-- Photographic film detects radiation. -- Gas occupies the whole container. -- The Geiger counter is used to detect and measure the intensity of radiation. -- An atom is the smallest particle to retain characteristics of the element. -- The atomic mass is the mass of the protons and neutrons in the atom. (The electrons hardly make any difference. One proton or neutron has as much mass as more than 1800 electrons.) -- A solid has a fixed shape. -- The neutron is electrically neutral. It has no charge. -- The proton has a positive charge. -- The electron has a negative charge. -- The atomic number is the number of protons in the nucleus of an atom.