Using VSEPR what is the shape of ammonia (NH3) and water (H2O) when bonded to a hydrogen bond?
To determine the shape of a molecule using the VSEPR (Valence Shell Electron Pair Repulsion) theory, we need to consider the number of electron groups (bonded atoms and lone pairs) around the central atom. Let's examine the shapes of ammonia (NH3) and water (H2O) when bonded to a hydrogen atom.
1. Ammonia (NH3):
- Ammonia has one nitrogen (N) atom bonded to three hydrogen (H) atoms.
- The central nitrogen atom has one lone pair of electrons in addition to the three bonded hydrogen atoms.
- Therefore, NH3 has four electron groups around the central nitrogen atom, consisting of three bonding pairs (N-H) and one lone pair (N).
According to VSEPR theory, when there are four electron groups around a central atom, the electron groups arrange themselves in a tetrahedral geometry with a bond angle of approximately 109.5 degrees. However, the presence of a lone pair tends to compress the bond angles slightly.
Hence, the shape of ammonia (NH3) is called trigonal pyramidal. The three hydrogen atoms are arranged in a pyramid-like shape around the central nitrogen atom, with a bond angle close to 107 degrees.
2. Water (H2O):
- Water has one oxygen (O) atom bonded to two hydrogen (H) atoms.
- The central oxygen atom has two lone pairs of electrons in addition to the two bonded hydrogen atoms.
- Therefore, H2O has four electron groups around the central oxygen atom, consisting of two bonding pairs (O-H) and two lone pairs (O).
Again, using the VSEPR theory, we find that four electron groups around the central atom tend to form a tetrahedral geometry with a bond angle of approximately 109.5 degrees. However, the presence of two lone pairs compresses the bond angles further.
Thus, the shape of water (H2O) is called bent or V-shaped. The two hydrogen atoms are arranged in a V-shape around the central oxygen atom, with a bond angle close to 104.5 degrees.
In summary:
- Ammonia (NH3) has a trigonal pyramidal shape, with a bond angle of approximately 107 degrees.
- Water (H2O) has a bent or V-shaped geometry, with a bond angle of around 104.5 degrees.