Atomic mass more significant figures than atomic weights. Standard atomic weight is related to atomic mass by the abundance ranking of isotopes for each element. It is usually "about" the same value as the atomic mass of the most abundant isotope, other than what looks like (but is not actually) a rounding difference. The atomic mass of atoms, ions, or atomic nuclei is slightly less than the sum of the masses of their constituent protons, neutrons, and electrons, due to binding energy mass loss (as per "E"="mc"). Relative "isotopic" mass (a property of a single atom) is not to be confused with
Neutron 1942) and the first nuclear weapon (Trinity, 1945). Free neutrons, while not directly ionizing atoms, cause ionizing radiation. As such they can be a biological hazard, depending upon dose. A small natural "neutron background" flux of free neutrons exists on Earth, caused by cosmic ray showers, and by the natural radioactivity of spontaneously fissionable elements in the Earth's crust. Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments. Atomic nuclei are formed by a number of protons, Z the atomic number, and a number of neutrons,