Neutrons have size (about 10-13 cm in diameter) and mass (1.6749 x 10-27 kg).
Protons and Neutrons
At the simplest level of description, a nucleus contains protons and neutrons.
Number of protons PLUS number of neutrons.
The mass number of an element is equal to the sum of the elements neutrons and protons.
Protons and neutrons make up the nucleus of an atom. Protons have a positive charge, and neutrons have no charge. Electrons orbit around the nucleus.
Protons and electrons are both smaller than neutrons. Neutrons are composed of three quarks, while protons have two up quarks and one down quark, and electrons are fundamental particles with no substructure.
Two isotopes have two neutrons: 3H and 4He.
It is helium, specifically the helium-4 isotope, that is composed of two protons and two neutrons.
An alpha particle is composed of two protons and two neutrons.
There are just two things that make up atomic nuclei, and those are, protons and neutrons. All nuclei have these, except for the hydrogen one isotope which contains only one proton and no neutrons.
A helium atom typically has two neutrons. This gives helium a total of four particles in its nucleus (two protons and two neutrons), which contributes to its atomic mass of approximately 4 atomic mass units.
Helium has two protons, and, in its neutral state, two electrons. It can have either one or two neutrons.