2.  The same element with a different number of neutrons:  EX- C-12(6n°), C-13(7n°) & C-14 (8n°)  12.001= the Real Mass: An average of all the isotopes of that element.

3.  The nucleus of an atom breaks down and emits radioactive particles:  EX- Alpha- slow, absorbed by a substance thicker than paper  Beta- almost the speed of light-more penetration: can go through thin copper sheet  Gamma- travels at the speed of light and can penetrate lead. 299,792,458 m/s!!  Uses in Industry: Check the thickness of paper and plastic, keep food fresh, radioactive tracing (track substances through the body) and radiotherapy (kills cancer cells)

4.  The amount of time it takes for half of the atoms in a radioactive sample to decay.  EX- C14= 5,700 yrs. & Be11= 13.81s  The half-life for a given isotope is always the same. It doesn’t depend on how many atoms you have or how long they have been sitting around!  EX- Be11(changes to B11)  16g----  8g -----  4g ----  2g 13.81s 13.81s 13.81s 3-Half-lives have passed!!