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The helium molecule $He X_{2}$ is not known under ordinary conditions because, according to MO theory,

Ait is paramagnetic
Bit has an odd number of electrons
Cits bond order is zero
Dits bond order is one

Answer: C

He X_{2}

has four electrons:

(σ 1 s)^{2} (σ^{*} 1 s)^{2}

. Bond order

= (2 - 2) /2 = 0

, so there is no net bonding and the molecule is not stable.

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