#beta#-decay is the decay of a neutron into a proton (or a proton into a neutron) with the release of an electron to conserve charge. It also releases a neutrino. This keeps the mass the same (neutrons and protons weigh about the same) but increases the atomic number by one.
#beta#-decay happens when a nucleus has too many neutrons and is unstable.
Atomic number, #Z#, is the number of protons, and mass number #X# is the total number of neutrons plus protons. In notation of element #A# this is written as
#""_ Z^XA#
A theoretical example of #beta#-decay using carbon-12 (#Z=6, X=12#) would increase the atomic number by one to make nitrogen and keep the mass the same.
#""_ 6^12C->""_ 7^12N+""_ -1^0e#
Check this is correct by adding up the numbers on the top and bottom - they should be the same on both sides
#12->12+0#
#6->7-1#
