How do you find out the number of Neutrons,Protons, and Electrons in an atom?

The number or protons and electrons of an atom are equil to its atomic number.  The number of neutrons is equil to the atomic weight - the atomic number.

The periodic table of elements can help you here.

In a neutral atom, the atomic number equals the number of protons. since the atom is neutral the number of protons equals the number of electrons.  the atomic mass minus the atomic number should give you the number of neutrons.

The number of protons and electrons are equal to the atomic number. To find the neutrins you need to subtract the atomic mass from atomic number.

Ok use the atomic number to find protons. So oxygen (O) has 8 as an atomic number. The atomic number is equal to the number of protons. so oxygen has 8 protons. assuming your atoms isn't an ion, then the number of protons is equal to the number of electrons. So it has 8 electrons too. For the neutons look at the atomic mass. round up. so lets pretend an element has the atomic mass on 60.31. Round to the nearest whole number. 60. the atomic mass is the mass of the nucleus which is protons and electrons. so-

mass = protons + neutrons

we want to get rid of the protons. so lets rewrite.

60 = 8 (we said oxygen has 8 protons) + n

52 = n

it has 52 neutrons.

keep in mind that I have no idea what the atomic mass of oxygen is so thats not accurate.

so do this-

atomic number = protons and electrons

atomic mass = protons + neutrons
subtract protons and that gives you neutrons.