Subatomic particles
Split a grain of gold, a drop of water, or a breath of air finely enough and you always arrive at the same three building blocks: protons, neutrons and electrons. These are the subatomic particles — the pieces smaller than the atom itself — and every one of the roughly hundred-odd elements is just a different recipe of the same three ingredients, mixed in different numbers. This article opens the atom up and asks what is inside it: which particles carry charge, where each one sits, and how scientists first found them. You will see why a neutral atom carries no overall charge even though it is built entirely from charged particles, and why the electron, proton and neutron were discovered decades apart rather than all at once. Keep the picture classical — nucleus at the centre, electrons in the space around it — with no orbits or shell numbers yet; those come later.
An atom is built from three subatomic particles — the positive proton and neutral neutron packed into a central nucleus, and the negative electron occupying the space around it — and a neutral atom has no overall charge only because its proton count and electron count are exactly equal.
The core idea
Every atom, however large or small, is assembled from just three fundamental particles, and the properties that matter at this stage are charge and location.
- Electron — carries a negative charge, has a mass so small it is almost negligible, and is found in the space surrounding the nucleus, not inside it.
- Proton — carries a positive charge, has a mass close to atomic mass unit, and sits inside the nucleus.
- Neutron — carries no charge at all (it is electrically neutral), has a mass close to atomic mass unit, and also sits inside the nucleus.
A quick memory hook: "proton" sounds like "positive," "neutron" sounds like "neutral," and the electron is left with the remaining negative charge. Because protons and neutrons are both far heavier than electrons and are both found inside the nucleus, they are often grouped together as nucleons; the electron is the odd one out, both in mass and in location. An atom as a whole is electrically neutral. This is not because it lacks charged particles — it is packed with them — but because the number of protons and the number of electrons are always exactly equal in a neutral atom, so their opposite charges cancel:
where: the number of protons is fixed for a given element and counts the positive charges in the nucleus, and the number of electrons counts the negative charges around it; when these two counts match, the atom's net charge works out to zero. Neutrons play no part in this balance at all — they add mass to the nucleus but carry no charge, so changing the number of neutrons never changes whether an atom is neutral.
Why it works
Two things follow directly from this picture, and both are worth reasoning through rather than memorising. First, why does a neutral atom show no charge at all, when it is made of charged particles? Charge cancellation works like a tug of war with equal teams: every proton's positive charge is matched by one electron's negative charge, so the pulls cancel exactly, leaving zero net charge. Add or remove a proton without matching it with an electron and the atom would stop being neutral — but as long as the two counts move together, the cancellation holds no matter how many neutrons are also present, since neutrons carry no charge to begin with. Second, why were the three particles found so far apart in time — electron in 1897, proton in the early 1920s, neutron only in 1932? The answer is about detectability, not importance. Charged particles push and bend under electric and magnetic fields, so J. J. Thomson could deflect a beam of electrons with a magnet and measure them directly; Ernest Rutherford could likewise trace the positively charged proton through its interactions in the nucleus. A neutron, carrying no charge, does not respond to electric or magnetic fields at all, so it leaves no such trail — James Chadwick had to infer its existence indirectly, from how it knocked other particles around, which took far more ingenuity and decades longer to work out.
Worked example
Question: An atom of the element sodium has 11 protons in its nucleus. State how many electrons it has if it is electrically neutral, and use the three subatomic particles to explain why the atom overall carries no charge even though 11 neutrons are also present in the same nucleus.
Step 1. Identify what "electrically neutral" requires. For any neutral atom, the number of electrons must equal the number of protons, so an atom with 11 protons must also have exactly 11 electrons.
Step 2. Check that the charges cancel. Each of the 11 protons contributes unit of charge and each of the 11 electrons contributes unit of charge, so the total charge is . The atom is neutral because these equal and opposite charges cancel exactly, not because it lacks charged particles.
Step 3. Explain the role of the neutrons. The 11 neutrons sitting alongside the protons in the nucleus carry no charge of their own, so they do not enter the charge calculation at all — they only add mass to the nucleus. Removing or adding neutrons would change the atom's mass but would never by itself make the atom charged.
The atom has 11 electrons, and it is neutral because its 11 protons and 11 electrons carry equal and opposite charges that cancel, while its neutrons contribute no charge at all.
Competitive stretch
Question: Match each particle to its charge, its location, and its discoverer: proton, neutron, electron.
Step 1. Sort by location first. Protons and neutrons are both found inside the nucleus; the electron alone is found in the space around the nucleus.
Step 2. Sort by charge. Proton is positive, neutron carries no charge, electron is negative.
Step 3. Attach the discoverer and rough order. Electron — J. J. Thomson, 1897. Proton — Ernest Rutherford, early 1920s. Neutron — James Chadwick, 1932, discovered last because it is uncharged and therefore hardest to detect. This gives the complete match: proton (positive, nucleus, Rutherford), neutron (neutral, nucleus, Chadwick), electron (negative, around the nucleus, Thomson).
Common traps
- Swapping the charges — remembering "proton = positive, neutron = neutral, electron = negative" by the sound of the names fixes this instantly.
- Saying an atom has "no charged particles" instead of "equal numbers of oppositely charged particles that cancel out."
- Assuming neutrons affect the atom's charge — they never do; they only add mass located in the nucleus.
- Getting the discovery order backwards — it is electron (1897), then proton (early 1920s), then neutron (1932), in that order, because uncharged particles are hardest to detect.
Ask the tutor: want to practise matching more elements' proton and electron counts, or hear the fuller story of how Chadwick tracked down the uncharged neutron?
If neutrons have no charge, what is the point of having them in the atom at all
Neutrons add mass and stability to the nucleus without disturbing its charge balance. A nucleus packed only with protons would have all-positive charges crowded together with nothing neutral in between, which affects how tightly the nucleus can hold together. For this stage, just remember that neutrons contribute to an atom's mass, sitting in the nucleus alongside protons, but never to its charge.
Why do we say electrons are found around the nucleus instead of inside it
Electrons are far lighter than protons and neutrons and are not held in the same dense, central clump. Experiments consistently show negative charge distributed in the space surrounding the nucleus rather than packed into it, which is why protons and neutrons are described as being inside the nucleus while electrons occupy the region around it. The exact shape of that surrounding space is a topic for later classes; for now, "around the nucleus" is the correct and sufficient description.
Do I need to remember the exact years the particles were discovered
Knowing the correct order — electron first in 1897, then proton in the early 1920s, then neutron in 1932 — matters more than memorising the exact years. What is worth remembering is why that order makes sense: charged particles are easier to detect with electric and magnetic fields, so the electron and proton were found earlier, while the uncharged neutron took much longer to confirm because it leaves no such trail.