What Is Inside a Nucleus?

At the centre of every atom is a tiny, dense nucleus.

The nucleus contains two types of particles:

• Protons
• Neutrons

Together, protons and neutrons are called nucleons.

The number and combination of these nucleons determine the identity and nuclear properties of an atom.

Nucleus = protons + neutrons

Electrons are located outside the nucleus and therefore are not nucleons.

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Atomic Number

The atomic number tells us the number of protons in the nucleus.

It is represented by the symbol:

Z

Therefore:

Z = number of protons

The number of protons determines which element an atom is.

For example:

Every carbon nucleus contains 6 protons.

If the number of protons changes, the element changes.

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Mass Number

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

It is represented by:

A

Therefore:

A = protons + neutrons

Since the number of protons is the atomic number:

A = Z + N

where:

A = mass number

Z = atomic number

N = number of neutrons

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Calculating the Number of Neutrons

We can rearrange:

A = Z + N

to give:

N = A − Z

Therefore:

Number of neutrons = mass number − atomic number

Example 1: Carbon-12

Carbon-12 has:

A = 12

Z = 6

Therefore:

N = 12 − 6

N = 6

Carbon-12 contains:

• 6 protons
• 6 neutrons

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Example 2: Sodium-23

Sodium has atomic number 11.

For sodium-23:

A = 23

Z = 11

Therefore:

N = 23 − 11

N = 12

Sodium-23 contains:

• 11 protons
• 12 neutrons

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Example 3: Uranium-238

Uranium has atomic number 92.

A = 238

Z = 92

Therefore:

N = 238 − 92

N = 146

Uranium-238 contains:

• 92 protons
• 146 neutrons

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Nuclear Notation

Scientists use nuclear notation to show the composition of a nucleus.

The general form is:

ᴬZX

where:

X = chemical symbol

A = mass number

Z = atomic number

For example, carbon-14 can be written as:

¹⁴₆C

This tells us:

A = 14

Z = 6

Therefore:

Protons = 6

Neutrons = 14 − 6 = 8

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Reading a Nuclear Symbol

Consider:

²³₁₁Na

We can read this systematically.

Step 1: Identify the element

Na = sodium

Step 2: Find the atomic number

Z = 11

Therefore:

Protons = 11

Step 3: Find the mass number

A = 23

Step 4: Calculate neutrons

N = A − Z

N = 23 − 11

N = 12

Therefore, the sodium-23 nucleus contains:

11 protons and 12 neutrons

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What Is a Nuclide?

A nuclide is a particular type of nucleus defined by its specific numbers of protons and neutrons.

Examples include:

¹²₆C

¹⁴₆C

²³₁₁Na

²³⁵₉₂U

²³⁸₉₂U

Each represents a particular nuclide.

The term nuclide is especially useful in nuclear physics because nuclear behaviour depends on both the number of protons and the number of neutrons.

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Naming Nuclides

Nuclides can also be written using the element name followed by the mass number.

For example:

¹²₆C = carbon-12

¹⁴₆C = carbon-14

²³₁₁Na = sodium-23

²³⁵₉₂U = uranium-235

²³⁸₉₂U = uranium-238

Notice that the atomic number is normally not included in the written name because the element already tells us the number of protons.

If we know something is uranium, we already know:

Z = 92

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What Are Isotopes?

Isotopes are atoms of the same element that contain the same number of protons but different numbers of neutrons.

Because they belong to the same element:

same Z

But because they have different numbers of neutrons:

different A

For example:

All three are carbon because they all contain:

6 protons

However, they contain different numbers of neutrons.

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Isotopes and Nuclear Stability

Different isotopes of the same element can have different nuclear properties.

Some isotopes are stable.

Others are unstable, meaning their nuclei can undergo radioactive decay.

For example:

Carbon-12 → stable

Carbon-14 → radioactive

Both are carbon atoms because they contain 6 protons, but their different numbers of neutrons affect the stability of their nuclei.

This relationship between proton number, neutron number, and nuclear stability will become extremely important later in this unit.

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What Is an Ion?

An ion is an atom or group of atoms that has gained or lost electrons.

This changes the particle's electric charge.

For example, a neutral sodium atom has:

11 protons

11 electrons

If it loses one electron:

11 protons

10 electrons

it becomes:

Na⁺

The nucleus has not changed.

It still contains 11 protons.

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Isotopes vs Ions

Isotopes and ions are very different concepts.

Isotopes

Differ in the number of neutrons.

The nucleus is different.

Example:

¹²₆C and ¹⁴₆C

Ions

Differ in the number of electrons.

The nucleus does not need to change.

Example:

Na and Na⁺

A useful rule is:

Change neutrons → isotope

Change electrons → ion

Change protons → different element

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Comparing the Three Changes

This is one of the most useful relationships to remember when interpreting atomic and nuclear notation.

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Nuclear Notation for Ions

An ionic charge can also be added to nuclear notation.

For example:

²³₁₁Na⁺

The nuclear information tells us:

A = 23

Z = 11

Therefore:

Protons = 11

Neutrons = 23 − 11 = 12

The +1 charge tells us that the atom has lost one electron.

Therefore:

Electrons = 10

Notice that the charge does not change the number of protons or neutrons.

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Worked Example: Magnesium Ion

Consider:

²⁴₁₂Mg²⁺

Step 1: Find the protons

Z = 12

Therefore:

12 protons

Step 2: Find the neutrons

N = A − Z

N = 24 − 12

N = 12

Step 3: Find the electrons

The charge is +2, meaning two electrons have been lost.

Electrons = 12 − 2 = 10

Therefore:

²⁴₁₂Mg²⁺ contains 12 protons, 12 neutrons, and 10 electrons.

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Worked Example: Chloride Ion

Consider:

³⁷₁₇Cl⁻

Protons

17

Neutrons

37 − 17 = 20

Electrons

A −1 charge means the atom has gained one electron.

17 + 1 = 18 electrons

Therefore:

³⁷₁₇Cl⁻ contains 17 protons, 20 neutrons, and 18 electrons.

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Finding a Missing Mass Number

Sometimes you may be given the numbers of protons and neutrons.

For example:

A nucleus contains:

17 protons

20 neutrons

Find its nuclear symbol.

Step 1: Identify the element

17 protons means:

Z = 17

Element 17 is chlorine:

Cl

Step 2: Calculate the mass number

A = Z + N

A = 17 + 20

A = 37

Step 3: Write the nuclear symbol

³⁷₁₇Cl

The nuclide is chlorine-37.

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Finding a Missing Atomic Number

Suppose a nucleus has:

A = 27

N = 14

We know:

A = Z + N

Therefore:

Z = A − N

Z = 27 − 14

Z = 13

Atomic number 13 is aluminium.

Therefore the nuclide is:

²⁷₁₃Al

or:

aluminium-27

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A Reliable Method for Nuclear Problems

When solving nuclear composition questions, use the following relationships:

Z = number of protons

A = protons + neutrons

N = A − Z

Then ask:

What is changing?

If the number of protons changes → element changes

If the number of neutrons changes → isotope changes

If the number of electrons changes → ion changes

This method can solve most introductory nuclear notation problems.

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Nuclear Composition at a Glance

Consider these particles:

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Did You Know?

The word nuclide is broader than the word isotope.

A nuclide identifies one particular combination of protons and neutrons.

An isotope describes a relationship between nuclides of the same element.

For example:

carbon-12 is a nuclide

carbon-14 is a nuclide

and:

carbon-12 and carbon-14 are isotopes of carbon

This distinction becomes useful when studying large numbers of radioactive nuclei.

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Key Terms

Nucleus – The dense central region of an atom containing protons and neutrons.

Nucleon – A proton or neutron.

Atomic number (Z) – The number of protons in a nucleus.

Mass number (A) – The total number of protons and neutrons in a nucleus.

Neutron number (N) – The number of neutrons in a nucleus.

Nuclear notation – A symbolic system showing the element, atomic number, and mass number of a nuclide.

Nuclide – A specific type of nucleus defined by its numbers of protons and neutrons.

Isotope – One of two or more forms of the same element containing different numbers of neutrons.

Ion – An atom or group of atoms with an overall electric charge because electrons have been gained or lost.

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Key Takeaways

• The nucleus contains protons and neutrons, collectively called nucleons.
• The atomic number Z equals the number of protons.
• The mass number A equals the total number of protons and neutrons.
• The neutron number can be calculated using N = A − Z.
• Nuclear notation can be written in the form ᴬZX.
• The number of protons determines the element.
• Nuclides are specific combinations of protons and neutrons.
• Isotopes have the same number of protons but different numbers of neutrons.
• Ions form when the number of electrons changes.
• Changing the number of neutrons changes the isotope; changing the number of electrons changes the ion; changing the number of protons changes the element.
• Nuclear symbols can be used to determine the numbers of protons, neutrons, and, when charge is shown, electrons.