If three helium nuclei combine to form a carbon nucleus, energy is liberated. Why can't helium nuclei combine on their own and minimise the energy?

A nucleus of uranium decays at rest into nuclei of thorium and helium . Then:

In a fusion reactor, the reaction occurs in two stages. (i) Two deuterium (""_(1)^(2)D) nuclei fuse to form a tritium (""_(1)^(3)T) nucleus with a proton as product. (ii) A tritium nucleus fuses with another deuterium nucleus to form a helium (""_(2)^(4)He) nucleus with neutron as another product. Find (a) the energy released in each stage. (b) the energy released in the combined reaction per deuterium and (c) what percentage of the mass energy of the initial deuterium is released? Given : ""_(1)^(2)D=2.014102u,""_(1)^(3)T=3.016049u,""_(2)^(4)He=4.002603u,""_(1)^(1)H=1.007825u,""_(0)^(1)n=1.008665u Take 1u = 931 MeV

Two lithium nuclei in a lithium vapour at room temperature do not combine to form a carbon nucleus because

The band gap between the valence and the conduction bands in zinc oxide (ZnO) is 3.0eV.Suppose an electron in the conduction band combines with a hole in the valence band and the excess energy is released in the form of electromagnetic radiation .Find the maximum wavelength that can be emitted in this process.

Assertion: Extreme temperature is necessary to execute nuclear fusion. Reason: In a nuclear fusion, the nuclei of the reactants combine releasing high energy.