Constituents of the Atom

An atom has a central $\text{nucleus}$ containing protons and neutrons (collectively called $\text{nucleons}$), surrounded by electrons. The nucleus makes up more than 99.9% of the atom’s mass but only occupies a tiny fraction of its volume — the atom is mostly empty space.

The proton and electron carry equal and opposite charge, $\pm 1.60\times 10^{-19}\text{ C}$, usually written as $\pm e$. The neutron is uncharged. In terms of mass, the proton and neutron are almost identical (about $1.67\times 10^{-27}$ kg, or 1 atomic mass unit), while the electron is roughly $1836$ times lighter.

A nuclide is written ${}_Z^{A}X$, where $Z$ is the $\text{proton number}$ (or atomic number) — the number of protons, which fixes which element it is — and $A$ is the $\text{nucleon number}$ (or mass number) — the total number of protons and neutrons. The number of neutrons is never written directly; you find it by subtraction.

Because it is the proton number that identifies the element, changing the number of neutrons doesn’t change what element you have. Two nuclides with the same $Z$ but different $A$ are $\text{isotopes}$ of each other — chemically identical, but with different masses and often different nuclear stability.

The $\text{specific charge}$ of a particle is its charge divided by its mass, measured in C kg⁻¹. It is not the same as charge — a particle with a small charge can have an enormous specific charge if its mass is tiny enough, and this is exactly what happens with the electron.

Specific charge matters practically too: it is what determines how strongly a charged particle is deflected by an electric or magnetic field (for a given field and speed, the deflection scales with charge-to-mass ratio), which is how experiments like Thomson’s original measurement of the electron identified it as a distinct, very light particle.