According to the **VSEPR Model**, bond angles result from surrounding elements and lone pairs around the central element positioning themselves at an optimal distance.

**Concept:** VSEPR Model for Bonding

Just realize according to the valence shell electron pair repulsion model or VSEPR model, we're going to say bond and lone pairs will position themselves around the central element so that they are as far apart as possible.

We said this before, if we have lone pairs, lone pairs are like pure energy. They want to push away all the bonds away from them. The bonds will position themselves as far away if possible from each other, but also as far away as possible from the lone pairs that exists around the central element. That's what the VSEPR model tries to explain. It's a way of describing what the molecular geometry is and why does it look the way that it does.

The **Valence Shell Electron Pair Repulsion **(VSEPR) theory gives bond angles based on the number of groups around the central element.

**Concept:** Ideal Bond Angles

Based on the electronic geometry that we have, we have certain types of idealized bond angles, so ideal bond angles. So if your electronic geometry is AX2, your electronic region is two. AX3, AX4, AX5, AX6. That's what we mean by the electron regions. That just means how many groups are around my central element.

We know if you're AX2, then you're linear, so your ideal bond angle is 180 degrees. If your trigonal planer, it's 120. If you're tetrahedral, it's 109.5. If you're trigonal bipyramidal or you're trigonal bipyramidal, you have all of these angles involved. Then, if you're octahedral, you have 90 and 180.

The more lone pairs on the central element then the more compressed the bond angle, and the greater the deviation from an *ideal *bond angle.

**Concept:** Lone Pairs & Bond Angles

What we should realize here is that those ideal bond angles aren't the same for every known compound. Here's the thing, we're going to say those ideal bond angles exist only if our central element has no lone pairs. Once our central element starts to get lone pairs, it's going to compress the bond angle. It's going to make it smaller.

Here we can see an example. Here we have methane which is CH4. Its electronic geometry will be AX4. It has no electron pairs around it, no lone pairs around the central element, so its ideal bond angle, its perfect bond angle would be 109.5.

But if we moved over to ammonia, NH3, we have our first lone pair involved. Lone pairs want to be as far away as everyone else. This is going to push the other bonds away from it. This causes them to compress or get smaller and that actually makes the bond angle smaller.

Water, now we have an additional bond angle, getting smaller, because now there are two lone pairs pushing away.

What you're supposed to take from this is those ideal bond angles are only if the central element has no lone pairs. Once the central element starts to have lone pairs the bond is going to get smaller and smaller.

Of course, your professor is not going to want you to memorize every single bond angle known to man. All you would have to say is, you don't need to know this exact bond angle, all you need to know is that the electronic geometry is AX4, so technically it's tetrahedral. The ideal bond angle is 109.5, but because that lone pair is there, all you'd have to really say is, you would expect the bond angle to be less than 109.5.

Here, since you have two lone pairs, you could say the same exact thing again, its electronic geometry is still AX4, ideally, it should be 109.5, but the lone pairs being there, make it less than 109.5.

This is what your professor would be looking for and this is what you would have to say.

**Example:** Determine the bond angles of each of the following compounds.

CO_{2}

**Example:** Determine the bond angles of each of the following compounds.

BrF_{4}^{+}

In cumulene, what are the C=C=C and H-C-H bond angles, respectively?

Enter the C=C=C bond angle followed by the H-C-H bond angle separated by a comma.

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Which of the following has bond angles slightly less than 120 ^{o}?

- NO
_{3}^{-} - HO
_{2}^{-} - NO
_{2}^{-} - CS
_{3}^{2-} - I
_{3}^{+}

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Which of the following molecules or ions has the smallest H-N-H bond angle?

a) NH_{4}^{+}

b) NH_{3}

c) NH_{2}^{−}

d) H_{2}N-NH_{2}

e) H_{2}N-CH_{3}

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Which of the following molecules would have the smallest angles at the central atom?

A. CH_{4}

B. NH_{3}

C. H_{2}O

D. CO_{2}

E. PF_{5}

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The bond angels in SF_{5}^{+ }are expected to be:

a. 90°

b. 120°

c. 90° and 120°

d. 90° and 180°

e. 90°, 120° and 180°

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The best predicted shape and bond angle of SbH _{3} is

1. trigonal pyramidal; 107°.

2. trigonal pyramidal; 109.5°.

3. tetrahedral; 109.5°.

4. trigonal planar; 120°.

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What is the actual bond angle between oxygen-sulfur bonds on molecules of SO _{3}?

(a) 109.5°

(b) < 109.5°

(c) > 109.5°

(d) 120°

(e) <120°

Watch Solution

Of the following molecules given below, which has the largest bond angle?

a) SO_{3}

b) SF_{2}

c) HCN

d) H_{2}S

e) PF_{3}

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Dot structures are given below for several compounds containing iodine (lone pairs are shown only for the central atom). Draw the structure and name the molecular geometry from the following list: linear, bent, T-shaped, see-saw, trigonal pyramidal, trigonal planar, trigonal bipyramidal, square planar, square pyramidal, tetrahedral, octahedral.

Watch Solution

Draw the Lewis structure of BrF_{3} and determine the bond angle between an equatorial F atom and an axial F atom

- = 90º
- < 90º
- > 120º
- = 120
- = 109.5º

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Which molecule has the smallest bond angles?

a) CO_{2}

b) H_{2}O

c) NH_{3}

d) BF_{3}

e) CCl_{4}

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The C—N—O bond angle in nitromethane, CH_{3}NO_{2}, is expected to be approximately

1) 60°

2) 90°

3) 109.5°

4) 120°

5) 180°

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Place the following in order of ** increasing** X-Se-X bond angle, where X represents the outer atoms in each molecule.

SeO_{2} SeCl _{6} SeF_{2}

A) SeCl_{6} < SeF_{2} < SeO_{2}

B) SeF_{2} < SeO_{2} < SeCl_{6}

C) SeF_{2} < SeCl_{6} < SeO_{2}

D) SeO_{2} < SeF_{2} < SeCl_{6}

E) SeCl_{6} < SeO_{2} < SeF_{2}

Watch Solution

What is the value of the smallest bond angle in XeBr _{4}?

a. 109.5

b. 120

c. 90

d. 180

e. 45

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If PBr_{3}Cl_{2} is a nonpolar molecule, determine the Cl-P-Br bond angle?

a. 120

b. 180

c. 90

d. 109

e. 55

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If PBr_{3}Cl_{2} is a nonpolar molecule, determine the Cl—P—Br bond angle.

a. 120

b. 180

c. 90

d. 109

e. 55

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Of the following, which molecule has the largest bond angle?

A. SO_{3}

B. SF_{2 }

C. HCN

D. H_{2}S

E. PF_{3}

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The bond angles in NO_{2}Cl (N is the central atom) are

1. 90°

2. < 190.5°

3. 109.5°

4. 120°

5. < 120°

Watch Solution

Which of the following molecules or ions has the ** smallest** H-N-H bond angle?

- NH
_{4}^{+}

- NH
_{3}

- NH
_{2}^{−}

- H
_{2}N-NH_{2}

- H
_{2}N-CH_{3}

Watch Solution