Radioactivity

​are atoms of the same elements that have different number of neutrons.

​Have the same atomic number but different mass number.

​is the average mass of the element's isotopes.

​The black and white photo shows Henri Becquerels photographic plate.

​the dark area on the plate was exposed to radiation given off by uranium in the mineral even though the mineral was not exposed to sunlight

​Marie Curie studied radioactivity and discovered two new radioactive elements

​polonium and radium

​Elements that spontaneously emit radiation

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​Henri Becquerel and Marie Curie discovered that the radiation released by uranium was made of energy and particles.

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​is a process that occurs when an unstable atomic nucleus changes into another more stabel nucleus by emitting radiation.

​An alpha particle is made of two protons and two neutrons

​when an atom releases an alpha particle, its atomic number decreases by two.

​Uranium-238 decays to thorium-235 through the process of alpha decay

​When beta decay occurs, a neutron in an atom changes into a proton and high-energy electron called a beta particle.

​In beta decay, the atomic number of an atom increases by one because it has gain a proton.

​Gamma rays do not contain particles, but they do contain a lot of energy.

​In fact, gamma rays can pass through thin sheet of lead! because gamma rays do not contain particles.

​The release of gamma rays does not change one element into another element.

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​The energy released by radioactive decay can be both harmful and beneficial to humans.

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​Radiation therapy can be beneficial to humans by destroying cancer cell.