Question 17
The subscript 2 is the atomic number for helium and subscript 4 is the mass number for Helium
explanation
when writing down an isotope of an element the mass number of that isotope is written in upper side of the chemical symbol, while atomic number is written on the lower side.
Since 4 is on upper side it is the mass number of He, while 2 is the atomic number since it is on lower side.
Question 18
The type of radiation emitted when polonium -212 form lead 208 is an alpha particle
explanation
Alpha decay is a decay in which an atomic nuclei emit an alpha particles and be transformed into different atomic nuclei with a mass number that is reduced by 4
polonium 212 is reduced by 4 to form lead 208 therefore it undergo alpha particle decay.
