Acertain radioactive element has a half-life of one hour. if you start with 20.0 grams of the element at noon, how much of the radioactive element will be left at 3 p. m.?
a. 10.0 grams b. 5.0 grams c. 2.5 grams d. 1.25 grams

Iknow the answer is a but i don't know why

Calculate the mass of silver needed to react with chlorine to produce 126g if silver chloride?

Which sentence about particles in matter is true? a. atoms are present in solids and liquids but not in gases. b. the particles of matter are in constant motion. c. the same kinds of atoms are found in different elements. d. when a solid changes to a liquid, the sizes of the particles change.