Question

h2 molecule exists but he2 molecule does not. why?

Answer 1

H₂ is formed due to combination of two hydrogen atoms. Atomic number of hydrogen is 1 .

Hence it’s electronic configuration is 1s¹.According to valence bond theory,halfilled atomic orbitals can overlapp to form a bond.Thus two hydrogen atoms along with halffilled 1s orbital,overlapp with each other to form H₂ molecule. H₂ is formed dew to s-s overlapp.

Whereas atomic number of helium is 2 .It’s electronic configuration is 1s².Filled orbital can not overlapp and hence cannot form a bond.Hence diatomic helium i.e.He₂ molecule does not exist.

Answer 2

when i was at your level this question was a nightmare for me

here's your answer

h2 molecule exist because the h atom contains only electronin its outer most orbit

now every atoms needs 2 or 8 electron to complete its duplet or octate(you can see this as a fundamental property in chemistry)

so the h atom bonds (pairs up) with another h atom to get another electron (by sharing electrons they both have 2 electron together)

but as you can see he atom already has 2 electron in it's outermost cell

so it doesn't require another electron to complete its outermost orbit as it is all ready filled. so it doesn't form any bonds(the atoms which have their outermost shell/orbit filled don't react as they don't need to fullfill their last orbit). these atoms are called inert gases.

hope it helps