Question

how do you find the mass number of carbon and other periodic elements

Answer 1

Mass number for all elements in periodic table is Sum of number of proton and number of neutron in the nucleus.

for each element number proton and number of neutron is same.

Mass number of Carbon = number of proton + number of Neutron

proton number i.e.atomic number for C = 6

Mass number of Carbon = 6+6 = 12

Mass number of Carbon = 12

similarly for other periodic element mass number can be determine

example

mass number of Nitrogen =number of proton + number of Neutron = 7 +7 = 14

mass number of oxygen =number of proton + number of Neutron =  8+8 = 16