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Which of the following statements is not correct regarding bonding molecular orbitals?
A Bonding molecular orbitals possess low energy than the atomic orbitals from which they are formed;
B Bonding molecular orbitals have low electron density between the two nuclei;
C Every electron in bonding molecular orbitals contributes to the attraction between the atoms;
D They are formed when the lobes of the combining atomic orbitals have the same sign.

I am confused about option D, I think this should be right as same sign means lobes are in the same phase, so they should form BMO. But the answer key states that the statement is incorrect.

Can anybody please explain me this and is my reasoning correct?

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  • $\begingroup$ Statement (D) doesn't quite make sense. Same sign? Same sign where? $\endgroup$ – Zhe Jul 24 '17 at 15:30
  • $\begingroup$ Consider the canonical bonding molecular orbitals for butadiene. One of the bonding molecular orbitals has a node, so the signs are not the same. $\endgroup$ – Zhe Jul 24 '17 at 16:02
  • $\begingroup$ @Gem. Perhaps because D) is intentionally unclear? A statement like ".... have the same sign in the interatomic region" which is more commonly said, as you did, "lobes overlap with the same sign". It is like that D) means that both lobes of two p orbitals have a + and a - lobe, the same signs. $\endgroup$ – Alchimista Jul 24 '17 at 16:45
  • $\begingroup$ So if 2 - lobe combine would they form a BMO $\endgroup$ – Gem Jul 24 '17 at 17:24
  • $\begingroup$ The question (of the exercise, not that by the OP) Is not well formulated. Zhe is correct with the example of butadiene. However the question seems to refers to biatomic molecules (choice B) "... between the two nuclei". Anyway the general answer is A and one has not to run in distinctions, nor to pi-sigma separation, etc. $\endgroup$ – Alchimista Jul 24 '17 at 17:25

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