II · THE DISCOVERY · HISTORY OF SCIENCE
Proton
Every atom in your body has protons at its centre. The number of protons defines what element an atom is: one proton makes hydrogen, six make carbon, eight make oxygen.
At a glance
- Discovered
- 1920
- Discoverer
- Ernest Rutherford
- Where
- University of Manchester
- Symbol
- p, H⁺, or ¹H⁺
- Charge
- +1 e (one elementary charge)
Look closer
The name means 'first'
Rutherford chose the Greek word for 'first' because the hydrogen nucleus — a single proton — was the lightest nucleus known, and because he had extracted it from nitrogen by collision. That suggested protons might be fundamental building blocks of all heavier nuclei, which was a reasonable interpretation in 1920 though the picture has since become more complicated.
The number of protons defines the element
One proton in the nucleus makes hydrogen. Six make carbon. Eight make oxygen. This count — called the atomic number, symbol Z — is what determines which element an atom is. Change the number of neutrons and you have a different isotope of the same element; change the number of protons and you have a different element entirely.
Protons are not elementary particles
When Rutherford named the proton, it was a candidate to be a fundamental particle — something not made of smaller parts. That understanding changed. Protons are now known to be composite: each contains three quarks, two up quarks and one down quark, bound by gluons. The quarks themselves contribute only about one per cent of the proton's mass; the rest comes from the binding energy of the forces holding them together.
The story
A proton is a subatomic particle with a positive electric charge. It sits in the nucleus of every atom, and the count of protons in that nucleus determines what element the atom is.
Ernest Rutherford discovered the proton at the University of Manchester in 1920. In the years before, he had been firing particles at nitrogen and observing what came out. He found that hydrogen nuclei — the lightest nuclei known — could be knocked out of nitrogen by atomic collisions. That suggested hydrogen nuclei might be a component of heavier elements, so Rutherford gave them a name: protons, from the Greek for 'first'.
The proton carries a positive charge of +1 e, where e is the elementary charge — the smallest unit of charge that appears in nature on its own. Its mass is slightly less than a neutron's and roughly 1836 times an electron's mass. Protons and neutrons, which have similar masses, are collectively called nucleons because they are the particles found in atomic nuclei.
The number of protons in a nucleus is called the atomic number, written Z. It is the defining property of an element. Hydrogen has one proton, carbon has six, oxygen has eight. If you add or remove a proton, you change the element. If you add or remove a neutron, you have a different isotope of the same element — the chemistry stays the same but the mass changes.
The protons in a nucleus provide the positive charge that holds the negatively charged electrons in orbit around it. The number of protons determines how many electrons a neutral atom will have, and that in turn determines the atom's chemical behaviour.
When Rutherford named the proton, it seemed likely to be a fundamental particle — something not made of anything smaller. That understanding has changed. Protons are now known to be composite particles. Each proton contains three quarks: two up quarks, each with a charge of +2/3 e, and one down quark with a charge of −1/3 e. The three charges sum to +1 e, the proton's total charge.
The quarks are held together by particles called gluons, which carry the strong nuclear force. The quarks themselves account for only about one per cent of the proton's mass. The rest — the overwhelming majority — comes from the binding energy of the strong force, which includes the kinetic energy of the quarks moving inside the proton and the energy of the gluon fields.
Free protons — protons not bound in a nucleus — will capture electrons if the temperature and energy are low enough. Free protons are used in particle accelerators, including the Large Hadron Collider, and in proton therapy, a form of cancer treatment.
Why it mattered then
The discovery of the proton in 1920 gave physicists a concrete picture of atomic structure. Before Rutherford's work, the nucleus was known to exist — he had demonstrated it in earlier experiments — but its internal composition was not clear. The proton provided a unit: a positively charged particle that could be counted, and whose count determined the element. That count, the atomic number, became the organising principle of the periodic table. Elements had been arranged by increasing atomic weight since the 19th century, but there were awkward exceptions where the order by weight did not match the order by chemical properties. The atomic number resolved those exceptions: the periodic table is ordered by proton count, not mass, and the chemical properties follow from the number of electrons, which in a neutral atom equals the number of protons. Rutherford's extraction of protons from nitrogen also suggested that nuclei were composite — built from smaller parts — which opened the question of what held them together. Protons repel each other electrically, so something else had to be binding them into a stable nucleus. That question led eventually to the theory of the strong nuclear force.
Why it matters now
The proton remains central to chemistry and physics. The atomic number — the count of protons — is still how elements are defined, and the periodic table is still ordered by it. Every chemical reaction, every molecule, every material property depends on how many protons the atoms involved carry, because that determines how many electrons they have and how those electrons behave. Protons are used in medical treatment. Proton therapy directs beams of protons at tumours; because protons deposit most of their energy at a specific depth, the technique can target cancerous tissue while sparing surrounding healthy tissue more effectively than some other forms of radiation. Particle accelerators use protons to probe the structure of matter. The Large Hadron Collider accelerates protons to nearly the speed of light and collides them, producing showers of particles that reveal the forces and particles that govern the subatomic world. The discovery that protons are made of quarks, and that most of their mass comes from binding energy rather than the quarks themselves, emerged from experiments of this kind. The internal structure of the proton is still being studied. Measurements of the proton's size — its charge radius — have produced slightly different values depending on the method used, and the discrepancy is not yet fully explained.
The surprising detail
Most of a proton's mass does not come from the quarks inside it. The three quarks — two up, one down — contribute only about one per cent of the total. The rest comes from the energy of the strong force holding the quarks together: the kinetic energy of the quarks zipping around inside the proton and the energy stored in the gluon fields binding them. Mass and energy are equivalent, as Einstein's equation states, so the binding energy has mass. A proton is, in that sense, mostly binding energy with a small amount of quark.
What is disputed
The proton's charge radius — a measure of its size — has been measured by different methods, and the results do not quite agree. The value is around 0.841 femtometres, but measurements using electrons give one value and measurements using muons (heavier cousins of electrons) give a slightly smaller one. The discrepancy is small but persistent, and physicists do not yet have a settled explanation for it.
Remember this
The number of protons in an atom's nucleus defines what element it is. Change that number and you change the element.
Test yourself
Why does the number of protons determine an element's chemical behaviour, rather than the number of neutrons?
The number of protons determines how many electrons a neutral atom has, because the positive charge of the protons must be balanced by the negative charge of the electrons. Chemistry is governed by how electrons behave — how they are shared or transferred between atoms — so the electron count is what matters. Neutrons add mass and affect nuclear stability, but they do not change the electron count or the charge, so adding or removing neutrons gives you a different isotope of the same element with the same chemistry. Adding or removing protons changes the electron count and therefore the chemistry, which makes it a different element.
Go deeper
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