Question

How do i know how many neutrons, electrons, and protons there is in an element?

Answer 1

proton =electron= atomic no.of elements
atomic mass= proton + neutron

if you know atomic no and it's mass you can find neuron by above formula

Answer 2

By their mass number and atomic number . For instance if you have carbon(6)atomic number and mass number (12) . Then it will have 6 electrons and protons and 12 neutrons.

Neutrons are the mass number where say electrons and protons are it's atomic number.