Atomic Structure | O-Level Chemistry

Last updated: 28 September 2026

Diagnose → Learn → Master: Every study-note section includes 15 free MCQs — start with 5 Diagnostic MCQs to identify gaps in your understanding, then complete 10 Mastery MCQs near the end to check what you have learned.

Atomic structure is one of the foundations of Chemistry. Before we can understand bonding, the Periodic Table or why atoms form ions, we need to understand what an atom contains and what each sub-atomic particle does.

Many questions in this chapter are really particle-counting problems. If you can keep track of protons, neutrons and electrons, you can work out the identity of an element, the isotope present and the charge on an ion.

The key is not to memorise several unrelated rules. Instead, give each particle a clear role: protons identify the element, protons and neutrons determine the nucleon number, and electrons determine whether the species is neutral or charged.

Diagnostic Quiz

How Well Do You Know Atomic Structure?

Try these five questions before reading the chapter. They are designed to reveal which atomic-structure ideas you already understand and which ones need more attention.

5 diagnostic questions
Atoms, particles and ions
Instant explanation for every answer option

Which sub-atomic particle has a negative charge?

Proton

A proton has a relative charge of +1, so it is positively charged.

Neutron

A neutron has no electrical charge. Its relative charge is 0.

Electron

Correct. An electron has a relative charge of −1.

Nucleus

The nucleus is a region of the atom containing protons and neutrons. It is not one of the three sub-atomic particles being compared.

The three sub-atomic particles have relative charges +1 for a proton, 0 for a neutron and −1 for an electron.

Where are protons and neutrons found in an atom?

In the nucleus

Correct. Protons and neutrons are located together in the nucleus.

In the outermost electron shell

Electron shells contain electrons, not protons and neutrons.

Between the electron shells

Protons and neutrons are concentrated in the nucleus rather than being distributed between electron shells.

Outside the atom

Protons and neutrons are parts of the atom and are found in its central nucleus.

The nucleus contains protons and neutrons. Electrons occupy shells around the nucleus.

A neutral atom contains 12 protons. How many electrons does it contain?

There is no reason to halve the proton number. Neutrality requires equal positive and negative charges.

Twelve protons and ten electrons would give an overall charge of +2, so the species would not be neutral.

Correct. A neutral atom has equal numbers of protons and electrons.

Doubling the proton number does not determine the electron count. A neutral atom requires one electron for each proton.

Each proton contributes +1 and each electron contributes −1. A neutral atom therefore has equal numbers of protons and electrons.

Which particle has a relative mass of approximately 1/1840?

Proton

A proton has relative mass approximately 1.

Neutron

A neutron also has relative mass approximately 1.

Electron

Correct. The electron is much lighter than either a proton or neutron, with relative mass approximately 1/1840.

Nucleon

A nucleon means either a proton or a neutron. Both have relative mass approximately 1.

Almost all the mass of an atom is concentrated in the nucleus because protons and neutrons are much heavier than electrons.

What does the proton number of an atom tell you?

The number of protons in its nucleus

Correct. The proton number is defined as the number of protons in the nucleus.

The total number of protons and neutrons

That is the nucleon number, not the proton number.

The number of neutrons

The neutron number is normally calculated from nucleon number minus proton number.

The total number of all sub-atomic particles

Proton number counts protons only. It does not include neutrons or electrons.

Proton number is especially important because the number of protons determines the identity of an element.

What is inside an atom?

An atom has two main regions. At its centre is a very small nucleus. Surrounding the nucleus are electrons arranged in shells, also called energy levels.

The nucleus contains protons and neutrons. Since these particles are found inside the nucleus, they are collectively called nucleons.

Although the nucleus occupies only a very small part of the atom, almost all of the atom’s mass is concentrated there.

Structure of an atom showing protons and neutrons in the nucleus and electrons arranged in electron shells around the nucleus — A simplified model of atomic structure: protons and neutrons are found in the nucleus, while electrons occupy shells around the nucleus.

The three sub-atomic particles

Particle	Relative charge	Approximate relative mass	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	−1	Approximately 1/1840	Electron shells around the nucleus

Mass and charge come from different particle counts.

Protons and neutrons account for almost all the mass of an atom. Electrical charge, however, depends mainly on the balance between positively charged protons and negatively charged electrons.

Why is a normal atom electrically neutral?

A neutral atom contains the same number of protons and electrons.

Suppose an atom contains 11 protons and 11 electrons. Its protons provide a total charge of +11 while its electrons provide a total charge of −11. These charges cancel:

overall charge = +11 + (−11) = 0

Neutrons do not affect the charge because each neutron has a charge of zero.

Why does the nucleus contain almost all the mass?

A proton and a neutron each have relative mass approximately 1, whereas an electron has relative mass only about 1/1840.

This does not mean that electrons have no mass. It means that their mass is so small compared with protons and neutrons that it is normally neglected in O-Level atomic-mass reasoning.

What do electron shells represent?

At O-Level, electrons are represented as being arranged in shells around the nucleus. These shells represent different energy levels.

The first electron shell can contain up to 2 electrons. For the first 20 elements, the second shell can contain up to 8 electrons.

The circles commonly drawn in shell diagrams are therefore a useful model. They should not be imagined as exact circular tracks followed by electrons like planets orbiting the Sun.

Proton number and nucleon number

Two numbers are especially important when describing an atom: proton number and nucleon number. Their names tell us what is being counted, but they count different things.

Proton number identifies the element

The proton number is the number of protons in the nucleus of an atom.

Every atom of the same element has the same number of protons. For example:

every carbon atom contains 6 protons;
every oxygen atom contains 8 protons;
every sodium atom contains 11 protons.

The proton number therefore acts like the element’s identity. If the number of protons changes, the element itself has changed.

Think of proton number as the identity number of the element.

Neutron numbers can differ between isotopes and electron numbers can change when ions form, but all atoms of one element retain the same proton number.

Nucleon number counts protons and neutrons together

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

nucleon number = number of protons + number of neutrons

Electrons are not included because electrons are not nucleons.

Calculating the number of neutrons

Rearranging the relationship gives:

number of neutrons = nucleon number − proton number

Worked example: sodium-23

Sodium-23 has proton number 11 and nucleon number 23.

The proton number tells us immediately that there are 11 protons.

The number of neutrons is:

23 − 11 = 12 neutrons

Since a sodium-23 atom is neutral, it must also contain 11 electrons.

Proton number: counts protons and identifies the element.

Nucleon number: counts protons + neutrons in one particular atom or isotope.

How to read nuclide notation

Chemists use nuclide notation to show the chemical symbol, nucleon number and proton number together.

Sodium-23 can be represented as:

In this notation:

Na is the chemical symbol;
23 is the nucleon number;
11 is the proton number.

The general form is:

Here, A represents the nucleon number, Z represents the proton number and X represents the chemical symbol.

If the species is an ion, its charge is written at the upper right of the chemical symbol.

How to read nuclide notation showing nucleon number A at the upper left, proton number Z at the lower left, chemical symbol X and ionic charge at the upper right — Nuclide notation shows the nucleon number, proton number, chemical symbol and, where present, the charge on the ion.

A reliable order for interpreting nuclide notation

Rather than looking at all the numbers at once, work in the same order every time:

Read the proton number to find the number of protons.
Subtract proton number from nucleon number to find the number of neutrons.
If the species is neutral, electrons = protons.
If an ion charge is shown, adjust the electron number using the charge.

Worked example: phosphorus-31

The proton number is 15, so the atom contains 15 protons.

The number of neutrons is:

31 − 15 = 16 neutrons

No charge is shown, so the species is neutral and contains 15 electrons.

Isotopes: the same element with different neutron numbers

Atoms of the same element must have the same number of protons. However, they do not necessarily have 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 the neutron numbers differ, their nucleon numbers also differ.

Example: chlorine-35 and chlorine-37

Chlorine has proton number 17. Consider these two atoms:

Isotope	Protons	Neutrons	Electrons
	17	35 − 17 = 18	17
	17	37 − 17 = 20	17

Both atoms are chlorine because both contain 17 protons. They are different isotopes because chlorine-35 contains 18 neutrons whereas chlorine-37 contains 20 neutrons.

Why are they still the same element?

An element is identified by its number of protons, not by its number of neutrons. Since both isotopes have 17 protons, both belong to chlorine.

Why do isotopes have the same chemical properties?

Chemical reactions involve electrons, particularly the electrons in the outermost occupied shell.

Neutral isotopes of the same element have the same proton number and therefore the same number and arrangement of electrons.

At O-Level, isotopes are therefore treated as having the same chemical properties.

Why can their physical properties differ?

Isotopes have different numbers of neutrons, so their masses are different. Physical properties that depend on mass can therefore differ.

Same proton number, different neutron number.

That single idea explains why isotopes remain the same element while having different nucleon numbers.

Atoms and ions: what changes when electrons move?

A neutral atom has equal numbers of protons and electrons. If the atom gains or loses electrons, this balance is disturbed and a charged particle called an ion forms.

During ordinary chemical reactions, the number of protons does not change. Only electrons are gained or lost.

Positive ions form by losing electrons

A positively charged ion is called a cation.

A magnesium atom contains 12 protons and 12 electrons. If it loses two electrons:

Mg → Mg²⁺ + 2e⁻

The Mg²⁺ ion still has 12 protons, but it now has only 10 electrons.

charge = 12 − 10 = +2

Negative ions form by gaining electrons

A negatively charged ion is called an anion.

An oxygen atom contains 8 protons and 8 electrons. If it gains two electrons:

O + 2e⁻ → O²⁻

The O²⁻ ion still has 8 protons but now contains 10 electrons.

charge = 8 − 10 = −2

Positive ion: electrons have been lost, so there are fewer electrons than protons.

Negative ion: electrons have been gained, so there are more electrons than protons.

A useful rule for finding electrons in ions

Start with the proton number. A neutral atom would have the same number of electrons as protons. Then adjust for the charge.

positive ion: electrons = proton number − positive charge

negative ion: electrons = proton number + magnitude of negative charge

The signs make sense if you remember what an electron is: an electron carries negative charge.

Worked example: aluminium-27 ion

Consider:

Proton number = 13, so there are 13 protons.

Number of neutrons:

27 − 13 = 14

A neutral aluminium atom would have 13 electrons. The 3+ charge tells us that three electrons have been lost:

13 − 3 = 10 electrons

Therefore, the ion contains 13 protons, 14 neutrons and 10 electrons.

Worked example: chlorine-35 ion

Proton number = 17, so there are 17 protons.

Number of neutrons:

35 − 17 = 18

The negative charge means one electron has been gained:

17 + 1 = 18 electrons

Therefore, the chloride ion contains 17 protons, 18 neutrons and 18 electrons.

A reliable method for particle-count questions

Questions involving atoms, isotopes and ions can look different, but the same reasoning method works almost every time.

Work in the order: protons → neutrons → electrons.

First use the proton number. Then calculate neutrons from the nucleon number. Only after that should you adjust the electron number for any ionic charge.

Protons	Neutrons	Electrons
11	12	11
12	12	10
17	20	18

You can also check an ionic charge by comparing protons with electrons:

charge = number of protons − number of electrons

Common atomic-structure mistakes

Common mistake	Why it is wrong	Better approach
Treating the nucleon number as the number of neutrons	Nucleon number counts both protons and neutrons.	Use neutrons = nucleon number − proton number.
Including electrons in the nucleon number	Electrons are not nucleons and are outside the nucleus.	Count protons + neutrons only.
Changing the number of protons when an ion forms	Chemical ion formation involves gaining or losing electrons.	Keep the proton number unchanged and adjust only the electron count.
Adding electrons for a positive ion	A positive ion forms because negatively charged electrons were lost.	Subtract electrons for a positive ion.
Subtracting electrons for a negative ion	A negative ion has gained negatively charged electrons.	Add electrons for a negative ion.
Defining isotopes only as atoms with different nucleon numbers	The definition must show that the atoms are of the same element.	State: same number of protons but different numbers of neutrons.
Saying electrons have no mass	Electrons do have mass, but it is very small compared with nucleons.	State that their relative mass is approximately 1/1840 and is often treated as negligible.

Atomic-structure checklist

Sub-atomic particles

Can I state the charge of each particle?
Can I state the approximate relative mass of each particle?
Do I know where each particle is found?
Can I explain why an atom is neutral?

Proton and nucleon numbers

Can I define proton number?
Can I define nucleon number?
Can I calculate the number of neutrons?
Do I know which number identifies the element?

Nuclide notation and isotopes

Can I identify A, Z and X in nuclide notation?
Can I give the full definition of isotopes?
Can I compare the particles in two isotopes?
Can I explain why isotopes have the same chemical properties?

Atoms and ions

Can I distinguish an atom from an ion?
Can I explain how cations form?
Can I explain how anions form?
Can I calculate the electrons in an ion?

Mastery Quiz

Atomic Structure Mastery Quiz

Now apply what you have learned. These questions focus on particle counting, isotopes and ion reasoning rather than simple definition recall.

10 mastery questions
Particle-counting and ion reasoning
Instant explanation for every answer option

Sodium-23 has proton number 11. How many protons, neutrons and electrons are present in a neutral sodium-23 atom?

11 protons, 11 neutrons, 12 electrons

The neutron number is 23 − 11 = 12, and a neutral atom must have 11 electrons rather than 12.

11 protons, 12 neutrons, 11 electrons

Correct. Protons = 11, neutrons = 23 − 11 = 12, and electrons = 11 because the atom is neutral.

23 protons, 12 neutrons, 23 electrons

Twenty-three is the nucleon number, not the proton number.

11 protons, 23 neutrons, 11 electrons

The nucleon number already includes the 11 protons. The number of neutrons is therefore 23 − 11, not 23.

Always work in the order protons → neutrons → electrons.

Magnesium-24 forms Mg²⁺. Magnesium has proton number 12. Which set of particle numbers is correct?

12 protons, 12 neutrons, 10 electrons

Correct. Neutrons = 24 − 12 = 12. Mg²⁺ has lost two electrons, so it has 12 − 2 = 10 electrons.

10 protons, 12 neutrons, 12 electrons

Ion formation does not remove protons. Magnesium still has 12 protons.

12 protons, 10 neutrons, 12 electrons

The neutron count is 24 − 12 = 12. The 2+ charge affects electrons, not neutrons.

12 protons, 12 neutrons, 14 electrons

Fourteen electrons would give a 2− charge. Mg²⁺ has lost two electrons and therefore has only 10.

A positive ion has fewer electrons than protons because electrons have been lost.

Chlorine-37 has proton number 17. How many electrons are present in a chlorine-37 chloride ion, Cl⁻?

Sixteen electrons would give more protons than electrons and hence a positive charge.

Seventeen electrons would make the chlorine species neutral.

Correct. A neutral chlorine atom has 17 electrons, and Cl⁻ has gained one additional electron.

Twenty is the number of neutrons in chlorine-37, not the number of electrons in Cl⁻.

The isotope affects the neutron number. The ion charge determines how the electron count differs from the proton number.

Two atoms contain 8 protons each. One contains 8 neutrons and the other contains 10 neutrons. What is the relationship between the atoms?

They are isotopes of the same element.

Correct. They have the same proton number but different neutron numbers, which is exactly the definition of isotopes.

They are ions of different elements.

The equal proton numbers show that they are the same element. Nothing in the information indicates an ionic charge.

They are different elements because their neutron numbers differ.

Element identity depends on proton number, not neutron number.

They are identical atoms.

They have different neutron numbers and therefore different nucleon numbers, so they are different isotopes.

When comparing possible isotopes, compare proton numbers first. They must be the same before the atoms can be isotopes of one element.

An ion contains 13 protons and 10 electrons. What is its charge?

3−

There are more protons than electrons, so the ion must be positive, not negative.

Correct. Charge = 13 − 10 = +3.

10+

The charge is determined by the difference between protons and electrons, not simply by the electron number.

23+

Adding the particle numbers does not give the charge. Their charges oppose each other.

A quick check is charge = protons − electrons.

An ion X²⁻ has 16 protons. How many electrons does it contain?

Fourteen electrons would mean two fewer electrons than protons, producing X²⁺ rather than X²⁻.

Sixteen electrons would exactly balance the 16 protons, producing a neutral atom.

Correct. A 2− ion has gained two electrons: 16 + 2 = 18.

The ion charge does not require doubling the proton number.

Negative ions contain more electrons than protons because electrons have been gained.

A neutral atom has proton number 19 and nucleon number 39. How many neutrons does it contain?

Nineteen is the proton number. The neutron number must be found by subtraction.

Correct. Neutrons = 39 − 19 = 20.

Thirty-nine is the total number of protons and neutrons, not the neutron count alone.

Adding proton number to nucleon number does not determine neutron number.

Nucleon number = protons + neutrons, so neutrons are found by subtracting the proton number.

Why do two isotopes of the same element have essentially the same chemical properties?

They have identical neutron numbers.

Isotopes specifically have different neutron numbers.

Their neutral atoms have the same electronic arrangement.

Correct. Chemical reactions depend mainly on electrons, and neutral isotopes of one element have the same electron arrangement.

They always have identical masses.

Their masses differ because their neutron numbers differ.

Their nuclei contain no neutrons.

Isotopes may contain neutrons. Their chemical similarity is linked to their electrons rather than the absence of neutrons.

Chemical behaviour is governed by electronic structure. Neutron number mainly changes nuclear mass.

A student says, “Mg²⁺ forms because a magnesium atom gains two protons.” Which correction is best?

Mg²⁺ forms because magnesium gains two neutrons.

Gaining neutrons would change the isotope, not produce the ordinary Mg²⁺ ion.

Mg²⁺ forms because magnesium loses two electrons.

Correct. Removing two negatively charged electrons leaves two more positive charges than negative charges.

Mg²⁺ forms because magnesium loses two protons.

Losing protons would change the proton number and hence the identity of the element.

Mg²⁺ forms because magnesium gains two electrons.

Gaining two electrons would make the species negatively charged, not 2+.

Ordinary ion formation changes the number of electrons while leaving the nucleus unchanged.

Species P has 17 protons, 18 neutrons and 18 electrons. Which description is correct?

It is a neutral atom with nucleon number 35.

The nucleon number is indeed 35, but 17 protons and 18 electrons do not give a neutral species.

It is a 1− ion with nucleon number 35.

Correct. Nucleon number = 17 + 18 = 35, and charge = 17 − 18 = −1.

It is a 1+ ion with nucleon number 36.

There is one more electron than proton, so the charge is negative. Also, nucleon number is 17 + 18 = 35.

It is a 2− ion with nucleon number 18.

The difference between protons and electrons is only one, and nucleon number includes both protons and neutrons.

This question combines nuclear counting and charge counting: nucleon number uses protons + neutrons, while charge compares protons with electrons.

Frequently asked questions

Questions students often ask about atomic structure

These questions bring together some of the details that often become confusing once atoms, isotopes and ions are discussed together.

Are electron shells actual circular paths around the nucleus?

No. The circles drawn in O-Level shell diagrams are a simplified model representing electron energy levels. They should not be interpreted as exact circular tracks followed by individual electrons.

Why are protons and neutrons called nucleons?

They are called nucleons because both particles are located in the nucleus. The nucleon number therefore counts protons and neutrons, but not electrons.

Why is the mass of an electron usually ignored?

An electron has a relative mass of only approximately 1/1840, compared with approximately 1 for a proton or neutron. Its contribution to the total atomic mass is therefore very small.

The electron does have mass; it is simply treated as negligible in most O-Level atomic-mass reasoning.

Can two different elements have the same nucleon number?

Yes. Nucleon number does not identify the element. The proton number does.

Two atoms could contain the same total number of protons and neutrons but different numbers of protons. They would therefore belong to different elements.

Why does changing the neutron number not change the element?

The identity of an element is determined by its proton number. Changing the neutron number changes the isotope and therefore its nucleon number, but the element remains the same as long as the proton number remains unchanged.

Can an isotope also form an ion?

Yes. Isotope identity depends on the numbers of protons and neutrons, whereas ionic charge depends on the balance between protons and electrons.

For example, chlorine-35 can exist as a neutral atom or as a Cl⁻ ion. In either case, its nucleus still contains 17 protons and 18 neutrons.

Why does a positive ion have fewer electrons rather than more protons?

Ordinary chemical reactions involve electrons, not changes to the nucleus. A positive ion therefore forms when negatively charged electrons are removed.

For example, Mg²⁺ still contains 12 protons. It is positive because it contains only 10 electrons.

What is the quickest way to check an ion’s charge?

Compare its numbers of protons and electrons: charge = protons − electrons.

If protons and electrons are equal, the species is neutral. More protons give a positive charge, while more electrons give a negative charge.

Is nucleon number the same as relative atomic mass?

No. The nucleon number counts the actual number of protons and neutrons in one particular atom and is therefore a whole number.

The relative atomic mass shown for an element can reflect a weighted average of its naturally occurring isotopes and may therefore be non-integral.

Atomic structure becomes much easier when each particle is given one clear role. The proton number identifies the element, protons and neutrons together determine the nucleon number, and the balance between protons and electrons determines the charge. Once you apply those ideas in that order, questions involving atoms, isotopes and ions become systematic particle-counting problems rather than separate rules to memorise.