Except for hydrogen, all atoms have neutrons in their [[1]].

Except for protium, all atoms have neutrons in their nucleus.

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. 

For protium, which is an isotope of hydrogen. Protium is the first isotope of hydrogen and it is actually referred to commonly as hydrogen. 

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 helium whose atomic number is $2$ and mass number is $4$. Therefore, the number of neutrons is =$4-2=2$.

For neon whose atomic number is $10$ and mass number is $20$. Therefore, the number of neutrons is =$20-10=10$.

For 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