Question

All atoms contain neutrons in their nuclei except:
A. Protium
B. Helium
C. Neon
D. Chlorine

Answer 1

Neutrons are chargeless particles present in the nuclei of atoms, and weight the same as protons. Both protons and neutrons contribute to the mass number of an atom. Thus, by subtracting the number of protons from the mass number, we can get the number of neutrons in an atom.

Complete answer:
As we know, the mass number of an atom is defined as the sum of the neutrons and protons present in the nucleus of that element. Therefore, to find the number of neutrons, we just have to subtract the number of protons from the mass number. The number of protons is also equal to the atomic number of the element.
For option A, we have protium, which is an isotope of hydrogen. Isotopes are atoms of the same element having the same atomic number but different mass numbers. Protium is the first isotope of hydrogen and it is actually what we refer to commonly as hydrogen. As we know, protium has an atomic number of $1$ and also a mass number of $1$, meaning that the number of neutrons is $1 - 1 = 0$.
For option B, we have helium whose atomic number is $2$ and mass number is $4$. Therefore, the number of neutrons is $4 - 2 = 2$.
For option C, we have neon whose atomic number is $10$ and mass number is $20$. Therefore, the number of neutrons is $20 - 10 = 10$.
For option D, we have chlorine whose atomic number is $17$ and mass number is $35$. Therefore, the number of neutrons is $35 - 17 = 18$.
Hence, as we can see from above, the only atom which doesn’t have neutrons is protium.
Therefore, the correct option to be marked is A.

Note:
Hydrogen has two other isotopes, deuterium and tritium, which have one and two neutrons in their nuclei respectively. Protium, which is the most abundant isotope, has only one proton and one electron. The main function of a neutron is to keep the atom stabilised between the positive charge effects of the proton and the negative charge effects of electrons. However, as protium only has one proton and electron, there is not much instability and no neutrons are present, which makes it the only element in the universe to have equal atomic number and mass number.