Neutron

Neutrons are neutral particles (charge = 0) found in the nucleus of every atom except hydrogen-1 (protium).

Relative mass: 1 (same as a proton). Actual mass: $1.675 \times 10^{-27}$ kg.

The number of neutrons can be found by subtracting the proton number from the nucleon number:

$N = A - Z$

Changing the number of neutrons creates a different isotope of the same element.