Question

To find the mass defect for uranium-238 several calculations must be made. Which is the calculation that should be made first? Assume atomic number for uranium-238 is 92.
A.
92(1.0073) x 92(1.0087) =
B.
92(1.0073) x 146(1.0087) =
C.
146(1.0073) x 238(1.0087) =
D.
238(1.0073) x 238(1.0087) =

Answer 1

mass number of uranium 238 is sum of number of proton and neutrons in it

While atomic number is just number of protons

So total number of protons in it = 92

total number of neutrons in it = 238  - 92 = 146

mass of proton = 1.0073 amu

mass of neutron = 1.0087 amu

now for the calculation of mass of nuclei is given as

$M = zm_p + (A-z)m_n$

$M = 92(1.0073) + 146(1.0087)$

so option B is correct