# Does fusion increase density?

1. Feb 19, 2016

### CrackerMcGinger

So, I was wondering this; since fusion takes two atoms and combines them into one nucleus, can that increase the density of the resulting nucleus?

2. Feb 19, 2016

### Staff: Mentor

The density of nuclei is nearly independent of the mass of them. More massive nuclei are also larger, the density is roughly the same.
Where does the density of a nucleus matter?

3. Feb 19, 2016

### HallsofIvy

That depends on many different things. If the fusion results in energy given off then the total mass of the two nuclei together will be slightly less than the two masses separately. But density is "mass divided by volume". How are you measuring the volume of the fused nuclei?

4. Feb 19, 2016

### CrackerMcGinger

It wouldn't if only two atoms were combined, but if a substantial quantity of atoms were combined, lets say 1,000,000,000 atoms, wouldn't the density increase?

5. Feb 19, 2016

### CrackerMcGinger

By the mass of the original nuclei compared to the mass of the fused and its new size. Or so I would think.

6. Feb 19, 2016

### Staff: Mentor

You cannot fuse a billion atoms to a single one. The largest stable atoms have about 200 nucleons, the largest short-living produced in a lab about 300.

A neutron star is a bit similar to a huge amount of nuclear matter, and guess what: even its density is similar to the density of normal nuclei.
How do you measure the size? Nuclei are not billard balls.

7. Feb 19, 2016

### CrackerMcGinger

Do you know how many electrons and protons are in that 300 nucleon atom?

8. Feb 19, 2016

### Staff: Mentor

No electrons at all. It is a nucleus, not a neutral atom. The produced nucleus can capture a few electrons before it decays.
Nuclei with up to 118 protons have been produced experimentally.

9. Feb 19, 2016

### CrackerMcGinger

What is the most stable heavy atom?

10. Feb 19, 2016

### Staff: Mentor

There is no "most stable", either nuclei are stable or they are not.
Lead-208 is the heaviest nucleus where no decay has been observed.
Zirconium-92 is the heaviest nucleus where no decay is possible at all, apart from proton decay (if protons are not stable).

11. Feb 19, 2016

### CrackerMcGinger

Is their any way to combine two atoms so that they don't lose their electrons and such?

12. Feb 19, 2016

### Staff: Mentor

You are mixing two completely different topics here.

The nuclei just don't care about electrons. Unless you keep the nuclei carefully isolated, they collect electrons until they form neutral atoms (or until they decay), but this is irrelevant for the nuclei themself.
If you put two atoms (nuclei with electrons) close together, they can form a chemical bond. That has nothing to do with nuclear fusion.

13. Feb 19, 2016

### Staff: Mentor

I think the OP should consider:
From wikipedia:
If you are asking does density go up with atomic number somehow? - then the Answer is no. The above is a counterexample.
Gold (Au) is 79, Tungsten(W) is 74, and Uranium (U) is 92, but Au and W are denser than U. Density has to do with how whole groups of atoms arrange themselves into crystals (or solids or gases), not how many protons are in the nucleus of an atom. And there are instances where a higher atomic numbered element is denser than some other element with a lower atomic number. It just is never always true.

Plus I think you are confusing the density with mass. density is the quotient: mass/volume It is a relative value, relative to something (normally water).

14. Feb 19, 2016

### Astronuc

Staff Emeritus
And atomic radii, which is determined by the electrons surrounding the nucleus.

Also, look at the densities of Os and Ir ( X-ray crystallographic data yielded densities of 22.56 g/cm3 for iridium and 22.59 g/cm3 for osmium.[11]' from Wikipedia). Hs and Mt probably have fairly high densities as well, but they don't stay around very long.
Ref: https://en.wikipedia.org/wiki/Iridium#Physical_properties

15. Feb 22, 2016

### snorkack

Is the density of feebly bound nuclei like deuterons the same as for better bound ones?
To a limit where the combined nucleus become unstable to positron emission or electron capture.
For example, it is possible to fuse as many as 10 alpha particles so that they do not lose any electrons - calcium 40 is stable. But 11th is too much - titanium 44 is unstable to electron capture with half-life 60 years.
It is possible to fuse further nuclei to titanium 44. Chromium 48 is capable of positron emission (half-life 21 hours), so is iron 52 (half-life 8 hours) and nickel 56 (half-life 6 days). Zinc 60 also decays (half-life 2 minutes). But the problem is that zinc 60 is so strongly positively charged that fusing that 15th alpha loses energy. Which is why only 14 alpha particles can fuse to nickel 56, which then decays to iron 56.
If there were time for unstable nuclei to decay and get rid of excess electrons and positive charge, the resulting less charged nuclei would be more stable and be capable of further fusion. For example, iron 56 could fuse with further two alphas (and get rid of further two electrons) and form nickel 64. However, nickel 56 tends to be formed in places where it does not have enough time to get rid of electrons, which is why iron is common but nickel rare.

16. Feb 22, 2016

### Staff: Mentor

How do you define the volume of deuterium? rms of mass? rms of charge? Volume that contains 90% of mass or charge? Cross sections? Something else? This is not a large issue for larger nuclei, but it is a massive problem for small ones.