Question

An isotope of $U - 235$ has $92$ protons in its nucleus. How many neutrons are there in the nucleus of $U - 238$ isotope?

Answer 1

In order to solve this equation we first need to know, the relationship between protons, electrons and neutrons in an element.
-The number of protons is the same as the atomic number.
-The number of neutrons can be different, even in atoms of the same element.
-Atoms of the same element that contain the same number of protons, but different numbers of neutrons, are called isotopes.
-Isotopes of any given element contain the same number of protons, therefore they have the same atomic number
-Isotopes of a given element contain different numbers of neutrons, hence, different isotopes have different mass numbers.

Complete answer:
Since we know that atomic mass of $U - 238$ is said to be $238$ itself which is denoted as M.
The number of protons will be the same for different isotopes of the same element which means it will be the same in $U - 235$ and $U - 238$ which is $92$ protons.
Since we know that the number of protons is equal to the atomic number.
Hence, $Z = 92$
${\text{Number of neutrons = M - Z}}$
$= 238 - 92$
$146 neutrons$
Hence the number of neutrons in $U - 238$ is $146neutrons$.

Note: Isotopes are atoms of the same element that have different neutron numbers but have the same number of protons and electrons. The difference in the number of neutrons between the various isotopes of an element denotes that the various isotopes have different masses. The superscript number to the left of the element abbreviation indicates the number of protons plus neutrons in the isotope. The mass number of an atom is equal to the sum of the number of protons and neutrons contained in its nucleus.