One of the two main building blocks (along with the proton) of the nucleus at the centre of an atom. Neutrons have essentially the same mass as a proton (very slightly larger) but no electric charge, and are made up of one “up” quark and two “down” quarks. The number of neutrons in an atom determines the isotope of an element. Outside of a nucleus, they are unstable and disintegrate within about ten minutes.