i know that the half life of strontium 90 half-life is 28.8 but how can i explain this???

Answer 1

over 28.8 years, half of it breaks down into other isotopes, elements and particles. Over the next 28.8 years, half of that half breaks down so we have 1/4. In another 28.8 years half of the quarter breaks down so we have 1/8 of the original etc.
The decay is known as beta- decay whereby a proton in the nucleus emits an electron thereby becoming a neutron reducing the atomic number by one.