Question

 A neutral atom of fluorine has a mass number of $19$ and an atomic number of $9$. How many neutrons does it have?
A. $10$
B. $9$
C. $19$
D. $28$
E. It is impossible to determine from the information given.

Answer 1

 Hint: The atomic number indicates numbers of protons present in the nucleus or number of electrons outside the nucleus while the mass number indicates total number of protons and neutrons are present in the nucleus. You have atomic number and mass number and number of neutrons can now be calculated by taking the difference between mass number and atomic number.

Complete step by step solution: 
Atomic number indicates the number of protons that is present in the nucleus and atomic number also indicates the number of electrons that revolve around a nucleus.
In a neutral atom, the number of protons is equal to the number of electrons. So, if the atomic number or number of protons is given, then the number of electrons will have the same value.
Mass number refers to the sum of the number of protons and neutrons that is present in the nucleus. The sum total of the number of protons and neutrons is also called as nucleons. So, the number of nucleons will determine the mass number of an atom.
So, From the above information we can say that,
$Atomic\ Number=Number \ Of \ Protons$
$Mass\ Number=Number \ Of \ Protons+Number \ Of \ Neutrons$ (Or)
$Mass\ Number=Atomic \ Number+Number \ Of \ Neutrons$
Here, atomic and mass numbers are given and we have to find out the number of neutrons of fluorine atoms.
Thus, it can be found out by using the formula,
$Number Of Neutrons=Mass Number-Atomic Number$
So, Atomic number of fluorine is given i.e. $9$ and mass number is $19$.
Thus, using the above formula, the number of neutrons will be,
$Number Of Neutrons=19-9=10$.
Therefore, the number of neutrons of a neutral atom of fluorine is $10$.
Hence, the correct option is A.

Note: Mass number is determined by the number of nucleons (i.e. the sum total of the number of protons and neutrons present in an atom) while the atomic number is determined by the number of protons present in the nucleus of an atom. So, the number of neutrons can be calculated by taking the difference of mass number and atomic number.