Bond Angles (Simplified) - Online Tutor, Practice Problems & Exam Prep

A bond angle is the angle formed between two adjacent atoms in a molecule. The ideal bond angle occurs when the central atom has no lone pairs, minimizing repulsion. For two electron groups, the bond angle is 180 degrees. With three electron groups, the ideal angle is 120 degrees, but lone pairs decrease this value. For four groups, the ideal angle is 109.5 degrees, which also decreases with lone pairs. Understanding these angles is crucial for predicting molecular shapes and behaviors in chemical reactions.

Bond Angles (Simplified) Concept 1

A bond angle is the angle formed by two adjacent neighboring atoms in a molecule. And we're going to say when the central element has 0 lone pairs, it possesses what we call an ideal bond angle. Now this ideal bond angle is the optimal angle elements take in order to minimize repulsion between one another. And we're going to say when the central element has one or more lone pairs, its ideal bond angle will be decreased.

So, for example, if you take a look here, we said that the bond angle is the angle by two adjacent neighboring atoms in a molecule. So you'll have a central element. It'll be connected to two surrounding elements, and you're going to say that the angle bond angle is this portion here. If we have three surrounding elements, the bond angle will be here, it would also be here, as well as here. And if we had a lone pair, the bond angle would be in here. Now again, remember, once we have a lone pair on our central element, the bond angle is going to decrease. This decrease will be represented as a blue image for a smaller bond angle. Right. So now that we know what a bond angle is and how lone pairs help to reduce our ideal bond angle values, let's click on the next video and take a deeper look in terms of the exact values with these bond angles.

Bond Angles (Simplified) Example 1

Here we're told if the H-C-H angle within CH₄ molecule is 109.5°, what is the H-N-H bond angle within NH₃? Right. So if we were to draw out CH₄, carbon would go in the center. Remember, bonds to hydrogens are only single bonds. This would represent CH₄. We have 4 bonding groups on the carbon with 0 lone pairs on the carbon.

NH₃: So NH₃ would have our bonds to hydrogens, but remember if we count up the total number of valence electrons, nitrogen has 5, and then we have 3 hydrogens, each one with 1 electron because they're in group 1a. So that'd be 8 total valence electrons. We have 2, 4, 6 here and we'd have a lone pair on the nitrogen for our remaining electrons. Here, the bond angle is 109.5°. But remember, we said that the presence of a lone pair reduces or decreases our bond angle. So we'd expect NH₃ to have a bond angle that is less than 109.5°. If we take a look at our options, the only one that has an angle that's less than 109.5° is option C, 107.3°.

Bond Angles (Simplified) Concept 2

Here we can say that bond angles can further differentiate molecules that possess the same number of electron groups. So when we have 2 electron groups, that means we have only one possibility: our central element having 2 surrounding elements. In that case, we have an ideal bond angle because our central element can't have a lone pair. Therefore, for an electron group of 2, the ideal bond angle is 180°. When we have 3 electron groups around the central element, we have 2 possibilities, one where the central element just has 3 bonding groups and 0 lone pairs. In this case, because there are no lone pairs on the central element, we have an ideal bond angle of 120°. But remember, another possibility exists where we could have 2 bonding groups and 1 lone pair. The presence of the lone pair means that our bond angle will decrease from its ideal value. All you need to say at this point is if the ideal bond angle for 3 electron groups is 120°, then when it gets decreased, it'll be less than 120°. You don't have to give an exact number. You can just say less than 120°.

Alright. So when we have 4 electron groups, we have 3 possibilities: we have 4 bonding groups, 0 lone pairs. Having 0 lone pairs means we have an ideal bond angle of 109.5°. But we also have 3 bonding groups and 1 lone pair. So here we just say that our bond angle now is less than 109.5°. And then we have our last possible option: 2 bonding groups, 2 lone pairs. Here, we expect the bonding angle to be again less than 109.5°. And in fact, we'd say that it's even a little bit less than 109.5° than this one because the presence of more lone pairs only helps to further reduce the bond angle. So just remember, when we have no lone pairs on the central element, we have an ideal bond angle. The inclusion of any lone pairs after this means that our bond angle will be decreased from this ideal bond angle value.

Bond Angles (Simplified) Example 2

Here we need to determine the Sn-H bond angle for the following compound. So we have Tin (Sn) connected to 2 hydrogens. Tin is in group 4A, so it has 4 valence electrons. Here we have 2 hydrogens, each one has 1 valence electron since they're in group 1A. That's a total of 6 valence electrons involved. Here, we would place Tin in the center. It is connected to the 2 hydrogens. Remember, hydrogens can only make single bonds. They don't follow the octet rule; they only follow the duet rule, so you don't have to add any additional electrons to them. At this point, we've used 4 of our electrons because each covalent bond has 2 electrons within it. That means we have 2 valence electrons remaining, which we simply place on the tin. Now how many electron groups does the tin have? It has one lone pair and 2 bonding groups, so 3 electron groups. Remember, for 3 electron groups, the ideal bond angle is 120 degrees. But when there is the presence of a lone pair on the central element, the ideal bond angle decreases. So, all you have to say here is that we have a bond angle that is less than 120 degrees for the following compound.

Determine the bond angle for the following compound:BeCl₂.

• A. 90°
• B. 180°
• C. 109.5°
• D. 120°

Determine the bond angle for the thiocyanate ion, SCN^–.

• A. 180°
• B. 90°
• C. 120°
• D. 109.5°

Determine the Cl–O–Cl bond angle for the OCl₂ molecule.

• A. >109.5°
• B. <109.5°
• C. >120°
• D. <120°

Here’s what students ask on this topic:

What is a bond angle in chemistry?

A bond angle in chemistry is the angle formed between two adjacent atoms in a molecule, with a central atom connecting them. It is a crucial parameter in determining the shape and geometry of a molecule. The ideal bond angle occurs when the central atom has no lone pairs, minimizing electron repulsion. For example, in a molecule with two electron groups, the bond angle is 180 degrees. Understanding bond angles helps predict molecular shapes and behaviors in chemical reactions.

How do lone pairs affect bond angles?

Lone pairs on the central atom affect bond angles by decreasing them from their ideal values. Lone pairs occupy more space than bonding pairs, causing greater repulsion. This repulsion pushes the bonding pairs closer together, reducing the bond angle. For instance, in a molecule with three electron groups, the ideal bond angle is 120 degrees. However, if one of these groups is a lone pair, the bond angle will be less than 120 degrees.

What is the ideal bond angle for a molecule with four electron groups?

The ideal bond angle for a molecule with four electron groups is 109.5 degrees. This angle is observed when the central atom has no lone pairs, resulting in a tetrahedral geometry. If lone pairs are present, the bond angle will be less than 109.5 degrees due to increased repulsion from the lone pairs.

Why is the bond angle 180 degrees for two electron groups?

The bond angle is 180 degrees for two electron groups because this arrangement minimizes electron repulsion. With only two electron groups, the atoms are positioned directly opposite each other, creating a linear geometry. This maximizes the distance between the electron groups, resulting in the least amount of repulsion and an ideal bond angle of 180 degrees.

How do bond angles help in predicting molecular shapes?

Bond angles help in predicting molecular shapes by providing information about the spatial arrangement of atoms around a central atom. By knowing the number of electron groups and the presence of lone pairs, one can determine the ideal bond angles and how they are adjusted due to lone pair repulsion. This information allows chemists to predict the overall geometry of the molecule, such as linear, trigonal planar, tetrahedral, etc., which is essential for understanding molecular behavior and reactivity.