Question Video: Determining the Bond Angle around the Central Atom in a Molecule with a Tetrahedral Shape | Nagwa Question Video: Determining the Bond Angle around the Central Atom in a Molecule with a Tetrahedral Shape | Nagwa

Question Video: Determining the Bond Angle around the Central Atom in a Molecule with a Tetrahedral Shape Chemistry • Second Year of Secondary School

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Molecules with a steric number of 4 have bond angles that differ by approximately 2.5° and have a tetrahedral, trigonal pyramidal, or bent (nonlinear) shape. What is the bond angle around the central atom in a molecule with a tetrahedral shape?

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Video Transcript

Molecules with a steric number of four have bond angles that differ by approximately 2.5 degrees and have a tetrahedral, trigonal pyramidal, or bent, nonlinear, shape. What is the bond angle around the central atom in a molecule with a tetrahedral shape?

First of all, the steric number identifies the number of groups around the central atom in a molecule. These groups may be bonded electron pairs or nonbonded electron pairs, otherwise known as lone pairs. If a molecule has a steric number of four, then it has four groups around the central atom. You may already be familiar with the AXE notation, which includes the central atom, number of bonded pairs, and number of nonbonded pairs in a molecule. The sum of the bonded pairs and nonbonded pairs, or 𝑚 plus 𝑛, must equal four for the steric number to be four.

Now, let’s discuss three shapes of molecules with a steric number of four. Molecules in the AX2E2 group have a bent or nonlinear shape. Molecules in the AX3E group have a trigonal pyramidal shape. Finally, molecules in the AX4 group have a tetrahedral shape.

Let’s discuss the bent shape first. Water is an example of a molecule with a bent shape. We can see that the molecule has two bonded electron pairs. Each pair is shared in a covalent bond between the central oxygen atom and a hydrogen atom. Water also has two lone pairs, which are valence electrons that are not shared in a covalent bond.

Now, let’s examine the trigonal pyramidal shape. Ammonia is an example of a molecule that has a trigonal pyramidal shape. The molecule has three bonded electron pairs. Each of these pairs is shared in a covalent bond between the central nitrogen atom and a hydrogen atom. Ammonia also has one lone pair.

Finally, let’s discuss the tetrahedral shape by looking at a molecule of methane. Methane has four bonded electron pairs, and each pair is shared between the central carbon atom and a hydrogen atom. Methane does not have any lone pairs on the central atom.

Now we’re ready to determine the bond angles around the central atom in each molecule. In methane, the four bonded pairs have equal repulsive forces and all HCH bond angles are identical, with a value of 109.5 degrees. In ammonia, the lone pair occupies more space than each bonded pair, so the HNH angle is only 107 degrees. Finally, in water, the two lone pairs take up even more space and repel each other more strongly than the bonded pairs. Therefore, the HOH bond angle is even less and has a value of 104.5 degrees.

In conclusion, the bond angle around the central atom in a molecule with a tetrahedral shape is 109.5 degrees.

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