Why doesn't the most common form of the hydrogen atom have a neutron?

In summary, the most common isotopes of elements have varying numbers of neutrons, with Helium having 2 neutrons and Lithium having 3. As we move to higher atomic numbers, the number of protons and neutrons in the nucleus may not be equal, as seen in elements like iron. However, the most basic atom, Hydrogen, also does not have equal numbers of protons and neutrons, leading to the question of why this is the case. Deuterium, a form of Hydrogen with one proton and one neutron, was produced in the Big Bang but is not as stable as Helium-4. The relative abundance of deuterium in the early universe was only about 1%, and its
  • #1
omoplata
327
2
Most common isotopes of He has 2 neutrons, Li has 3 neutrons and so on right, until Z increases to higher numbers and we get to elements like iron, where the nucleus doesn't have equal numbers of protons and neutrons anymore. But why isn't the number of protons and neutrons equal in the most common form of Hydrogen, which is the most basic atom there is?

Thanks.
 
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  • #2
Deuterium was produced in the Big Bang. And it's quite easy to make, but apparently the trick is keeping your deuterium once you have produced it. Helium-4 is so doggone stable that conditions in which deuterium can form are also conditions in which deuterium fuses to Helium-4.
 
  • #3
The relative abundance of deuterium in the early universe was at its highest still only about 1%.

The amount of deuterium you can make is limited by the number of neutrons you have, and once you get neutrons, as Bill said, they tend to get locked up into heavier elements like Helium.
 
  • #4

Deuterium is also the most weakly bound compound nucleus and breaks apart easily inside stellar cores.
 
  • #5
Orion1 said:

Deuterium is also the most weakly bound compound nucleus and breaks apart easily inside stellar cores.

Then the question arises "why is it so weakly bound?"Does this has something to do with charge/mass ratio of the nucleus?
 
  • #6
tenchotomic said:
Then the question arises "why is it so weakly bound?"Does this has something to do with charge/mass ratio of the nucleus?

I think so. It is due to there only being 2 particles to attract each other. All other nuclei have 3+ nucleons or are unstable.
 
  • #7
In response to the OP's question, I think the basic answer is that the abundance of light nuclei is determined by big-bang nucleosynthesis, which occurred at high temperatures. At those temperatures, there was a tendency for deuterium to break up. In comparison, helium is doubly magic http://en.wikipedia.org/wiki/Magic_number_(physics) , so it's extremely stable, and any helium that formed in big bang nucleosynthesis was likely to hold together.

Another point to note is that although there are reactions in stars that destroy deuterium, there are none that create it. Therefore the abundance of deuterium in our universe is like a ticking clock that started counting down after the big bang. The fact that its abundance isn't zero is actually very strong evidence for the finite age of the universe. If the universe was infinitely old, then there would be no deuterium left.

tenchotomic said:
Then the question arises "why is it so weakly bound?"Does this has something to do with charge/mass ratio of the nucleus?

No, electromagnetic interactions are irrelevant to the stability of light nuclei. When you have a set of Z protons interacting, the number of interactions is Z(Z-1)/2, i.e., it basically grows like Z^2. In a heavy nucleus like uranium, with Z=92, electrical repulsion is a major player, but not for small Z. For Z=1 the Coulomb energy vanishes, which would tend to make the nucleus *more* stable.

For light nuclei, the most stable ratio of Z/A is 1/2. This isn't because the protons are charged. It's because of the Pauli exclusion principle, which favors equal numbers of protons and neutrons. Again, this should actually favor the stability of deuterium.

A couple of generic reasons why we should expect deuterium to be unstable:

(1) Nuclei are most bound in the region around iron. Nuclei lighter and heavier than iron are less bound, which is why you can generate energy by fusion of light nuclei and fission of heavy nuclei.

(2) Odd-odd nuclei are always unstable compared to odd and even-even nuclei.

What's actually quite surprising is that deuterium is *stable* with respect to beta decay. Almost all odd-odd nuclei are unstable.
 
  • #8
I'd argue its largely related to the condition that created the other nucleons.

In stars there are lots of protons and neutrons, and the gravity is strong enough to bring them close together, where the nuclear force is notable.

Outside of stars, there are many more protons than neutrons if for no other reason, due to substantially different half lives. Plus there are very few forces bringing the two together. I.e, if a nucleon is "made" in open space, its a very rare event. Usually protons just carry on their own way.

So essentially it isn't that hydrogen is special, its that all of the other nuclei are.

Incidentally what binds the deuteron is the nuclear spins. They align in the singlet state, which is the only bound state of the deuteron.
 
  • #9
diggy said:
Outside of stars, there are many more protons than neutrons if for no other reason, due to substantially different half lives. Plus there are very few forces bringing the two together. I.e, if a nucleon is "made" in open space, its a very rare event. Usually protons just carry on their own way.

This is all wrong. It sounds like you're imagining deuterium being produced in the present-day universe. The deuterium that presently exists in the universe was formed in big-bang nucleosynthesis: http://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis
 
  • #10
The point I was trying to make is that protons and neutrons don't just come together and form deuterium for no reason, i.e. free protons are the norm, and bound states (such as deuterium, and heavier nucleons) are the exception.
 
  • #11
diggy said:
The point I was trying to make is that protons and neutrons don't just come together and form deuterium for no reason, i.e. free protons are the norm, and bound states (such as deuterium, and heavier nucleons) are the exception.

Big-bang nucleosynthesis happened in the early universe, which was very dense. There is also no Coulomb barrier between a neutron and a proton. So I'm serious, what you said was just completely wrong. If you want to learn something about big-bang nucleosynthesis, a very cool popular-level book is The First Three Minutes, by Steven Weinberg.
 
  • #12
diggy said:
In stars there are lots of protons and neutrons, and the gravity is strong enough to bring them close together, where the nuclear force is notable.

There are few, if any, free neutrons in the cores of stars. Neither the triple alpha process, the proton-proton chain, nor the CNO cycle release free neutrons. The neutrons that are made are a result of beta decay and are completely bound to the nuclei in which they form.

Incidentally what binds the deuteron is the nuclear spins. They align in the singlet state, which is the only bound state of the deuteron.

I believe it is the strong force that binds both nucleons together in a deuteron. The alignment of the spin is simply the lowest energy level between the available options.
 
  • #13
bcrowell said:
Big-bang nucleosynthesis happened in the early universe, which was very dense. There is also no Coulomb barrier between a neutron and a proton. So I'm serious, what you said was just completely wrong. If you want to learn something about big-bang nucleosynthesis, a very cool popular-level book is The First Three Minutes, by Steven Weinberg.

You are right that is the better picture. I was thinking in terms of post big bang. But as you say, everything was cooking together at the start. Incidentally do you know what the binding energy of the deuteron would have to be for the universe to be deuteron rich and hydrogen poor?
 

FAQ: Why doesn't the most common form of the hydrogen atom have a neutron?

1. Why doesn't the most common form of the hydrogen atom have a neutron?

The most common form of the hydrogen atom, also known as hydrogen-1 or protium, does not have a neutron because it has a single proton in its nucleus and no neutrons. This is because hydrogen is the lightest element and only contains one proton. This form of hydrogen is stable and does not require a neutron for stability.

2. What is the role of neutrons in an atom?

Neutrons play a crucial role in the stability of an atom. They are found in the nucleus of an atom and help to hold the protons together through the strong nuclear force. Neutrons also help to balance out the positive charge of the protons, making the atom more stable.

3. How many forms of hydrogen are there?

There are three known forms of hydrogen: hydrogen-1 (protium), hydrogen-2 (deuterium), and hydrogen-3 (tritium). Each form has a different number of neutrons in its nucleus, with protium having zero neutrons, deuterium having one neutron, and tritium having two neutrons.

4. Can hydrogen-1 have a neutron?

No, hydrogen-1 (protium) cannot have a neutron. The number of neutrons in an atom is determined by its atomic number, which for hydrogen is 1. This means that hydrogen-1 will always have one proton and no neutrons.

5. Why is hydrogen-1 the most common form of hydrogen?

Hydrogen-1 (protium) is the most common form of hydrogen because it is the lightest and simplest element. It is also the most stable form of hydrogen, making it the most abundant in the universe. This is because it only contains one proton, which is the basic building block of all elements.

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