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Kebsis
2004-Jul-30, 03:37 AM
You know, I got the sneaking suspicion that I've asked this question before on this board, but I don't remember for sure.

Anyway, I was wondering how an atom can have mass when the parts that it's made up of, electrons and such, are massless?

ToSeek
2004-Jul-30, 03:40 AM
Electrons have mass, just not very much. Most of an atom's mass comes from the neutrons and protons, which have (comparatively) a lot of mass.

2004-Jul-30, 03:45 AM
Now I'm wondering what the density of an atom is. Not the nucleus, the entire atom.

Kebsis
2004-Jul-30, 03:45 AM
Really? I was always under the impression that electrons moved at the speed of light. Are the things that electrons are made of have mass as well?

2004-Jul-30, 03:52 AM
Really? I was always under the impression that electrons moved at the speed of light. Are the things that electrons are made of have mass as well?

As far as I know, an object with mass can't move at the speed of light unless it has infinite mass. Which brings up another question... If electrons are moving so fast, shouldn't their mass be very high?

Tobin Dax
2004-Jul-30, 04:08 AM
Now I'm wondering what the density of an atom is. Not the nucleus, the entire atom.

Essentially, the mass of the nucleus over a cubic angstrom or so (larger for large atoms). Electrons don't contribute a whole lot to the mass of an atom.

swansont
2004-Jul-30, 10:48 AM
Really? I was always under the impression that electrons moved at the speed of light. Are the things that electrons are made of have mass as well?

As far as I know, an object with mass can't move at the speed of light unless it has infinite mass. Which brings up another question... If electrons are moving so fast, shouldn't their mass be very high?

You have cause and effect backward. An infinite mass wouldn't cause an object to move at the speed of light. But a massive object would require infinite energy to move that fast.

Eta C
2004-Jul-30, 12:42 PM
From the source of all wisdom on matters particle related, the Particle Data Group entry on the electron (http://www-pdg.lbl.gov/2004/listings/s003.pdf). The table first gives the rest mass in atomic mass units, then in the more normally cited MeV/c^2. The electrons rest mass (after some rounding) is .511 MeV/c^2. For comparison, the proton and neutron rest masses are almost 1000 MeV/c^2. So most of the mass of an atom is concentrated in its nucleus.

Edited to add the word "rest" to avoid confusion with relativity.

Wally
2004-Jul-30, 12:49 PM
Really? I was always under the impression that electrons moved at the speed of light. Are the things that electrons are made of have mass as well?

As far as I know, an object with mass can't move at the speed of light unless it has infinite mass. Which brings up another question... If electrons are moving so fast, shouldn't their mass be very high?

There's "at rest" mass and "inertial" mass (apparent mass due to speed). When talking of infinite mass at relativistic speeds, you're talking the later, not the former.

tracer
2004-Jul-30, 04:32 PM
Electrons have mass, just not very much. Most of an atom's mass comes from the neutrons and protons, which have (comparatively) a lot of mass.
Okay, then ... what gives a proton or a neutron its mass? Their rest masses are a lot higher than the rest masses of the individual quarks they're made up of, as I recall.

Hamlet
2004-Jul-30, 05:26 PM
You know, I got the sneaking suspicion that I've asked this question before on this board, but I don't remember for sure.

Anyway, I was wondering how an atom can have mass when the parts that it's made up of, electrons and such, are massless?

The protons, neutrons and electrons that make up an atom all have mass with the lions share being in the nucleus.

There is a theory proposed by Peter Higgs that there is a field, appropriately called the Higgs Field (http://encyclopedia.thefreedictionary.com/Higgs%20field) that gives rise to mass via particle interactions with the Higgs boson.

Some current particle accelerators have had tantlizing events that could be from a Higgs boson, but , so far there is nothing conclusive. Maybe one of the newer accelarators will have a chance.

I haven't been able to wrap my head around how the Higgs Field produces mass, but I keep trying. :D

Spaceman Spiff
2004-Jul-30, 07:37 PM
Electrons have mass, just not very much. Most of an atom's mass comes from the neutrons and protons, which have (comparatively) a lot of mass.
Okay, then ... what gives a proton or a neutron its mass? Their rest masses are a lot higher than the rest masses of the individual quarks they're made up of, as I recall.

Protons and neutrons are composed of combinations of 3 "up" and "down" quarks: protons (uud) and neutrons (ddu).

Experiments have demonstrated that the inertial masses of the individual quarks in an atom's nucleus are a tiny fraction (together perhaps 1.5%) of the nucleus' rest mass. The remainder of the rest mass of an atom's nucleus arises from the binding energy via the strong nuclear force (under the exchange of gluons) of the individual quarks within the protons and neutrons (the strong nuclear force gets stronger as quarks separate from one another). The sum of the electrons' rest mass is just a fraction of this. If this is so, then not only are the atoms that compose you mostly empty space, but all but 1.5% of your mass is quark-quark binding energy :o

Apparently, though it's not yet been substantiated, the Higgs' Boson is responsible for this remainder (1.5%) inertia. I think I've got that right.

swansont
2004-Jul-31, 07:17 PM
Electrons have mass, just not very much. Most of an atom's mass comes from the neutrons and protons, which have (comparatively) a lot of mass.
Okay, then ... what gives a proton or a neutron its mass? Their rest masses are a lot higher than the rest masses of the individual quarks they're made up of, as I recall.

Protons and neutrons are composed of combinations of 3 "up" and "down" quarks: protons (uud) and neutrons (ddu).

Experiments have demonstrated that the inertial masses of the individual quarks in an atom's nucleus are a tiny fraction (together perhaps 1.5%) of the nucleus' rest mass. The remainder of the rest mass of an atom's nucleus arises from the binding energy via the strong nuclear force (under the exchange of gluons) of the individual quarks within the protons and neutrons (the strong nuclear force gets stronger as quarks separate from one another). The sum of the electrons' rest mass is just a fraction of this. If this is so, then not only are the atoms that compose you mostly empty space, but all but 1.5% of your mass is quark-quark binding energy :o

Apparently, though it's not yet been substantiated, the Higgs' Boson is responsible for this remainder (1.5%) inertia. I think I've got that right.

Binding energy reduces the mass of the composite particle, not the other way around. That is, the individual quarks have more mass than the composite particle. How much the mass decreases when forming a bound system tells you how tightly bound the system is. So sayng that most of the mass of a nucleus is binding energy makes no sense.

Celestial Mechanic
2004-Aug-01, 04:09 AM
Binding energy reduces the mass of the composite particle, not the other way around. That is, the individual quarks have more mass than the composite particle. How much the mass decreases when forming a bound system tells you how tightly bound the system is. So sayng that most of the mass of a nucleus is binding energy makes no sense.
This is true only if the particles can separate, that is, they are not confined. But the quarks, antiquarks, and gluons in a hadron are all confined and cannot exist apart from one another. Any attempt to pull one component away from the others requires so much energy that ultimately a quark/antiquark pair is produced and a meson of some sort flies off.

The best estimates of the up and down quark masses are 3-5 MeV for the up quark and 5-7 MeV for the down quark. The stress energy tensor of the gluons must be the main component of the nucleon mass (938.5 MeV for proton, 939.3 for neutron).

It is worth noting that there are possibly some particles that are made up of bound states of gluons alone, called "glueballs". I don't know if any definite glueball states have been confirmed. Eta C might be able to say something about the topic of this thread.

Spaceman Spiff
2004-Aug-02, 02:27 PM
Binding energy reduces the mass of the composite particle, not the other way around. That is, the individual quarks have more mass than the composite particle. How much the mass decreases when forming a bound system tells you how tightly bound the system is. So sayng that most of the mass of a nucleus is binding energy makes no sense.
This is true only if the particles can separate, that is, they are not confined. But the quarks, antiquarks, and gluons in a hadron are all confined and cannot exist apart from one another. Any attempt to pull one component away from the others requires so much energy that ultimately a quark/antiquark pair is produced and a meson of some sort flies off.

The best estimates of the up and down quark masses are 3-5 MeV for the up quark and 5-7 MeV for the down quark. The stress energy tensor of the gluons must be the main component of the nucleon mass (938.5 MeV for proton, 939.3 for neutron).

It is worth noting that there are possibly some particles that are made up of bound states of gluons alone, called "glueballs". I don't know if any definite glueball states have been confirmed. Eta C might be able to say something about the topic of this thread.

Yep, what Celestial Mechanic said (and I said) is correct. Figuring somebody might pick up on this and perhaps get confused I added the comment that the strong nuclear force between the quarks (and the rate they exchange gluons) increases as one tries to pull them apart. Thanks, Celestial, for the excellent additional information.

genebujold
2004-Aug-02, 05:11 PM
Electrons have mass, just not very much. Most of an atom's mass comes from the neutrons and protons, which have (comparatively) a lot of mass.

Very little energy is bound in the electrons, and is equivalent to their combined quantum level energy levels from the lowest to infinity.

That's why stripping an atom of it's electrons is so easy.

A great deal of energy is bound in the protons and neutrons, which is why splitting an atom is so difficult.