Answer:
The period 4 alkaline-earth metal has the name Calcium. An isotope of this element having 20 neutrons has a mass number of 40.
Explanation:
Looking at the periodic table 4th row for the alkaline-earth metal, we can find Calcium which has an atomic number of 20 (or protons number, or z). The mass number is given by the sum of protons and neutrons. If the number of protons in this case is 20 and the number of neutrons is also 20, the mass number A = 40.
You cannot find it if you do not know the information. Was there a molar mass given or do you know if it is asking for the molar mass?
An electron this is the smallest particle and it takes up the least mass because its negative charge makes it harder for electrons to have a place in an atom.
