Answer:
Step-by-step explanation:
One
I've never heard of a mass defect before you asked this question. Interesting concept.
Givens
Number of neutrons =207 - 82 = 125
Number of protons = 82
The amu of a proton is 1.00728
The amu of a neutron is 1.00866
Solution
Find out the actual mass of the Lead atom
Actual mass = 82*1.00728 + 125*1.00866
Actual mass = 208.67946
The mass defect = 208.67946 - 207 = 1.67946
That is the proper way to calculate mass defect.
Apparently the way you are supposed to do it is
Mass defect = 208.67946 - 206.975897= 1.703563 which means that C is the closest answer. You will have to look at your notes to see if amu for protons and neutrons are what I used.
Two
The number of protons is the number of Uranium on the periodic table.
92 protons. 92 comes from the position on the periodic table.
You can get the same thing by Taking its relative mass (238) and subtracting the neutrons (146) which is 238 - 146 = 92