Define the "half-life" of a radioisotope

The amount of time it takes for 1/2 of it to decay or dissapear

this is liike compound interest because it takes the same amount of time to decay 1/2 of it, regardless of the amount

example

lets say half life of element x is 1 year
if yo has 32 grams, after  1 year, it decays to 16g
2 years decays to 8
3 years decays to 4
4 years decays to 2
5 years decays to 1
etc

hope this helps (I'm not the best at chem)

Electrons are emitted

Molarity is moles of solute over liters of solution. idk if that is what you are talking about, but you can convert moles of solute to grams of solute using dimensional analyzes by using molar mass found on the periodic table.