Types of radioactive decay
In alpha decay, a nucleus emits an alpha particle (), so the mass number drops by 4 and the atomic number drops by 2. In beta decay, a neutron converts into a proton and an electron is emitted, so the atomic number increases by 1 while the mass number is unchanged. Gamma decay releases energy as electromagnetic radiation with no change to mass or atomic number.
Example 1: Write the nuclear equation for the alpha decay of Uranium-238 (atomic number 92).
-- the mass number falls by 4 (238 to 234) and the atomic number falls by 2 (92 to 90, which is thorium).
Half-life
A radioactive sample's remaining amount after time is , where is the half-life.
Example 2: A radioactive sample has a half-life of 10 days. If the initial mass is 80 g, find the mass remaining after 30 days.
30 days is 3 half-lives, so .
The photoelectric effect
Shining light on certain metal surfaces can knock electrons out of the metal, but only if the light's frequency is above a minimum "threshold frequency" for that metal -- increasing the light's brightness without raising its frequency does not help, which was one of the first clues that light behaves as discrete packets of energy (photons) with energy , rather than a continuous wave.