plz refer to explaination . [because I'm facing some difficulty in answering this question.
First of all , the question seems incomplete 'how many neutrons are in an atom' .
we got 118 elements in periodic table & around 170 at wikipedia . Out of those more than 100 are identified .
mostly elements have different number of neutrons . But some consective elements may have same no. of neutrons .
The elements I'd been talking about were the most abundantly occuring isotope .
But elements got many isotopes like hydrogen got 3 main isotopes ; protium,deuterium, tritium.
deuterium has neutrons equal to Helium with mass number 3 and tritium has equal neutrons as helium with mass no. 4. so, plz mention specific element .
additional- neutrons is a fermion , but not a lepton , it is composed of two down and 1 up quark.
Hope this helps!