Give the atomic symbol for an element that does not exist as a molecule or extended structure. that is, an element that exists only as discrete atoms.

Generally an elements exist as a molecule of extended structure as it can form bond with the atom / atoms of same element

They form bond in order to full fill their octet / duet (o=in case of hydrogen) and to attain stability

It is considered that the molecules with complete octet of atom is stable

However, in case of noble gases (like helium, neon , argon, Krypton etc) they already have that complete octet and they have already attained that stability.

So they do not undergo bond formation with the atoms of same type hence they do not form He2 like H2

Or we can say that these elements  ( He , Ne and Ar) exists as discrete atom