If one neutron is added to Helium nucleus, the result is? a) Lithium b) Helium c) Hydrogen d) Carbon

When a neutron is added to helium nucleus, the new atom keeps the same number of electrons. This means that it's chemical properties stay pretty much the same. An stable atom that has extra neutrons but behaves the same chemically as those atoms without the extra neutrons is an isotope. (Well, they are isotopes to each other). So the new atom is a Helium isotope.

The only way to change an atom's name or the element is to add/subtract protons to/from its nucleus, respectively. If we add another neutron to a helium nucleus, we just get a new isotope of helium, called helium-5.

It will be denoted by #""_2^5"He"# or just #""^5"He"#.

In fact, this is an extremely unstable isotope of helium, with a half-life of just #2.1*10^-20 \ "s"=2.1*10^-11 \ "ns"#.

Source:

https://en.wikipedia.org/wiki/Isotopes_of_helium#Heavier_helium_isotopes

Helium is an atomic nucleus which contains two protons. There are nine known isotopes of Helium which have between zero and eight neutrons. Only Helium-3 and Helium-4 are stable.

If a neutron is added to a Helium-3 nucleus it becomes the highly stable Helium-4.

If a neutron is added to a Helium-4 nucleus it becomes the highly unstable Helium-5. Helium-5 is the most unstable isotope of Helium. It decays by emitting a neutron with a half life of #7*10^-22# seconds.