What Defines the Size of an Atom and How Do We Know Force Carriers Exist?

[Helicobacter]

1. How is the size of an atom defined? Shouldn't it be the size of the universe? Is it defined as: The max span of an isolated electron cloud, such that the c.d.f. will equal .95?
2. How do we know force carriers/bosons exist? Have they been directly observed or are they just hypothesized to exist?
3. Since the proton is encapsulated within the electron cloud, I would assume that field is finite (at a certain radius there is a critical value where you can guarantee 100% that the proton will be in that region). Is this true?

 

[blechman]

1. Size of atom is <r>, the quantum expectation value of the radius. There is certainly a probability for the electrons to be "outside" the atom with this definition, but that's not much more than semantics ("size" versus "size" versus "size"). The probability of the electron being "outside" the atom falls off exponentially with distance.

2. Photons can be observed directly in optics experiments. Gluons can be seen directly in jets at particle colliders. W/Z bosons can be made at particle colliders and identified through their decays.

3. The proton is NOT "guaranteed 100%" to be ANYWHERE! But like your first question, that is merely semantics. The probability of the proton being separated from the electron cloud is vanishingly small!

 

[Helicobacter]

thx for ur asnwer

reg. your last statement

i claim: a particular proton has 100% probability of being anywhere in the universe at time t.
am i wrong?

and we all see constantly pics of the pdf of the electron cloud but never of the proton cloud...how does the shape of the pdf of the proton cloud look like?

4. also another question that I am wondering about: why are certain atoms radioactive? i mean doesn't the strong interaction make the added neutrons stick to the nucleus (very hard/strong)?

 

[blechman]

proton is not guaranteed to be at any FINITE REGION, is what I meant.

Radioactivity: strong force is strong, it's true, but not always strong enough. If an atom has too many neutrons, the neutrons will want to decay to protons (e.g.: beta decay). If the atom is too large, it will want to break apart due to electromagnetic repulsion (e.g.: alpha decay).

 

[Helicobacter]

i thought in beta decay what is shed out by the atom is an electron?

 

[blechman]

neutron --> proton + electron + (anti)neutrino.

the proton stays in the nucleus.

 

[Helicobacter]

so that means it will transform into another atom??

and how does nature decide that it will shed this combination of particles? i mean its nice and convenient: we have neutrinos and electrons and protons in nature already so its good to stay consistent with the same "lego pieces" but its still itneresting to note that we don't have "cucumber particle" and "banana particle" from the neutron...i think i have asked this before and i didnt get an answer i could understand... why does nature say: well let's go with particles i already know..thanks for the reaction formula.

 

[blechman]

radioactive decays (except gamma decays) always change atomic number, so yes, you will have another atom.

I have no idea how to interpret that last paragraph of yours. We spent the last century discovering new particles in experiments. After a lot of research and a lot of money spent, we now have a consistent framework called the "Standard Model of Particle Physics" that tells us all of the particles and their interactions in nature. We are still trying to discover more of these particles at the LHC. Nothing "convenient" about anything! Nature does things HER way!

 

[Helicobacter]

the question is: why is the neutron not kicked out of the atom per se. why does a transformation need to occur first of all, and secondly why is it exactly these 3 particles and not any other n particles with the same mass as the neutron (say 4 particles each with mass [neutron mass]/4)?