Atomic Structure: What's Inside Every Atom?

Chemistry · Atomic Structure — revision notes

Everything around you — your phone, the air you breathe, your own body — is built from atoms. And every single atom is built from just three types of particle, arranged the same basic way every time. Once you know those three particles and one piece of shorthand notation, you can read off the full "recipe" of any atom or ion just by looking at its symbol. That's the whole game of this topic, and by the end of this guide you'll be able to do it on sight.

Meet the Three Subatomic Particles

Before the details, ask: why do chemists even bother splitting the atom into three particles instead of treating it as one blob? Because those three particles behave completely differently — where they sit, what charge they carry, how heavy they are — and that is what explains almost everything else in chemistry (bonding, ions, isotopes, all of it). So getting these three particles solid now pays off for the rest of the course.

Picture an atom like a tiny solar system: a packed, heavy centre with something much lighter moving around it. The centre is called the nucleus (the dense core of the atom), and it's home to two of the three particles.

Protons live inside the nucleus. Each one carries a relative charge of +1 and a relative mass of 1.

Neutrons also live inside the nucleus, packed in right next to the protons. Each one carries no charge at all (relative charge 0 — hence "neutr-on") but still has a relative mass of 1, the same as a proton.

Electrons are the odd ones out: they don't live in the nucleus at all. They move around outside it, in regions called shells (energy levels — zones where electrons are likely to be found, not solid tracks like a race track). Each electron carries a relative charge of −1, and here's the detail students often lose marks on: an electron's mass is not zero, but negligible — about 1/1840 of a proton's mass. It's so small compared to a proton or neutron that for O-Level calculations we treat it as 0. You never need the exact fraction for marks; "negligible" or "≈ 0" is all that's examined.

Quick memory trick: ProTon = Positive. NeuTron = Neutral. Electron = nEgativE.

Your turn. A particle has a relative charge of 0 and a relative mass of 1. Which subatomic particle is it? Hint (only if stuck): use the memory trick — which word means "no charge"?

Answer - have a real go first. It's a neutron. Charge 0 rules out protons (+1) and electrons (−1); mass 1 rules out electrons (negligible). The trap here is forgetting that neutrons and protons share the same relative mass (1) — students sometimes assume "neutral" must also mean "light," but charge and mass are two separate properties.

The Structure of an Atom

Now put the three particles together into one atom. Since protons and neutrons both live in the nucleus, the nucleus ends up dense and heavy (that's where basically all the mass is). The electrons then arrange themselves around that nucleus in layers — like the layers of an onion — and each layer is called a shell.

A carbon atom: the nucleus (6 protons + 6 neutrons) sits in the centre; the electrons sit in shells around it (2 in the inner shell, 4 in the outer). Remember the shells are regions where electrons are found — energy levels — not solid rings the electrons ride on.

Here's a fact worth pausing on: a neutral atom (one with no overall charge) always has exactly as many electrons as protons. Why must that be true? Because charge has to balance to zero — each proton's +1 needs an electron's −1 to cancel it, so the two counts must match exactly. (You'll see in a later section what happens when they don't match — that's how ions form.)

Why don't the electrons just fly off, given they're constantly moving? The nucleus is positively charged and the electrons are negatively charged, and opposite charges attract. That electrostatic pull is what holds the atom together — the same basic idea as gravity holding planets in orbit around the Sun, except here it's electric attraction instead of gravity, and it works over unimaginably tiny distances.

Your turn. An atom of fluorine has 9 protons. If it is neutral, how many electrons does it have? Hint (only if stuck): what has to happen to the total charge for it to be neutral?

Answer - have a real go first. 9 electrons. Neutral means total charge = 0, and the only way 9 protons (each +1) can be cancelled out is by exactly 9 electrons (each −1). A common slip is to assume "neutral" means zero electrons — but neutral means the charges cancel out, not that there are none of them.

Proton Number and Nucleon Number

You now know an atom has protons, neutrons and electrons — but chemists need a fast way to say exactly which atom they mean without listing every particle each time. Two numbers do that job.

Proton Number (Atomic Number) — symbol Z. This is simply the number of protons in the nucleus. It's the number that identifies the element: every carbon atom in the universe has exactly 6 protons, every oxygen atom exactly 8. Change the proton number and you've changed the element entirely — it's not carbon any more, it's something else.

Nucleon Number (Mass Number) — symbol A. This is the total count of protons plus neutrons in the nucleus. Protons and neutrons are collectively called nucleons (particles found in the nucleus) — that's where the name comes from. Electrons are never counted in A: they don't live in the nucleus, and their mass is negligible anyway, so leaving them out doesn't lose any accuracy.

Exam answer — how to earn the 'define' mark for outcome (c): these are 'define' questions, so a bare number or one word will NOT earn the mark — write a full sentence. - Proton number = the number of protons in the nucleus of an atom. - Nucleon number = the total number of an atom's nucleons — that is, its protons plus its neutrons combined.

Your turn. An atom has 17 protons and 18 neutrons. What are its proton number and its nucleon number? Hint (only if stuck): nucleon number adds two things together — which two?

Answer - have a real go first. Proton number Z = 17 (just the proton count). Nucleon number A = 35 (protons + neutrons = 17 + 18). The most common mistake here is writing A as "18" — copying the neutron count instead of adding protons and neutrons together; A is always the combined total, and for this reason A is almost always the bigger of the two numbers.

Nuclide Notation — Reading the Shorthand

Writing "17 protons, 18 neutrons" every time is slow, so chemists compress Z and A into one compact symbol called nuclide notation. A nuclide is a specific type of atom defined by its proton and nucleon numbers, and it's written like this:

 A
 Z X

Where: - X is the chemical symbol of the element. - A (top number) is the nucleon number (mass number) — total protons + neutrons. - Z (bottom number) is the proton number (atomic number) — number of protons.

You'll also see the same information written as X-A, for example C-12 means carbon with a nucleon number of 12. Both notations carry the same information; nuclide notation just also shows Z explicitly.

The top-left number (A) is the nucleon number; the bottom-left number (Z) is the proton number. Subtract them (A − Z) to get the neutrons.

Worked Example

Let's decode carbon-12, written as ¹²₆C, one piece at a time.

• Read off Z first, since it's given directly: Z = 6, so there are 6 protons — no calculation needed, it's just the bottom number.
• Read off A next, also given directly: A = 12, meaning protons + neutrons together = 12.
• Now find neutrons by subtraction, because A already includes the protons we just counted, so removing them leaves only the neutrons: neutrons = A − Z = 12 − 6 = 6 neutrons.
• Finally, electrons: the question doesn't say "ion," so this is a neutral atom, and a neutral atom always has electrons = protons. So electrons = 6.

That gives the general formula you'll use constantly:

Number of neutrons = A − Z (nucleon number minus proton number)

Your turn. Sodium is written as ²³₁₁Na. How many protons, neutrons, and electrons does a neutral sodium atom have? Hint (only if stuck): find protons and neutrons first, then use "neutral = electrons equal protons" for the last one.

Answer - have a real go first. 11 protons, 12 neutrons, 11 electrons. Protons come straight from Z (11); neutrons = A − Z = 23 − 11 = 12; electrons = protons because it's neutral (11). The most common error is subtracting the wrong way round (Z − A), which gives a negative number — a good sign something's gone wrong, since neutron count can never be negative.

Isotopes

Here's a question worth sitting with: can two atoms both genuinely be "carbon," yet not be identical? Yes — and working out how is exactly what isotopes are about.

Isotopes are atoms of the same element (same proton number, Z) that have different numbers of neutrons — and therefore different nucleon numbers, A, even though the element itself hasn't changed.

Analogy: picture identical twins — same face, same family, same "identity" (same element, same proton number) — but one is carrying a heavy backpack the other isn't (different number of neutrons, so different mass). They're still unmistakably the same person's twin; carbon-12 and carbon-14 are still unmistakably carbon.

Example: Isotopes of Carbon

All three are carbon (6 protons each), but the number of neutrons rises from 6 to 7 to 8. Same element, different mass — that's what makes them isotopes.

Notice what stays fixed and what changes: the proton number (6) never moves — that's why all three are still carbon — while only the neutron count climbs.

This matters for a reason that trips a lot of students up: because all three isotopes have the same number of electrons, and chemical behaviour is driven entirely by electrons (not by the nucleus), isotopes of an element have identical chemical properties. They differ only in mass, not in how they react.

Your turn. Chlorine has two common isotopes: ³⁵₁₇Cl and ³⁷₁₇Cl. Do they react the same way chemically? Why or why not? Hint (only if stuck): what actually controls chemical reactions — protons, neutrons, or electrons?

Answer - have a real go first. Yes, they react identically. Both have 17 protons, so both have 17 electrons (neutral atoms) — and since electrons control chemical behaviour, matching electron counts means matching chemistry, regardless of the extra neutrons. The common wrong answer is "no, because they have different masses" — mass affects things like density or rate of diffusion, but it does not affect which reactions happen or how, because reactions are an electron story, not a nucleus story.

Working Out Particles in Ions

So far every atom has been neutral. But atoms can lose or gain electrons to become ions — atoms with an overall electric charge. This sounds like it might complicate everything, but actually only one of the three particle counts ever changes.

The key rule to hold onto: protons and neutrons are unaffected when ions form — the nucleus doesn't change at all. Only the electron count changes. That makes sense once you think about how ions form: gaining or losing an electron is easy (electrons sit loosely in outer shells), but gaining or losing a proton or neutron would mean physically altering the nucleus — that's a nuclear reaction, not ordinary chemistry.

From there, the direction of the change follows logically: - A positive ion has fewer electrons than protons, because losing negative charge is what makes something more positive. - A negative ion has more electrons than protons, because gaining negative charge is what makes something more negative.

How to find the number of electrons in an ion

Start from the number of protons (= Z, and this never changes for an ion). Then adjust for the charge: - For a charge of +n (it lost n electrons): electrons = Z − n - For a charge of −n (it gained n electrons): electrons = Z + n

Rather than memorising a signed formula, it's more reliable to reason it through: ask "what did this ion do?" — positive means it lost electrons (so subtract), negative means it gained electrons (so add).

Worked Example

Magnesium ion: ²⁴₁₂Mg²⁺

• Z = 12, so there are 12 protons — and since the nucleus is unaffected by ion formation, this is true whether or not it's an ion.
• A = 12 + neutrons = 24, so neutrons = 24 − 12 = 12 neutrons — same subtraction as for a neutral atom, because A and Z never account for charge in the first place.
• The charge is 2+, which means it lost 2 electrons (losing negative charge makes it more positive), so electrons = 12 − 2 = 10 electrons.

Oxide ion: ¹⁶₈O²⁻

• Z = 8 → 8 protons.
• A = 16, neutrons = 16 − 8 = 8 neutrons.
• The charge is 2−, which means it gained 2 electrons (gaining negative charge makes it more negative), so electrons = 8 + 2 = 10 electrons.

Notice something neat: both ions above end up with 10 electrons, even though they started from different proton counts — this is completely normal and not a coincidence to be suspicious of; it just means both ions happen to have the same electron arrangement.

Your turn. A chloride ion is written as ³⁵₁₇Cl⁻. How many protons, neutrons, and electrons does it have? Hint (only if stuck): protons and neutrons come from A and Z exactly as normal — only the electron step changes for a charged particle.

Answer - have a real go first. 17 protons, 18 neutrons, 18 electrons. Protons = Z = 17 (unaffected by the ion); neutrons = A − Z = 35 − 17 = 18 (also unaffected); electrons = 17 + 1 = 18, because a 1− charge means it gained one electron. The most common mistake is subtracting instead of adding for a negative charge — remembering "negative ion, gained electrons, so more electrons than protons" avoids the mix-up.

Putting It All Together

You now know: 1. The three particles, where they live, their charges and masses. 2. How the atom is structured — nucleus in the middle, electrons in shells, held together by electrostatic attraction. 3. What proton number (Z) and nucleon number (A) mean, and why nucleon number is a sum but proton number isn't. 4. How to read nuclide notation like ²³₁₁Na or Na-23, and how to unpack it step by step. 5. What isotopes are, why same-element atoms can still differ, and why their chemistry stays identical. 6. How to find particle numbers for ions by reasoning through what gaining or losing electrons actually means.

That's the whole topic — and you've got this.

Key formulas

• Nucleon number (A) = number of protons + number of neutrons
• Number of neutrons = A − Z, where A is the nucleon number (mass number) and Z is the proton number (atomic number)
• Electrons in a neutral atom = Z (proton number), because equal protons and electrons means the charges cancel
• Electrons in a positive ion with charge n+ = Z − n (lost n electrons); electrons in a negative ion with charge n− = Z + n (gained n electrons)

Common mistakes

• Mixing up mass number (A) and atomic number (Z) — remember Z is at the bottom and stands for the proton count; A is at the top and is always the bigger number for most elements.
• Forgetting that neutrons are found by subtracting Z from A, not the other way round.
• Thinking isotopes have different chemical properties — they don't, because chemistry depends on electron number, which stays the same.
• Forgetting to adjust electron count for ions — a neutral atom and its ion have the same protons and neutrons, but different numbers of electrons.
• Giving electrons a relative mass of 1 — their mass is negligible (about 1/1840), so it's effectively 0 for these calculations.

Exam tips

• Always write out Z and A clearly when answering structure questions — examiners want to see you know which is which.
• For a 'define' question on proton number or nucleon number, write the full sentence (see the callout above) — a bare number or one word will not earn the mark.
• For isotope questions, state explicitly that the proton number is the same AND the nucleon number (or neutron number) is different — both parts are needed for full marks.
• When finding particles in an ion, work in order: protons first (= Z), then neutrons (= A − Z), then electrons last (adjust for charge). This order stops you making errors.
• If a question says 'nuclide notation', it wants the full symbol with A on top and Z on the bottom — not just the name or the mass number alone.
• Double-check your neutron calculation by adding your protons and neutrons back together — they must equal A.