The term UNIT IN PHYSICS can be broken down into four crossword solutions: ATOM (4 letters), ERG (3 letters), ION (3 letters), and RAD (3 letters). An ATOM is the smallest particle of an element that retains its chemical properties. An ERG is a unit of energy or work equal to 10^-7 joules. An ION is an ATOM or molecule with a net electric charge due to the loss or gain of one or more electrons. Finally, a rad is a unit of absorbed radiation dose equal to 0.01 joule per kilogram.