It's called "Ionic compound"
(4) physical properties, because they are in different phases.
The unit of 16.8 /kjg contains a mistake.
I just can help you by assuming the right units.
I will assume 16.8 kj /g. This is a rate, which means that 1 g of reactant will produce 16.8 kj of heat.
To calculate the mass of reactant that will produce 800j first you multiply by the conversion factor to obtain kj, and the you divide by the rate of heat, 16.8 kj/g
heat * conversion factor / rate
(800j) * (1kj/1000j) / (16.8 kj / g) <--- answer
Hence, it is less preferrable to choose an isotope to enter your body, which will emit radioactivity for a long time, so we tend to choose isotopes, the radioactivity of which ceases quickly, so that the least possible amount of damage is caused to the cells