Last updated: 15 August 2026
Before studying atomic orbitals, ionisation energies or chemical bonding, we need to understand what lies at the centre of every atom. The nucleus is extremely small, but it determines which element the atom belongs to and contributes almost all its mass.
This topic is therefore more than a list of three subatomic particles. You need to interpret proton numbers, nucleon numbers and ionic charges so that you can work out exactly how many protons, neutrons and electrons are present in atoms, ions and isotopes.
These ideas also provide the foundation for later topics. Once the distinction between the nucleus and the electrons outside it is clear, electronic structure, ionisation energy and mass spectrometry become much easier to understand.
Diagnostic Quiz
Try these five questions before reading the chapter. They check whether the basic ideas about protons, neutrons, electrons and isotope notation are already secure.
- 5 quick questions
- Checks important atomic-structure foundations
- Every answer option is explained
The three main subatomic particles
An atom is built from three principal subatomic particles: protons, neutrons and electrons. They differ in their location, relative charge and relative mass.
| Particle | Location | Relative charge | Relative mass |
|---|---|---|---|
| Proton | Inside the nucleus | +1 | 1 |
| Neutron | Inside the nucleus | 0 | 1 |
| Electron | Outside the nucleus | −1 | Approximately 1/1840 |
These are relative values. A proton is assigned relative mass 1 and relative charge +1, and the other particles are compared with it.
An electron really does have mass. Its mass is usually neglected when considering the approximate mass of an atom because it is tiny compared with the masses of protons and neutrons.
Where are the mass and charge found?
Protons and neutrons are packed into the nucleus. Since both have relative mass 1, while an electron has relative mass approximately 1/1840, nearly all the mass of an atom is concentrated in its nucleus.
The nucleus is positively charged because it contains protons. The negative charge is associated with electrons outside the nucleus.
- Nearly all the mass is concentrated in the nucleus.
- Positive charge is concentrated in the nucleus.
- Negative charge is found outside the nucleus.
In a neutral atom, the number of positively charged protons equals the number of negatively charged electrons. Their charges cancel to give an overall charge of zero.
Behaviour of subatomic particles in an electric field
An electric field can be produced between a positively charged plate and a negatively charged plate. When a charged particle passes through the field, it experiences an electrostatic force.
The behaviour of the three subatomic particles therefore immediately reveals whether they are positively charged, negatively charged or neutral.
| Particle beam | Observation | Reason |
|---|---|---|
| Protons | Deflected towards the negative plate | Protons have positive charge |
| Neutrons | Continue in a straight line | Neutrons have no charge |
| Electrons | Deflected towards the positive plate | Electrons have negative charge |
Direction of deflection depends on the sign of the charge
Opposite charges attract and like charges repel. Therefore:
- a positively charged proton is attracted towards the negative plate;
- a negatively charged electron is attracted towards the positive plate; and
- a neutron is not deflected because it carries no overall charge.
Angle of deflection depends on charge-to-mass ratio
Direction is only half of the story. We also need to explain why one charged particle may bend more strongly than another.
Under the same experimental conditions:
The vertical lines around charge mean that we are considering the magnitude of the charge. The positive or negative sign determines the direction of bending; the magnitude of the charge-to-mass ratio determines how strongly the particle is deflected.
We can see why by considering the electrostatic force:
For a given electric field strength, E, a particle with a larger charge magnitude experiences a larger force.
From Newton’s second law:
So, under the same conditions:
The sign of the charge determines the direction of deflection. The magnitude of |q|/m determines how strongly the particle is deflected when other conditions such as field strength and particle speed are the same.
Why is an electron deflected more than a proton?
A proton and an electron have charges of equal magnitude:
- proton relative charge = +1;
- electron relative charge = −1.
Their charge magnitudes are therefore equal. Their masses, however, are very different:
- proton relative mass = 1;
- electron relative mass ≈ 1/1840.
The electron therefore has a charge-to-mass ratio approximately 1840 times greater than that of the proton. Under comparable conditions, the electron undergoes a much greater acceleration and its path bends much more strongly.
Do not draw equal curves: the proton and electron bend in opposite directions, but the electron should show much greater deflection.
Their charge magnitudes are equal. The difference is caused by the electron’s much smaller mass.
Proton number and nucleon number
The composition of a nucleus can be described using two important numbers: the proton number and the nucleon number.
Proton number, Z
The proton number, Z, is the number of protons in the nucleus of an atom. It is also commonly called the atomic number.
The proton number determines the identity of the element. Every carbon nucleus contains 6 protons, every oxygen nucleus contains 8 protons, and every sodium nucleus contains 11 protons.
If the number of protons changes, the element changes. A nucleus containing 8 protons is oxygen regardless of its number of neutrons or electrons.
Nucleon number, A
The nucleon number, A, is the total number of protons and neutrons in the nucleus. It is also commonly called the mass number.
Protons and neutrons are collectively called nucleons because both are particles found in the nucleus.
Therefore:
Worked example: sodium-23
Sodium-23 has proton number 11 and nucleon number 23.
Number of protons = 11
Number of neutrons = 23 − 11 = 12
Number of electrons in a neutral sodium atom = 11
The proton number tells you which element is present. The nucleon number tells you which isotope of that element is present.
Writing nuclide notation
A particular atom can be represented using nuclide notation. The nucleon number, A, is written as a small superscript at the upper left of the element symbol, while the proton number, Z, is written as a small subscript at the lower left.
For example, sodium-23 is represented as:
From ²³₁₁Na:
- Na identifies the element as sodium.
- 23 is the nucleon number, A.
- 11 is the proton number, Z.
- Number of neutrons = 23 − 11 = 12.
Counting particles in atoms and ions
The proton and nucleon numbers tell us the numbers of protons and neutrons. To determine the number of electrons, we must also consider whether the particle is neutral or charged.
Neutral atoms
A neutral atom contains equal numbers of protons and electrons.
Positive ions
A positive ion, or cation, forms when an atom loses one or more negatively charged electrons.
Aluminium forms Al³⁺ by losing three electrons:
Therefore, ²⁷₁₃Al³⁺ contains:
- 13 protons;
- 27 − 13 = 14 neutrons; and
- 13 − 3 = 10 electrons.
Negative ions
A negative ion, or anion, forms when an atom gains one or more electrons.
For example, ³⁵₁₇Cl⁻ contains:
- 17 protons;
- 35 − 17 = 18 neutrons; and
- 17 + 1 = 18 electrons.
This provides a useful checking method. A particle containing 13 protons and 10 electrons has:
It must therefore be a 3+ ion.
Ordinary chemical reactions involve electrons. Losing or gaining electrons changes the charge of a particle, but the proton and neutron numbers remain unchanged.
A reliable three-step method
- Protons: read Z.
- Neutrons: calculate A − Z.
- Electrons: begin with Z and adjust for the ionic charge.
| Species | Protons | Neutrons | Electrons |
|---|---|---|---|
| ²³₁₁Na | 11 | 12 | 11 |
| ²⁴₁₂Mg²⁺ | 12 | 12 | 10 |
| ³⁷₁₇Cl⁻ | 17 | 20 | 18 |
| ⁵⁶₂₆Fe³⁺ | 26 | 30 | 23 |
Incorrect idea: Al³⁺ contains three more protons than an aluminium atom.
Correct reasoning: both contain 13 protons. Al³⁺ is positive because it has lost three electrons.
Isotopes: the same element with different nuclei
All atoms of a particular element have the same proton number. However, atoms of the same element do not necessarily contain the same number of neutrons.
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
Because their neutron numbers differ, isotopes have different nucleon numbers.
Example: chlorine-35 and chlorine-37
Chlorine has proton number 17. Consider ³⁵₁₇Cl and ³⁷₁₇Cl.
| Isotope | Protons | Neutrons |
|---|---|---|
| ³⁵₁₇Cl | 17 | 35 − 17 = 18 |
| ³⁷₁₇Cl | 17 | 37 − 17 = 20 |
Both atoms are chlorine because both contain 17 protons. They are different isotopes because they contain different numbers of neutrons.
Why are isotopes still the same element?
The identity of an element is determined by the number of protons in its nucleus, not by the number of neutrons.
Changing the number of neutrons changes the nuclear mass but does not change the proton number. The atom therefore remains the same element.
Chemical properties of isotopes
Chemical reactions involve electrons, particularly the outer electrons. Neutral isotopes of the same element have the same proton number and therefore the same number and arrangement of electrons.
Isotopes are therefore treated as having the same chemical properties at this level.
Physical properties of isotopes
Isotopes have different masses because their nuclei contain different numbers of neutrons.
Physical properties that depend on mass can therefore differ. For example, isotopes may differ slightly in density or rate of diffusion.
Isotopes have essentially the same chemical properties because their electronic structures are the same. Mass-dependent physical properties may differ because their nuclei have different masses.
The isotopes of hydrogen
Hydrogen gives a particularly clear example because its commonly discussed isotopes contain zero, one or two neutrons.
| Isotope | Symbol | Protons | Neutrons |
|---|---|---|---|
| Hydrogen-1 | ¹₁H | 1 | 0 |
| Hydrogen-2, or deuterium | ²₁H | 1 | 1 |
| Hydrogen-3, or tritium | ³₁H | 1 | 2 |
All three are hydrogen because every nucleus contains one proton.
Isotopes and ions are not the same thing
Students sometimes confuse isotopes and ions because both describe different forms of the same element. The particle that changes, however, is different.
| Feature | Isotopes | Ions |
|---|---|---|
| What differs? | Number of neutrons | Number of electrons |
| What remains unchanged? | Number of protons | Numbers of protons and neutrons |
| Main result | Different nucleon number | Different overall charge |
| Example | ³⁵Cl and ³⁷Cl | Na and Na⁺ |
Isotope: change the number of neutrons.
Ion: change the number of electrons.
In both cases, the proton number remains unchanged because the element remains the same.
Worked reasoning with atoms, ions and isotopes
Example 1: ⁵⁶₂₆Fe³⁺
Determine the numbers of protons, neutrons and electrons in ⁵⁶₂₆Fe³⁺.
The proton number is 26:
The neutron number is:
A neutral iron atom would contain 26 electrons. The 3+ charge means that three electrons have been lost:
Answer: 26 protons, 30 neutrons and 23 electrons.
Example 2: identifying an unknown particle
A particle contains 16 protons, 18 neutrons and 18 electrons.
A nucleus containing 16 protons belongs to sulfur.
The particle is therefore ³⁴₁₆S²⁻.
Example 3: isotope or ion difference?
Compare ²⁴₁₂Mg²⁺ and ²⁶₁₂Mg.
Both have proton number 12, so both are magnesium. Their nucleon numbers differ:
- ²⁴Mg contains 24 − 12 = 12 neutrons;
- ²⁶Mg contains 26 − 12 = 14 neutrons.
They therefore represent different isotopes of magnesium. The 2+ charge on the first particle changes only its number of electrons; it does not determine its isotope identity.
Common mistakes with atomic particles
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Treating the nucleon number as the number of neutrons | A includes both protons and neutrons | Calculate neutron number using A − Z |
| Changing the proton number when an ion forms | Ion formation involves electrons, not protons | Keep the proton number unchanged |
| Adding electrons for a positive ion | A positive ion forms by losing electrons | Subtract the positive charge from Z |
| Subtracting electrons for a negative ion | A negative ion forms by gaining electrons | Add the magnitude of the negative charge to Z |
| Defining isotopes as atoms with different proton numbers | Different proton numbers mean different elements | Isotopes have the same proton number but different neutron numbers |
| Saying an electron has no mass | An electron has a small but non-zero mass | Use relative mass approximately 1/1840 |
| Drawing equal deflection for electrons and protons | The electron has a far greater charge-to-mass ratio | Draw the electron path bending much more strongly |
| Using the sign of charge to compare how much particles are deflected | The sign controls direction rather than magnitude | Compare |q|/m for the extent of deflection |
| Saying all properties of isotopes are identical | Mass-dependent physical properties may differ | State that their chemical properties are essentially the same |
Atomic-particle checklist
Read the notation
- Identify the element symbol.
- Read A as the nucleon number.
- Read Z as the proton number.
- Check whether an ionic charge is present.
Count the particles
- Protons = Z.
- Neutrons = A − Z.
- Neutral-atom electrons = Z.
- Adjust the electrons for the ionic charge.
Recognise isotopes
- Check that the proton numbers are equal.
- Check that the neutron numbers differ.
- Expect different nucleon numbers.
- Do not use ionic charge to identify isotopes.
Analyse electric-field deflection
- Use the charge sign to determine direction.
- Neutral particles are not deflected.
- Greater |q|/m gives greater deflection.
- Electrons bend much more than protons.
Mastery Quiz
Now apply the chapter to atoms, ions, isotopes and electric-field observations. These questions require particle counting and chemical reasoning rather than simple definition recall.
- 10 application-focused questions
- Includes multi-step particle counting
- Every answer option is explained
Frequently asked questions
Questions students often ask about the atomic nucleus
These questions connect the particle-counting rules to ideas that often cause confusion later in A-Level Chemistry.
Is the nucleon number the same as relative atomic mass?
No. The nucleon number belongs to one particular isotope and is a whole number because it counts protons and neutrons.
Relative atomic mass is normally a weighted mean that takes account of the naturally occurring isotopes of an element. It therefore need not be a whole number.
Can two different elements have the same nucleon number?
Yes. Two nuclei can contain the same total number of nucleons but different numbers of protons. They are different elements because an element is identified by its proton number, not its nucleon number.
Do isotopes always have the same number of electrons?
Neutral isotopes of the same element have the same number of electrons because they have the same proton number. However, an isotope can also exist as an ion after gaining or losing electrons.
Why does losing an electron not produce another element?
The identity of an element is determined by the number of protons in its nucleus. Losing an electron changes the charge of the particle but leaves the proton number unchanged.
Are the chemical properties of isotopes exactly identical?
For ordinary A-Level Chemistry questions, isotopes are generally treated as having the same chemical properties because they have the same electronic structure.
Small isotope effects can exist in more advanced chemistry because the masses of the nuclei differ, but these are normally outside the scope of this discussion unless information is specifically provided.
Can an ordinary chemical reaction change the nucleus?
No. Ordinary chemical reactions involve the rearrangement, loss, gain or sharing of electrons. The numbers of protons and neutrons in the nuclei remain unchanged.
Why does a neutron travel straight through an electric field?
A neutron has no overall electric charge. It therefore experiences no electrostatic force from the charged plates and is not deflected.
Why does the sign of the charge not appear in the magnitude of the deflection relationship?
The sign determines the direction in which the particle bends. To compare how much different particles bend, we compare the magnitude of the charge, |q|, relative to the mass.
This is why the extent of deflection is related to |q|/m, whereas the positive or negative sign tells us which plate the particle moves towards.
Is angle of deflection always simply proportional to charge divided by mass?
The relationship is used when particles are compared under the same conditions. Factors such as electric-field strength, particle speed and the distance travelled through the field must also be controlled.
With those factors kept constant, a larger magnitude of |q|/m corresponds to greater deflection.
The nucleus provides the starting point for understanding atomic structure. The proton number identifies the element, the nucleon number counts the protons and neutrons in its nucleus, and differences in neutron number produce isotopes. Electrons contribute very little to atomic mass, but gaining or losing them produces ions. With these distinctions secure, we can now move beyond the nucleus and examine how electrons are arranged in energy levels and atomic orbitals.