Where EXACTLY is the "Mass" of an Atom? Electron? Proton? Neutron?

u r right. No one knows that what's the electron, proton & neutrons are made up of. But mass is there. Mass is not energy. Energy & mass r 2 different things. Don't confuse!

Mass is defined in terms of inertia - the need for a force to make something change its direction or speed of motion.

Mass and energy are under some circumstances interconvertible, as expressed in the equation

E = mc^2

but a lot of conditions have to be satisfied for this to happen large scale. For example, you can't turn an electron into pure energy, but an electron and the positron will react to generate energy in the form of a pair of gamma rays (challenge problem for readers: why do you need TWO gamma rays, not just one?).

The reference to Higgs was spot-on.

This is all very much physics. No one really knows, but we haven't given up and shouldn't give up of finding out quite a bit more.

BTW, a neutron weighs slightly more than a proton, and roughly 2000 times as much as an electron. You can calculate the energy released in a nuclear reaction by subtracting the mass of the products from the mass of the parent, and using Einstein's formula. The mass of a proton involves the mass of the quarks, of the gluons holding the quarks together, and the energy involved.

Energy always has mass. Matter such as protons and atoms take a lot of energy, and are extremely heavy compared to light (yes, light, or photons, I'm pretty sure it also has mass)

Protons are way heavier than electrons, and neutrons are a tiny, small bit heavier than protons because they are a proton and an electron combined. People often use atomic masses to measure weight rather than like attograms or something like that.

For example, 1 atomic mass... I'm pretty sure it's the weight of a neutron. So hydrogen would have the atomic mass of 1. (well, there is other isotopes of hydrogen but your average hydrogen is by far more common)

One theory involves the Higgs particle, which many hope the LHC will finally demonstrate to exist. According to some, there is a Higgs field or "ocean" that permeates the entire universe. The resistance of this field to acceleration is what creates inertia and what we describe as mass, whether you are talking about quarks, protons or ocean liners.

The mass is in the nucleus, which consists of protons and neutrons.

I think some French Scientists have proven Einstein's equation

E = mc^2

So, we now believe that mass is a form of energy.

At the middle of an atom there is the nucleus that is made of protons and neutrons. The 99.95% of the mass of our atom is concentrated in this little space.

It's in the proton. Energy is present within the subatomic particles, and this might relate somehow to Eintein's theory of energy and metter being related, thet they are convertable from one source to another. So as energy exists, it can exist as matter in some instances, thus producing the mass that is measured.

I don't think its the end of physics and start of philosophy, but i think that it is getting into the realm where they don't really know. The LHC is looking for the higgs boson and that is something to do with mass. Maybe some research about the higgs boson would help you :)

The mass of an atom is exatly in Proton bcz.. its atomis mass is greater that Neutron...

In the proton, in the nucleus.