Proton

The proton is the positively charged particle in the Atomic nucleus. How many protons an Atom has is what element it is: one proton makes hydrogen, six makes carbon, 92 makes uranium. It carries exactly the opposite Electric charge of an Electron, which is why ordinary matter is electrically neutral.

A proton is not fundamental. It is three Quarks — two up and one down — bound by the strong force.

Properties

property	value	note
electric charge	+1 e = +1.602 176 634 × 10⁻¹⁹ C	exact by SI definition since 2019
rest mass	1.672 621 925 95 × 10⁻²⁷ kg	CODATA 2022
rest mass (energy)	938.272 089 43 MeV	same mass, physicists' units
rest mass (atomic units)	1.007 276 466 621 u	PDG 2024
mass vs the electron	1836.152 673 426 × heavier	CODATA 2022
rms charge radius	8.407 5 × 10⁻¹⁶ m = 0.840 75 fm	CODATA 2022
magnetic moment	2.792 847 344 6 μN	PDG 2024
spin	½	a fermion
quark content	u u d	PDG 2024
mean life	no decay ever seen	see below

Almost all of its mass is not its parts

Add up the three quarks using the PDG's 2024 masses:

piece	mass
up quark	2.16 MeV
up quark	2.16 MeV
down quark	4.70 MeV
sum of quarks	9.02 MeV
the proton	938.27 MeV

The quarks account for about 1 % of the proton. The other 99 % is energy — the motion of the quarks and the gluon field binding them — showing up as mass through E = mc². The proton is mostly not made of stuff. It is made of a knot in a force field.

Does it ever decay?

Nobody has ever watched a proton fall apart, and people have watched very hard. Detectors full of thousands of tonnes of water sit underground waiting for a single event. The limits, from PDG 2024:

• > 9 × 10²⁹ years for the proton simply vanishing into invisible products.
• > 2.4 × 10³⁴ years for the classic grand-unified prediction p → e⁺ π⁰.

For scale, the universe is 13.797 ± 0.023 billion years old (PDG 2024, Astrophysical Constants). That second limit is over a trillion trillion times longer.

Proton decay is not ruled out — many grand unified theories require it. It just has not happened where anyone was looking.

Where to read next

• Neutron — the proton's neutral partner, and what turns one into the other
• Quark — the three pieces inside
• Atomic nucleus — how protons and neutrons stick together despite repelling each other
• Isotope — same proton count, different neutron count