# Mass of atoms

#### logearav

Joined Aug 19, 2011
243
Revered members,
We use spring balance, physical balance, electronic balance to find mass of a substance. But what is the apparatus used to find the mass of an atom? Because, mass of proton and mass of neutron constitute the mass of an atom. But, in nuclear physics, i read the real mass is less than the actual mass(mass of proton and neutron). So how one can find the real mass of an atom.?

#### Papabravo

Joined Feb 24, 2006
15,129
There is a classic High School Physics experiment where a magnetic field is used to measure the radius of the trajectory of electrons inside a vacuum tube. From these measurements the mass of the electron can be calculated. The masses of other elementary particles can be calculated from their energies and their trajectories in collisions. The mass of an atom is just the sum of the masses of the individual components.

#### Kermit2

Joined Feb 5, 2010
4,162

#### Papabravo

Joined Feb 24, 2006
15,129
Twas a nice article.

Joined Jul 7, 2009
1,583
The mass of an atom is just the sum of the masses of the individual components.
If you're wondering why the mass of an atom doesn't add up to the mass of the individual nucleons and electrons, look up "mass defect".

#### Wendy

Joined Mar 24, 2008
22,240
A helium detector is a dedicated piece of equipment that is based on a mass spectrometer. It works by using a cathode (much like a tube), injecting the sample gas onto a filament (the cathode), and having it leave the cathode with a charge. The sample gas then passes over a powerful magnet that bends the trajectory, the exact trajectory is largely dependent on the mass of the atom. If the atom is of the right mass, a helium atom, it will go through a slit in sheet of metal and hit a target, which is electronically active and detects the charged particle of helium.

You have had several queries about particle and quantum physics where you try to treat them as macro particles (macro = large, us). They are not the same, they follow different rules. Some of the best minds of our age are trying to establish links between quantum and macro, but the rules simply don't mix or work.

#### logearav

Joined Aug 19, 2011
243
Thanks for all your replies, revered members. @Bill, Will Bainbridge mass spectrometer find the mass of atoms?

#### Wendy

Joined Mar 24, 2008
22,240
Find them? I don't think so, it already knows them. I think it uses a look up table. I really don't know though, no hands on there.

I've been fortunate enough to service helium detectors. It is an interesting mix of high tech and low tech, low tech in that some of the machining is rather crude.

The cathode is a filament same as what is found in a light bulb. It burns out like one too, so I had to occasionally go in there and replace it. The target is cheap sheet metal cut in a die and folded, it needs cleaned when the machine is open with alcohol. A powerful, very machined magnet sits between the two. The whole arrangement is around 8" long, and 4" wide.

We used thin solder wire to form a gasket on the head, something similar to the head on a car, but much smaller and massive aluminum block. It got very thin indeed. I tended to be the guy who did it most often (engineers choice) because the other guys would not use nitrile gloves and I did. I didn't like them either, but the need was obvious, and when I finished the machine it settled in a very small fraction of the time it needed when the other guys did it.

Interesting physics is all around us, just keep your eyes open and pay attention.

And never, ever stop reading.

Joined Jul 7, 2009
1,583
Your question is a good one -- it's not an easy task to measure the mass of an atom. I recently read about someone using nanotubes to measure the mass of gold atoms; the gold atom would change the resonant frequency of a vibration mode of the nanotube. This is a nifty answer because frequencies can be measured very precisely. Of course, if you think about it a bit, you also need to know a measure of one of the mechanical properties of the nanotube, probably something like the modulus of elasticity. So one problem turns into another.

In chemistry, we often glibly work with moles, atomic weights, and Avogadro's number (Avogadro's number is a fundamental number that connects our macroscopic world with the microscopic world of the atom). But, of course, Avogadro's number is an experimentally-determined quantity, so the story doesn't end there.

The mass spectrometer that Bill mentions is a powerful tool, but they actually measure the relative mass of atoms in terms of ze/M, where z is the ionization level, e is the electronic charge, and M is the mass of the atom. Thus, you can use a mass spectrometer to measure the mass of all atoms relative to one atom, in theory (in practice, it's complicated by messy experimental problems). Some mass spectrometers are sensitive enough to measure the differences in isotopic composition -- and isotopes compound the difficulties of measuring and reporting atomic weights. You might want to look up Vienna Standard Mean Ocean Water to learn more about the molecular weight of water, as it addresses such issues.

So, you're left with the basic question of how do you measure the mass of that single atom that you've measured everything else with respect to? I can't answer that in detail because I don't know -- so unless someone pipes up, you're going to have to do some research (but I'll bet you learn a bunch of chemistry and physics along the way). First, start with an elementary chemistry textbook. For example, Pauling's General Chemistry, 3rd ed., discusses on page 103 how the mass of $$^{40}K$$ can be measured by knowing the energy of the emitted electron when the nuclide decays and using $$E = mc^2$$. It also involves the knowledge of the atomic mass of the daughter nuclide $$^{40}Ca$$, so again the measurement involves something in relation to something else. But it gives a clue concerning the measurement of nuclear energies. You probably also will profit by visiting the NIST website, as there will undoubtedly be pointers to some of the work of high-precision atomic weight work.

#### Papabravo

Joined Feb 24, 2006
15,129
What all of this comes down to is that we have a collection of experimental methods for making indirect measurements. At the most basic level we must deal with the uncertainty principle that limits the precision with which such measurements can be made. The very act of observing the nucleus disturbs it in such a way as to make those observations imprecise. This is the nature of quantum electrodynamics and the even stranger world of quantum chromodynamics as we try to fill in the last pieces of the standard model.