).
The nucleus has an overall **positive charge** due to the protons.
Protons and neutrons are collectively called **nucleons**.
Electrons orbit the nucleus and have a **negative charge**.

Proton Number (Z) and Nucleon Number (A)

The **proton number** (Z) is the number of protons in an atom; it determines the element.
The **nucleon number** (A) is the total number of protons and neutrons in the nucleus.
Number of neutrons = **nucleon number − proton number**.
In a neutral atom, the number of electrons equals the number of protons.

Nuclide Notation

Nuclide notation is written as **^A_Z X** $(e.g., ^{197}_{79} Au)$ .
The top number **A** is the nucleon number; the bottom number **Z** is the proton number.
A **nuclide** is a group of atoms with the same number of protons and neutrons.

Isotopes

**Isotopes** are atoms of the same element with the same number of protons but different numbers of neutrons.
Isotopes have the same proton number but different nucleon numbers.
Some isotopes are **radioactive** and emit α, β, or gamma radiation.
Examples: carbon-14, uranium-235, technetium-99m.

Relative Charge of Subatomic Particles

**Relative charge** compares a particle's charge to the fundamental charge $(1.6 \times$ 10⁻¹⁹ C).
Proton: relative charge **+1**.
Neutron: relative charge **0**.
Electron: relative charge **−1**.

Nuclear Charge (Extended Tier Only)

**Nuclear charge** is the total relative charge of the nucleus.
It equals the number of protons multiplied by +1.
For example, a carbon nucleus (6 protons) has a nuclear charge of **+6**.

Nuclear Fission (Extended Tier Only)

**Nuclear fission** is the splitting of a large, unstable nucleus into two smaller nuclei.
A neutron collides with the nucleus, causing it to split and release **2–3 neutrons** and **gamma rays**.
The products are **daughter nuclei** and neutrons with high kinetic energy.
Example: fission of uranium-235.

Nuclear Fusion (Extended Tier Only)

**Nuclear fusion** is the joining of two light nuclei to form a heavier nucleus.
In stars, hydrogen nuclei fuse to form helium, releasing huge amounts of energy.
Fusion requires extremely high **temperature** and **pressure**.
It is difficult to reproduce on Earth.

Bohr model of a lithium atom showing the nucleus (3 protons, 4 neutrons) and electron shells.

Bohr model of a uranium-235 atom (92 protons, 143 neutrons) used in nuclear fission.

Practice questions

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1.What particles are found in the nucleus of an atom?
Easy
- AProtons and neutrons
- BProtons and electrons
- CNeutrons and electrons
- DProtons, neutrons, and electrons
2.How many neutrons are in a gold atom with mass number 197 and atomic number 79?
Medium
- A130
- B142
- C118
- D106
3.What is the relative charge of a chromium nucleus with 24 protons?
Medium
- A+24
- B+52
- C+1
- D0
4.The number of protons in an atom is equal to the number of electrons.
Easy
True or false?
5.What is the term for atoms of the same element with different numbers of neutrons?
Easy
6.Complete the sentence.
Medium
In nuclide notation, the top number A represents $the ____$ number.
7.Match each particle to its relative charge.
Medium
- Proton
- Neutron
- Electron
- +1
- 0
- -1
8.Put the following steps of nuclear fission in the correct order.
Hard
- A neutron collides with an unstable nucleus
- The nucleus splits into two smaller nuclei and neutrons
- Gamma rays are emitted
- The nucleus becomes unstable

Slides

The Nucleus

The nucleus is at the centre of an atom.
It contains protons (positively charged) and neutrons (no charge).
The nucleus has an overall positive charge.

What particles are found in the nucleus of an atom?

Protons and electrons
Neutrons and electrons
Protons and neutrons
Protons, neutrons and electrons

Proton Number and Nucleon Number

The proton number (Z) is the number of protons in the nucleus.
The nucleon number (A) is the total number of protons and neutrons.
Number of neutrons $= A - Z$ .

An atom with proton number 6 and nucleon number 12 $has ____$ protons, ____ neutrons $and ____$ electrons.

Nuclide Notation

Nuclide notation is written $as _{Z}^{A}X$ .
The top number A is the nucleon number.
The bottom number Z is the proton number.
Example: $_{3}^{7}Li$ has 3 protons and 4 neutrons.

How many neutrons are in an atom of $gold-197 (_{79}^{197}Au)$ ?

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.
They have the same proton number but different nucleon numbers.
Some isotopes are radioactive and emit radiation when they decay.

Which of these are true about isotopes?

Isotopes have the same number of protons.
Isotopes have the same number of neutrons.
Isotopes have different nucleon numbers.
All isotopes are radioactive.

Relative Charge

Protons have a relative charge of +1.
Neutrons have a relative charge of 0.
Electrons have a relative charge of -1.
The nucleus has a positive charge equal to the number of protons.

The relative charge of a neutron $is ____$ .

Nuclear Charge (Extended)

Nuclear charge is the total relative charge of the nucleus.
It equals the number of protons $\times (+1)$ .
For example, carbon (6 protons) has a nuclear charge of +6.

What is the nuclear charge of an atom with 20 protons?

Nuclear Fission

Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei.
It is triggered by a neutron colliding with the nucleus.
The process releases neutrons and gamma rays, along with a lot of energy.

What is released during nuclear fission?

Only energy
Two smaller nuclei and neutrons
Only neutrons
Helium nuclei

Nuclear Fusion

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus.
It occurs in stars, where hydrogen fuses to form helium.
Fusion requires extremely high temperatures and pressures.

Which statements about nuclear fusion are correct?

Fusion combines light nuclei into a heavier nucleus.
Fusion is the same as fission.
Fusion powers stars.
Fusion is easy to achieve on Earth.

Summary

The nucleus contains protons and neutrons.
Proton number Z defines the element; nucleon number A is total protons + neutrons.
Isotopes have same Z, different A.
Fission splits large nuclei; fusion joins light nuclei.

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