Question

Which of the following is true for the following isotopes of uranium ${U^{235}}\;$ and ${U^{238}}$?
A) Both contain the same number of neutrons
B) Both contain the same number of protons, electrons, and neutrons.
C) Both contain the same number of protons and electrons but ${U^{238}}$ contain three more neutrons ${U^{235}}\;$.
D) ${U^{238}}$ contains three fewer neutrons than ${U^{235}}\;$.

Answer 1

Isotopes have the same atomic number and are nearly placed with their original member in the periodic table but they have different atomic masses. The atomic number gives us the total number of protons present in the nucleus whereas atomic mass gives us the sum of a total number of protons and neutrons in the nucleus. We need to analyze each option to determine which statement is true regarding the two given isotopes of uranium.

Complete answer:
Natural uranium consists primarily of three radioactive isotopes ${U^{234}},{U^{235\;}}$ and $\;{U^{238}}$.
Let us now see the mass number of both isotopes of uranium to analyze each statement.
${U^{235}}\;$- It consists of mass numbers$235$. That is it contains $92$ protons and $143$ neutrons. And ${U^{238}}$ consists of mass numbers $238$. That is it contains $92$ protons and $146$ neutrons.
Also, we know that neutral atoms consist of the same number of protons and electrons. Therefore the number of protons in the uranium will be equal to the number of electrons in the uranium.
Therefore we can conclude that both the isotopes of uranium have the same number of protons and electrons. Also, we can see that ${U^{238}}$ consists of three protons higher than ${U^{235}}\;$. Therefore both contain the same number of protons and electrons but ${U^{238}}$ contain three more neutrons than ${U^{235}}\;$.
Therefore, option (C) is the correct answer.

Note:
Option A cannot be true because they both do not have the same number of neutrons. Also, option B says that they both have the same number of protons, electrons, and neutrons. This can also be not true because they both do not have the same number of neutrons. Option D says that ${U^{238}}$ contains three fewer neutrons than ${U^{235}}\;$. This is not true because only ${U^{235}}\;$ contains three fewer neutrons than ${U^{238}}$ therefore this option is also not true.