Isotope Structure

Inside the Nucleus

The nucleus is so dense that almost all of an atom's mass (over 99.9%) is concentrated in a tiny central region with a radius approximately 10,000 times smaller than the radius of the atom itself.
The nucleus is built from two types of subatomic particles, collectively called nucleons.
These nucleons are positively charged protons and neutral neutrons, both of which have a relative mass of 1 unit (u).
Electrons orbit outside the nucleus and have a negligible mass (roughly 1/2000 the mass of a nucleon), so they do not significantly contribute to the atom's total mass.
The number of protons is the defining feature of an element; if the proton count changes, the atom becomes an entirely different element.

Atomic and Mass Numbers

Scientists use specific numbers to describe exactly what is inside a nucleus.

The Atomic (Proton) Number ( $Z$ ) represents the exact number of protons in the nucleus and determines the element's position on the Periodic Table.
The Mass (Nucleon) Number ( $A$ ) is the total combined number of protons and neutrons in the nucleus.
In a neutral atom, the number of orbiting electrons is exactly equal to the atomic number.
If an atom gains or loses electrons to become a charged ion, its nuclear structure ( $A$ and $Z$ ) remains entirely unchanged.

Standard Nuclear Notation

To clearly describe the nuclear structure of an element, scientists use standard nuclear notation: $^{A}_{Z}X$ .

First, the chemical symbol ( $X$ ) is written in the centre.
Next, the mass (nucleon) number ( $A$ ) is placed as a superscript at the top-left. It is always the larger of the two numbers.
Finally, the atomic (proton) number ( $Z$ ) is placed as a subscript at the bottom-left.

Example: Calculating particles in Lithium-7 ( $^{7}_{3}\text{Li}$ )

Identify the numbers: $A = 7$ and $Z = 3$ .
Protons: Equal to the atomic number ( $Z$ ), so there are 3 protons.
Neutrons: Calculated as $A - Z$ . $\text{Number of Neutrons} = 7 - 3 = 4\text{ neutrons}$
Electrons: In a neutral atom, this equals the number of protons, so there are 3 electrons.

The Structure of Isotopes

Isotopes are atoms of the same element that have an identical number of protons but a different number of neutrons.

Illustrating Isotopes side-by-side

To describe the structure of isotopes, you must compare how their $A$ and $Z$ values differ using standard notation. Consider the isotopes of Carbon:

Feature	Carbon-12 ( $^{12}_{6}\text{C}$ )	Carbon-14 ( $^{14}_{6}\text{C}$ )
Atomic Number ( $Z$ )	6	6
Mass Number ( $A$ )	12	14
Proton Count	6	6
Neutron Count ( $A-Z$ )	6	8

Step-by-Step Comparison:

Identify the Element: Both have the same atomic number ( $Z = 6$ ), meaning they are both Carbon atoms with identical chemical properties.
Compare the Nuclei: Carbon-14 has a higher mass number ( $A = 14$ ) than Carbon-12 ( $A = 12$ ).
Explain the Difference: The change in mass is due entirely to the different number of neutrons (8 vs 6).
Physical Properties: Because they have different masses, they have different physical properties, such as density.
Stability: The extra neutrons in Carbon-14 create an imbalance, making the nucleus unstable and prone to radioactive decay.

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Exam Tips

Common Mistake
Students often describe isotopes simply as 'atoms with a different mass number', but fail to explicitly state they are atoms of the SAME element.
2
For a 2-mark definition of isotopes, examiners expect the 'Gold Standard' answer: you must state they have the 'same number of protons' AND a 'different number of neutrons'.
3
On exam papers, the mass (nucleon) number is always the larger of the two numbers next to the chemical symbol, even if the printing layout looks slightly different to standard notation.
4
Edexcel exams use 'mass number' interchangeably with 'nucleon number', and 'atomic number' interchangeably with 'proton number', so be prepared to recognise both pairs of terms.

Key Terms(13)

Nucleon: A collective term for any subatomic particle found inside the nucleus, meaning either a proton or a neutron.
Proton: A subatomic particle found in the nucleus of an atom, possessing a positive charge and a relative mass of 1.
Neutron: A subatomic particle found in the nucleus of an atom, possessing no charge (neutral) and a relative mass of 1.
Electron: A subatomic particle with a negative charge and negligible mass, which orbits the nucleus of an atom in shells.
Atomic (Proton) Number: The number of protons in the nucleus of an atom, which determines the element's identity.
Mass (Nucleon) Number: The total number of protons and neutrons in the nucleus of an atom.
Isotopes: Atoms of the same element that have the same number of protons but different numbers of neutrons.
Ion: An atom that has gained or lost electrons, resulting in an overall electrical charge, without changing its nuclear structure.
Radioactive decay: The process by which an unstable nucleus releases energy and/or particles to become more stable.
Standard nuclear notation: A system of writing the chemical symbol with the mass number as a superscript and the atomic number as a subscript, both to the left of the symbol.
Relative mass: The mass of a subatomic particle compared to the mass of a nucleon (defined as 1 unit).
Chemical properties: Characteristics of a substance that relate to its chemical reactions, determined by electron configuration.
Physical properties: Characteristics such as mass, density, and boiling point that do not involve changing the substance's chemical identity.

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Key Terms(13)

Nucleon: A collective term for any subatomic particle found inside the nucleus, meaning either a proton or a neutron.
Proton: A subatomic particle found in the nucleus of an atom, possessing a positive charge and a relative mass of 1.
Neutron: A subatomic particle found in the nucleus of an atom, possessing no charge (neutral) and a relative mass of 1.
Electron: A subatomic particle with a negative charge and negligible mass, which orbits the nucleus of an atom in shells.
Atomic (Proton) Number: The number of protons in the nucleus of an atom, which determines the element's identity.
Mass (Nucleon) Number: The total number of protons and neutrons in the nucleus of an atom.
Isotopes: Atoms of the same element that have the same number of protons but different numbers of neutrons.
Ion: An atom that has gained or lost electrons, resulting in an overall electrical charge, without changing its nuclear structure.
Radioactive decay: The process by which an unstable nucleus releases energy and/or particles to become more stable.
Standard nuclear notation: A system of writing the chemical symbol with the mass number as a superscript and the atomic number as a subscript, both to the left of the symbol.
Relative mass: The mass of a subatomic particle compared to the mass of a nucleon (defined as 1 unit).
Chemical properties: Characteristics of a substance that relate to its chemical reactions, determined by electron configuration.
Physical properties: Characteristics such as mass, density, and boiling point that do not involve changing the substance's chemical identity.

Isotope Structure

Inside the Nucleus

The nucleus is so dense that almost all of an atom's mass (over 99.9%) is concentrated in a tiny central region with a radius approximately 10,000 times smaller than the radius of the atom itself.
The nucleus is built from two types of subatomic particles, collectively called nucleons.
These nucleons are positively charged protons and neutral neutrons, both of which have a relative mass of 1 unit (u).
Electrons orbit outside the nucleus and have a negligible mass (roughly 1/2000 the mass of a nucleon), so they do not significantly contribute to the atom's total mass.
The number of protons is the defining feature of an element; if the proton count changes, the atom becomes an entirely different element.

Atomic and Mass Numbers

Scientists use specific numbers to describe exactly what is inside a nucleus.

The Atomic (Proton) Number ( $Z$ ) represents the exact number of protons in the nucleus and determines the element's position on the Periodic Table.
The Mass (Nucleon) Number ( $A$ ) is the total combined number of protons and neutrons in the nucleus.
In a neutral atom, the number of orbiting electrons is exactly equal to the atomic number.
If an atom gains or loses electrons to become a charged ion, its nuclear structure ( $A$ and $Z$ ) remains entirely unchanged.

Standard Nuclear Notation

To clearly describe the nuclear structure of an element, scientists use standard nuclear notation: $^{A}_{Z}X$ .

First, the chemical symbol ( $X$ ) is written in the centre.
Next, the mass (nucleon) number ( $A$ ) is placed as a superscript at the top-left. It is always the larger of the two numbers.
Finally, the atomic (proton) number ( $Z$ ) is placed as a subscript at the bottom-left.

Example: Calculating particles in Lithium-7 ( $^{7}_{3}\text{Li}$ )

Identify the numbers: $A = 7$ and $Z = 3$ .
Protons: Equal to the atomic number ( $Z$ ), so there are 3 protons.
Neutrons: Calculated as $A - Z$ . $\text{Number of Neutrons} = 7 - 3 = 4\text{ neutrons}$
Electrons: In a neutral atom, this equals the number of protons, so there are 3 electrons.

The Structure of Isotopes

Isotopes are atoms of the same element that have an identical number of protons but a different number of neutrons.

Illustrating Isotopes side-by-side

To describe the structure of isotopes, you must compare how their $A$ and $Z$ values differ using standard notation. Consider the isotopes of Carbon:

Feature	Carbon-12 ( $^{12}_{6}\text{C}$ )	Carbon-14 ( $^{14}_{6}\text{C}$ )
Atomic Number ( $Z$ )	6	6
Mass Number ( $A$ )	12	14
Proton Count	6	6
Neutron Count ( $A-Z$ )	6	8

Step-by-Step Comparison:

Identify the Element: Both have the same atomic number ( $Z = 6$ ), meaning they are both Carbon atoms with identical chemical properties.
Compare the Nuclei: Carbon-14 has a higher mass number ( $A = 14$ ) than Carbon-12 ( $A = 12$ ).
Explain the Difference: The change in mass is due entirely to the different number of neutrons (8 vs 6).
Physical Properties: Because they have different masses, they have different physical properties, such as density.
Stability: The extra neutrons in Carbon-14 create an imbalance, making the nucleus unstable and prone to radioactive decay.

Sign up to continue reading

Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
Students often describe isotopes simply as 'atoms with a different mass number', but fail to explicitly state they are atoms of the SAME element.
2
For a 2-mark definition of isotopes, examiners expect the 'Gold Standard' answer: you must state they have the 'same number of protons' AND a 'different number of neutrons'.
3
On exam papers, the mass (nucleon) number is always the larger of the two numbers next to the chemical symbol, even if the printing layout looks slightly different to standard notation.
4
Edexcel exams use 'mass number' interchangeably with 'nucleon number', and 'atomic number' interchangeably with 'proton number', so be prepared to recognise both pairs of terms.

Key Terms(13)

Nucleon: A collective term for any subatomic particle found inside the nucleus, meaning either a proton or a neutron.
Proton: A subatomic particle found in the nucleus of an atom, possessing a positive charge and a relative mass of 1.
Neutron: A subatomic particle found in the nucleus of an atom, possessing no charge (neutral) and a relative mass of 1.
Electron: A subatomic particle with a negative charge and negligible mass, which orbits the nucleus of an atom in shells.
Atomic (Proton) Number: The number of protons in the nucleus of an atom, which determines the element's identity.
Mass (Nucleon) Number: The total number of protons and neutrons in the nucleus of an atom.
Isotopes: Atoms of the same element that have the same number of protons but different numbers of neutrons.
Ion: An atom that has gained or lost electrons, resulting in an overall electrical charge, without changing its nuclear structure.
Radioactive decay: The process by which an unstable nucleus releases energy and/or particles to become more stable.
Standard nuclear notation: A system of writing the chemical symbol with the mass number as a superscript and the atomic number as a subscript, both to the left of the symbol.
Relative mass: The mass of a subatomic particle compared to the mass of a nucleon (defined as 1 unit).
Chemical properties: Characteristics of a substance that relate to its chemical reactions, determined by electron configuration.
Physical properties: Characteristics such as mass, density, and boiling point that do not involve changing the substance's chemical identity.

Previous Subtopic: Atomic SizeAtomic Size Next Subtopic: Nucleus CharacteristicsNucleus Characteristics

Back to Isotope Structure

Put your knowledge into practice — try past paper questions for Physics

Key Terms(13)

Nucleon: A collective term for any subatomic particle found inside the nucleus, meaning either a proton or a neutron.
Proton: A subatomic particle found in the nucleus of an atom, possessing a positive charge and a relative mass of 1.
Neutron: A subatomic particle found in the nucleus of an atom, possessing no charge (neutral) and a relative mass of 1.
Electron: A subatomic particle with a negative charge and negligible mass, which orbits the nucleus of an atom in shells.
Atomic (Proton) Number: The number of protons in the nucleus of an atom, which determines the element's identity.
Mass (Nucleon) Number: The total number of protons and neutrons in the nucleus of an atom.
Isotopes: Atoms of the same element that have the same number of protons but different numbers of neutrons.
Ion: An atom that has gained or lost electrons, resulting in an overall electrical charge, without changing its nuclear structure.
Radioactive decay: The process by which an unstable nucleus releases energy and/or particles to become more stable.
Standard nuclear notation: A system of writing the chemical symbol with the mass number as a superscript and the atomic number as a subscript, both to the left of the symbol.
Relative mass: The mass of a subatomic particle compared to the mass of a nucleon (defined as 1 unit).
Chemical properties: Characteristics of a substance that relate to its chemical reactions, determined by electron configuration.
Physical properties: Characteristics such as mass, density, and boiling point that do not involve changing the substance's chemical identity.