Protons

Protons are positively charged particles found in the nucleus.

Properties

• charge of +1
• relative mass of 1
• located in the nucleus

The number of protons determines which element an atom is.

For example:

Changing the number of protons changes the element itself.

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Neutrons

Neutrons are particles with no electrical charge.

Properties

• charge of 0
• relative mass of 1
• located in the nucleus

Neutrons help:

• stabilise the nucleus,
• contribute to the atom's mass.

Atoms of the same element may have different numbers of neutrons. These are called isotopes, which you will study later.

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Electrons

Electrons are negatively charged particles that occupy the electron cloud surrounding the nucleus.

Properties

• charge of –1
• very small relative mass (about 1/1836 of a proton)
• located outside the nucleus

Electrons are responsible for:

• chemical bonding,
• electricity,
• the formation of ions.

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Comparing the Three Particles

This table summarises the key differences between the three particles.

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Why Are Atoms Electrically Neutral?

Most atoms have no overall electrical charge.

This happens because:

• every proton has a charge of +1,
• every electron has a charge of –1.

When the number of protons equals the number of electrons, the positive and negative charges cancel.

Example

Carbon atom:

• 6 protons
• 6 electrons

Overall charge:

+6 + (–6) = 0

The atom is electrically neutral.

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Worked Example 1

An atom contains:

• 8 protons
• 8 electrons

Question

What is its overall charge?

Solution

Positive charge:

+8

Negative charge:

–8

Total:

0

The atom is neutral.

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Determining the Number of Subatomic Particles

The atomic number tells us the number of protons.

For a neutral atom:

• Number of protons = Atomic number
• Number of electrons = Number of protons

The mass number equals:

Mass number = Protons + Neutrons

Therefore:

Neutrons = Mass number − Atomic number

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Worked Example 2

An atom has:

• atomic number = 11
• mass number = 23

Determine the number of:

• protons
• neutrons
• electrons

Solution

Protons:

11

Neutrons:

23 − 11 = 12

Electrons:

11 (because the atom is neutral)

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What Are Ions?

Sometimes atoms gain or lose electrons.

When this happens, they become ions.

An ion is an atom with an overall electrical charge.

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Positive Ions (Cations)

When an atom loses electrons:

• protons outnumber electrons,
• the atom becomes positively charged.

Example:

Sodium atom:

11 protons

11 electrons

↓

Loses one electron

↓

11 protons

10 electrons

Overall charge:

+1

Written as:

Na⁺

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Negative Ions (Anions)

When an atom gains electrons:

• electrons outnumber protons,
• the atom becomes negatively charged.

Example:

Chlorine atom:

17 protons

17 electrons

↓

Gains one electron

↓

17 protons

18 electrons

Overall charge:

–1

Written as:

Cl⁻

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Atoms vs Ions

Remember:

Changing the number of electrons changes the charge.

Changing the number of protons changes the element.

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Worked Example 3

A particle contains:

• 12 protons
• 10 electrons

Question

Is it an atom or an ion?

Solution

Positive charges:

+12

Negative charges:

–10

Overall charge:

+2

Since it has an overall charge, it is an ion, specifically a 2+ cation.

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Worked Example 4

A particle has:

• 17 protons
• 18 electrons

Question

What is its overall charge?

Solution

Positive charges:

+17

Negative charges:

–18

Overall charge:

–1

The particle is a negative ion (anion).

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Common Mistakes

Students often make these errors.

❌ Thinking electrons are inside the nucleus.

✔ Electrons occupy the electron cloud surrounding the nucleus.

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❌ Confusing mass number with atomic number.

✔ Atomic number = protons.

✔ Mass number = protons + neutrons.

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❌ Believing atoms become ions by gaining or losing protons.

✔ Atoms become ions by gaining or losing electrons.

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❌ Forgetting that neutral atoms have equal numbers of protons and electrons.

✔ Equal positive and negative charges give an overall charge of zero.

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Real-World Connection

Ions are essential for life and technology. Sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), and chloride (Cl⁻) ions help transmit nerve impulses, regulate muscle contractions, and maintain fluid balance in the human body. Lithium ions power rechargeable batteries used in smartphones, laptops, and electric vehicles, while calcium ions contribute to the strength of bones and teeth.

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Did You Know?

Although electrons are incredibly small, they move at extremely high speeds around the nucleus. Scientists cannot describe their exact paths like planets orbiting the Sun. Instead, modern quantum theory predicts the probability of finding an electron in different regions of the electron cloud.

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Key Terms

• Proton — a positively charged particle found in the nucleus.
• Neutron — a neutral particle found in the nucleus.
• Electron — a negatively charged particle found in the electron cloud.
• Subatomic particle — a particle smaller than an atom.
• Atomic number — the number of protons in an atom.
• Mass number — the total number of protons and neutrons.
• Ion — an atom that has gained or lost electrons and has an overall charge.
• Cation — a positively charged ion formed by losing electrons.
• Anion — a negatively charged ion formed by gaining electrons.
• Electrically neutral — having equal numbers of positive and negative charges.

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Key Takeaways

• Atoms contain three main subatomic particles: protons, neutrons, and electrons.
• Protons and neutrons are found in the nucleus, while electrons occupy the surrounding electron cloud.
• Protons determine the identity of an element, while electrons determine its electrical charge.
• Neutral atoms contain equal numbers of protons and electrons.
• Mass number equals the total number of protons and neutrons.
• Atoms become ions by gaining or losing electrons, not by changing the number of protons.
• Understanding subatomic particles provides the foundation for studying isotopes, chemical bonding, and chemical reactions.