Question

The number of neutrons in an atom $X$ of an atomic number $Z$ and mass number $A$ is:
A. Zero
B. $Z$
C. $A - Z$
D. $A$

Answer 1

Here we have to see what atomic number and mass number means:
Atomic number:
Atomic number is the number of protons inside the nucleus of an atom. The number of protons determines the identity of the element (i.e. an element with six protons is a carbon atom, no matter how many neutrons may be present).
Mass number:
The number of protons and the number of neutrons together forms the mass number.

Complete step by step answer:
Neutral atoms of a component contain an equivalent number of protons and electrons. The quantity of protons decides a component's nuclear number ( $Z$ ) and recognizes one component from another. For instance, carbon's nuclear number ( $Z$ ) is $6$ since it has $6$ protons. The quantity of neutrons can shift to deliver isotopes, which are molecules of a similar component that have various quantities of neutrons. The quantity of electrons can likewise be distinctive in molecules of a similar component, in this way creating particles (charged atoms). For example, iron, $Fe$, can exist in its impartial state, or in the $+ 2$ and $+ 3$ ionic states.
A component's mass number ( $A$ ) is the entirety of the quantity of protons and the quantity of neutrons. The little commitment of mass from electrons is dismissed in computing the mass number. This estimate of mass can be utilized to effortlessly ascertain the number of neutrons a component has by just taking away the quantity of protons from the mass number. Protons and neutrons both weigh around one nuclear mass unit or $amu$. Isotopes of a similar component will have similar atomic numbers yet unique mass numbers.
Hence, from the above discussion it can be concluded that the number of neutrons in an atom $X$ of an atomic number $Z$ and mass number $A$ is: $A - Z$.

So, the correct answer is “Option C”.

Note:
Here we may get confused with the atom $X$ which is just the name of the atom and is not relevant for this question. We just have to remember that the quantity of neutrons is equivalent to the difference between the mass number of the particle and the atomic number.