Protons, Neutrons, and Electrons in Atoms, Ions, and Isotopes

Hello. In the previous Chemistry lesson, you used the kinetic particle model to explain matter in terms of particles, their motion, energy, and forces of attraction. Now we zoom in on one of those particles: the atom.

This lesson develops the counting rules you need for atomic-structure questions in Cambridge O Level Chemistry. By the end, you should be able to determine the numbers of protons, neutrons, and electrons in a neutral atom, an isotope, or an ion—and work backwards when some information is missing.

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Inside an atom

Atoms have a tiny central nucleus containing protons and neutrons. Electrons occupy shells around the nucleus. The shell rings in textbook diagrams are a useful model for showing electron arrangement; they are not literal circular tracks followed by electrons.

The three subatomic particles have these essential properties:

Particle	Location	Relative charge	Relative mass
Proton	Nucleus	$+1$	$1$
Neutron	Nucleus	$0$	$1$
Electron	Shells outside nucleus	$-1$	Very small

Two memory links are useful:

• Proton begins with “p,” like positive.
• Neutron begins with “n,” like neutral.

Because electrons have a negligible mass compared with protons and neutrons, an atom’s mass number depends on its nucleus, not its electrons.

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The two numbers that unlock particle counts

Every element has an atomic number, usually represented by $Z$. It tells you the number of protons:

$$\text{number of protons}=Z$$

The number of protons defines the element itself. For instance, every atom with $17$ protons is chlorine. Changing the number of neutrons or electrons does not turn chlorine into another element; changing its proton number would.

The mass number, represented by $A$, is the total number of protons and neutrons:

$$A=\text{number of protons}+\text{number of neutrons}$$

Therefore, calculate neutrons by subtracting:

$$\text{number of neutrons}=A-Z$$

Nuclear notation puts these numbers around an element symbol:

$${}_Z^A\mathrm{X}$$

• The upper-left number is the mass number, $A$.
• The lower-left number is the atomic number, $Z$.
• The central symbol, $\mathrm{X}$, identifies the element.

For sodium-23:

$${}_{11}^{23}\mathrm{Na}$$

Sodium has $11$ protons because $Z=11$. It has:

$$23-11=12$$

neutrons.

Because no charge is shown, this is a neutral sodium atom. Its $11$ positive protons are balanced by $11$ negative electrons. Its complete particle count is:

Particle	Number
Protons	$11$
Neutrons	$12$
Electrons	$11$

A frequent exam error is to use the relative atomic mass from the periodic table as though it were a mass number. Relative atomic mass is often a decimal because it is an average for the element’s naturally occurring isotopes. A mass number is always a whole number and refers to one particular atom or isotope.

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Neutral atoms and ions: electrons determine charge

A neutral atom has equal numbers of protons and electrons. Its total positive charge cancels its total negative charge:

$$\text{number of electrons in a neutral atom}=Z$$

An ion is a charged atom formed when electrons are gained or lost.

• A positive ion, or cation, has lost electrons. It therefore has fewer electrons than protons.
• A negative ion, or anion, has gained electrons. It therefore has more electrons than protons.

In general:

$$\text{charge}=\text{number of protons}-\text{number of electrons}$$

You can rearrange this to calculate electrons:

$$\text{number of electrons}=Z-\text{charge}$$

Here, treat the charge as signed. A negative charge makes the subtraction become addition.

Example: an aluminium ion

Consider:

$${}_{13}^{27}{\mathrm{Al}}^{3+}$$

Start with the nucleus:

$$\text{protons}=13$$ $$\text{neutrons}=27-13=14$$

The $3+$ charge means the atom has lost three electrons:

$$\text{electrons}=13-3=10$$

So the aluminium ion contains $13$ protons, $14$ neutrons, and $10$ electrons.

Example: a chloride ion

Now consider:

$${}_{17}^{35}{\mathrm{Cl}}^{-}$$

First find the particles in the nucleus:

$$\text{protons}=17$$ $$\text{neutrons}=35-17=18$$

The charge is $1-$, meaning there is one extra electron:

$$\text{electrons}=17-(-1)=18$$

So a chloride ion contains $17$ protons, $18$ neutrons, and $18$ electrons.

The important scientific point is that ions form by changing electrons only. A chloride ion is still chlorine because it still has $17$ protons. Its neutron count is also unchanged when it gains an electron.

A quick charge check prevents many errors:

Particle comparison	Overall charge
Protons = electrons	Neutral
Protons > electrons	Positive
Electrons > protons	Negative

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Isotopes: same element, different neutron number

Isotopes are atoms of the same element with different numbers of neutrons.

Since all isotopes of an element have the same atomic number, they have the same number of protons. Neutral atoms of those isotopes also have the same number of electrons. Their mass numbers differ because their neutron numbers differ.

For the two chlorine isotopes:

$${}_{17}^{35}\mathrm{Cl}$$ $${}_{17}^{37}\mathrm{Cl}$$

Isotope	Protons	Neutrons	Electrons in neutral atom
Chlorine-35	$17$	$35-17=18$	$17$
Chlorine-37	$17$	$37-17=20$	$17$

An isotope is not an ion:

• An isotope differs in its number of neutrons.
• An ion differs in its number of electrons.

An atom can, however, be both an isotope and an ion. The chloride ion ${}_{17}^{35}{\mathrm{Cl}}^{-}$, for example, is chlorine-35 and has an ionic charge.

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A reliable exam method

For any particle-count question, use the same order every time:

1. Identify the atomic number, $Z$, from the nuclear symbol, question, or periodic table.
2. Use $Z$ to find the number of protons.
3. Find neutrons using $A-Z$.
4. Check whether the particle is neutral or charged.
5. Calculate electrons from the charge.
6. Verify the answer: proton number minus electron number must equal the stated charge.

For example, suppose a particle has mass number $56$, atomic number $26$, and charge $2+$.

$$\text{protons}=26$$ $$\text{neutrons}=56-26=30$$ $$\text{electrons}=26-2=24$$

The particle has $26$ protons, $30$ neutrons, and $24$ electrons. Its $2+$ charge makes sense because it has two more protons than electrons.

You may also encounter reverse questions. If an ion has $18$ electrons and a $4+$ charge, it must have $22$ protons: four more protons than electrons. Its atomic number is therefore $22$, which identifies it as titanium.

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Key takeaways

The atom’s three particle counts are governed by a small set of rules:

$$\text{protons}=\text{atomic number}$$ $$\text{neutrons}=\text{mass number}-\text{atomic number}$$ $$\text{electrons}=\text{atomic number}-\text{signed charge}$$

Remember the distinctions:

• Protons determine the element.
• Neutrons distinguish isotopes.
• Electrons determine whether the particle is neutral or charged.
• Neutral atoms have equal numbers of protons and electrons.
• Positive ions have lost electrons; negative ions have gained electrons.

Next, the course moves to Physics, where you will select suitable measuring instruments and record measurements using appropriate units and precision.