Proton
Definition
A proton is a particle that carries a positive charge among the particles that make up the atomic nucleus. The charge of a single proton is the elementary charge $+e$.
Explanation
Protons, together with neutrons, make up the atomic nucleus. The protons and neutrons in the atomic nucleus are collectively called nucleons. The main properties of the proton are as follows1.
| Property | Value |
|---|---|
| Charge | $+e$ |
| Mass | $m_{\mathrm{p}} \approx 1.672\ 62 \times 10^{-27}\ \mathrm{kg}$ |
| Ratio to electron mass | $m_{\mathrm{p}} / m_{\mathrm{e}} \approx 1836.15$ |
| Charge radius | about $0.84\ \mathrm{fm} = 8.4 \times 10^{-16}\ \mathrm{m}$ |
Here $1\ \mathrm{fm} = 10^{-15}\ \mathrm{m}$. A proton is about $1836$ times heavier than an electron, and has almost the same mass as a neutron. This is why most of the mass of an atom is concentrated in the atomic nucleus.
Atomic Number
The number of protons in the atomic nucleus is called the atomic number and is denoted by $Z$. Which element an atom is, is determined solely by the number of protons. The number of electrons changes when the atom becomes an ion, and the number of neutrons differs from isotope to isotope, but if the number of protons changes, it becomes a different element. For example, an atom with $1$ proton is hydrogen, with $6$ protons is carbon, and with $8$ protons is oxygen. An electrically neutral atom has the same number of protons and electrons, so it also has $Z$ electrons.
Structure
The electron is an elementary particle with no known smaller constituents, but the proton is not. The proton is a composite particle made up of two up quarks and one down quark. Since the charge of an up quark is $+\dfrac{2}{3}e$ and the charge of a down quark is $-\dfrac{1}{3}e$, the charge of the proton is as follows.
$$ \frac{2}{3}e + \frac{2}{3}e - \frac{1}{3}e = +e $$
See Also
미국 국립표준기술연구소: https://physics.nist.gov/cgi-bin/cuu/Value?mp ↩︎