Answer:
Considering the half-life of 10,000 years, after 20,000 years we will have a fourth of the remaining amount.
Explanation:
The half-time is the time a radioisotope takes to decay and lose half of its mass. Therefore, we can make the following scheme to know the amount remaining after a period of time:
Time_________________ Amount
t=0_____________________x
t=10,000 years____________x/2
t=20,000 years___________x/4
During the first 10,000 years the radioisotope lost half of its mass. After 10,000 years more (which means 2 half-lives), the remaining amount also lost half of its mass. Therefore, after 20,000 years, the we will have a fourth of the initial amount.
Exothermic reaction is when the system releases energy to the surrounding, the energy (in the perspective of the system) will have a sign of negative, as the system is transferring energy towards the surrounding.
Endothermic reaction is when the system is absorbing energy from the surrounding, the energy (in the perspective of the system) will have a sign of positive, as energy is transferred to the system.
