[thebrain13]

nt

[damocles]

Yes -- He-3 is a stable isotope of helium.

[thebrain13]

that has 3 neutrons. Is it possible for helium-1 exist? and if not why not?

[CliffordK]

that has 3 neutrons. Is it possible for helium-1 exist? and if not why not?

Not exactly.

Helium, of course, by definition has exactly 2 protons, and can vary with the number of neutrons.

Here is a list of the helium isotopes.

Helium-4 is (He) is the most common form of helium on Earth since it is a nuclear decay product.  It has 2 neutrons and 2 protons for a molecular weight of 4.

Helium-3 (He) is also stable, it has 1 neutron, and 2 protons for a molecular weight of 3.

Helium-2 (diprotium), (He) has no neutrons, and 2 protons.  It is very unstable, and decays almost immediately to deuterium, or Hydrogen-2 (H).

He → H + e⁺ + ν_e + Energy, where e⁺ is a positron (antimatter), and ν_e is an electron neutrino.

By definition, one can not have a Helium-1 because that would mean 2 protons and (-1) neutrons for a molecular weight of 1.

Helium with more neutrons is unstable, with a half-life of less than a second.
Helium-5 (He) would have 2 protons, 3 neutrons, and decays with a half-life of about 700 ×10⁻²⁴ s

[thebrain13]

oh thanks, my mistake. smacks head. what about with the bigger elements? is there a system that determines the least amount of neutrons that an element can have?

[Soul Surfer]

Yes the atomic number defines the number of protons but the atomic weight the number of protons plus the number of neutrons so subtract the atomic number from the atomic weight of an element and you get the approximate number of neutrons. 

I say approximate because many elements are mixtures of isotopes and do not have atomic weights close to whole numbers. 

There are also other reasons why atomic weights are not exact whole numbers.  The atomic mass unit is defined as one twelfth of the mass of a carbon atom (six portions six neutrons and six electrons) but the neutron is slightly heavier than the proton because of its lower binding energy. and most atoms have more neutrons than protons.  You also have to correct for the binding energy of the atom which has a small effect.

[evan_au]

There is a nice graph here: http://en.wikipedia.org/wiki/Stable_isotopes

• For stable light atoms, the number of protons & neutrons is approximately equal.
• For stable heavy atoms, the number of neutrons exceeds the number of protons. You can think of the electrostatic repulsion between the protons, which tends to disrupt the nucleus; this is counteracted by the strong nuclear force from both protons & neutrons which tends to hold the nucleus together.

If there are "too many" protons to be stable, a proton tends to turn into a neutron (emitting a positron=β+ or swallowing an inner electron).
If there are "too few" protons to be stable, a neutron tends to turn into a proton (emitting an electron=β-).
...But other decay modes exist, like emitting a proton or neutron; heavy atoms can also decay by emitting an alpha particle or fission (splitting in halves).