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I would just like to clarify the guidelines of drawing MOs formed from interactions between AOs. Strangely, this topic is rarely touched on in most books on molecular orbital theory. The following are some examples of what I consider to be guidelines:

  • When there is an in phase combination of atomic orbitals, i.e. the lobes in contact are of the same phase, the two lobes would merge into one bigger lobe.
  • When there is an anti phase combination of atomic orbitals, i.e. lobes of different phase, the two lobes would seem to be "pushed away" from each other. The electron density seems to shift away from the internuclear region. For example, in the case of a p orbital, the lobe involved in the anti phase combination would shrink in size while the other lobe would increase in size.
  • For bonding MOs, the lobes closer to the more electronegative atom must be necessarily larger. For antibonding MOs, the lobes closer to the less electronegative atom must necessarily be larger.

Is there a well-established set of such guidelines which beginners in MO theory can follow when drawing resultant MOs?

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  • $\begingroup$ I use JD Lee's Concise Inorganic Chemistry from page 93 onwards with special emphasis on page 97 and 98 $\endgroup$ – YUSUF HASAN Jan 10 at 17:58
  • $\begingroup$ The problem is an extension of drawing a function of one spatial variable, say x. Consider an in phase orbital to be positive, an anti phase orbital to be negative. Assume the orbitals have same occupancy (integrated density), then at some point there has to be a node in the summed orbitals, a point with zero density. Cancellations will occur elsewhere to different extents depending on orbital shapes. Extend this line of thought to 3d. $\endgroup$ – Buck Thorn Jan 10 at 19:10

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