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VSEPR for 3 electron clouds

In this video, we apply VSEPR theory to molecules and ions with three groups or “clouds” of electrons around the central atom. To minimize repulsions, three electron clouds will always adopt a trigonal planar electron geometry. If none of the clouds is a lone pair, the molecular geometry will also be trigonal planar. If one of the clouds is a lone pair, the molecular geometry will be bent. Created by Jay.

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• At , why don't both the oxygen's make double bonds with sulfur? I know this breaks the octet rule, but sulfur is in the 3rd period, so isn't it more important to have no formal charge than obey the octet rule?
• It is because Oxygen is more electronegative, and therefore will consume more of the electrons. Sulfur, being less electronegative, cannot have more electrons around it than the more electronegative oxygen
• Can anyone clarify the last rule of 'ignoring lone pairs of electrons'?
I thought that the lone pairs of electrons had a slightly stronger repulsive force than the bonded electrons and that they would reduce the bond angle between the 2 Oxygens to less than 120 (I know he denotes it as ~120 degrees). Surely in order to get the 'bent' shape, we have to specifically consider the lone electron pairs as opposed to ignoring them?
• The lone pairs help determine the electron pair geometry.
Thus, one lone pair and two bonding pairs give a trigonal planar electron geometry with bond angles of about 120°.
But molecular geometry considers only the bonding pairs.
Thus SO₂ has a trigonal planar electron geometry, but the two S-O bonds are at an angle of about 119°.
Since we consider only the bonds, the molecular geometry is bent.
• I don't get that if a central atom can complete an octet then why is the formal charge an issue?
I mean what happens due to the neutralization of the formal charge? How does it contribute to the stability? Or is formal charge a greater stability factor than octet?
• But like in the video it you have too much electrons in a molecule and you can't put them in any other thing that's bonded to the center, put some electrons in the center.
• Why O-S=O and not O=S=O?
• Watch the next video in the playlist - it specifically addresses this question.
• At it says bent, so is it 120 degrees or 104.5 degrees?
• In this instance, the bond angle will be 120 degrees. If there was a lone pair on the central atom, then the bond angle would be reduced to 104.5 degrees. See the video titled "VSEPR for 4 electron clouds" for an example of this
• At he says the bond angle for sulfur dioxide (bent) is 120 degrees, the same as for a trigonal planar structure. In my chemistry course, however, my teacher explained that the lone electron pair on the sulfur atom will exert a stronger repulsive force and thus push the oxygen atoms further away, resulting in a smaller bond angle of about 115 degrees.
• Your teacher is partly correct. Lone pairs do take up more space than bonds (or "push" the bonds away from them, if you like to think of it that way). However, the bond angle is 119.3°, not 115°. But please be aware that when we have lone pairs "pushing" the bonds away from them, the number of degrees they shift by is not a set thing that would apply to all molecules -- in some molecules or ions the amount of "pushing" is quite small, in others it is more pronounced. It just depends on the details.

(Your teacher may have been thinking of the nitrite anion, NO₂¯, which does have a bond angle of about 115°)
• At he mentions that Boron doesn't need to follow the octet rule but it can. Does that mean that BF3 would have four resonance structures? The one that doesn't follow the octet rule AND the three that do?
• It cannot because of the number of total Valence Electrons. Boron can follow the octet rule, but the total number of Valence Electrons prevents this. Boron normally "wants" 6-8 electrons.
• for evaluating the formal charge on oxygen u did it like 6-7 n got a -1 thn for sulfur y r we substracting 6 from 5? it shud have been 5-6 . as for oxygen u substracted its group no from the no of electrons it is surrouned by but the opposite was followed for sulfur.. i did nt get it
• Formal charge = valence number of neutral atom - (dots + bonds)
• in BF3.......doesnt boron lose 3 electrons which leaves 2 electrons tthus filling the first shell
(1 vote)
• No, the electrons are shared. The compounds of boron are not often covered in General Chemistry because they often involve different types of bonds than those usually covered at this level of study.

Also note that boron does not obey the octet rule (at least not in the usual sense).