The bond angle of water is decreased from 109Degree 28' to 104Degree 24',why?
The two sets of unpaired electrons on the water molecule repel each other making the H-O-H angle less than the 108.29 theoretical value.
The valence shell of Carbon (forming CH4), Nitrogen (forming :NH3) and Oxygen (forming HOH with 2 non-bonded pair) undergoes hybridization into 4sp^3 Hybrid Orbitals which look like skewed figure 8s... This is like taking the valence shell orbitals and putting them all in a blender, mixing really well and pouring the results out into 4 equal piles. These are hybrid orbitals. They all look alike, all have same energy and each can hold up to two electrons. Now, just before bonding takes place, the 4 hybrid orbitals connect at their apex of the fig-8 and assume positions in space of a regular tetrahedron. For CH4 all bonds are equal and angles between hybrids = 109.50 degrees. For Ammonia (:NH3) one of the hybrids is carrying 1 non-bonded pair of electrons and the other hybrids have 1 paramagnetic (unpaired) electron which bonds with Hydrogen. The negative character of the non-bonded pair has a stronger negative character than the bonded electron pairs. This results in e/e repulsions to push the bond angels to 107-degress and Oxygen in H:O:H (Visualize two non-bonded pair above and below the oxygen.) The stong negative character of the 2 non-bonded electron pairs repel the bonded electron pairs to 105-degrees.
Note that substrates that have high electronegative character (Halogens) attached to one of the hybrid orbitals of carbon can function to attract the bonded electron pair and increase the bond angles to greater than 109-degrees... Example, CH3Cl (Methy Chloride).
For more info... Google Valence Bond Theory of Molecular Bonding.
The bond angle of water, which is the angle formed between the two hydrogen atoms and the oxygen atom in a water molecule, is commonly known to be approximately 109 degrees and 28 minutes (written as 109° 28').
The decrease in the bond angle of water from 109° 28' to 104° 24' can be explained by the presence of external factors such as electronegativity and steric effects.
1. Electronegativity:
Oxygen is more electronegative than hydrogen, which means it attracts the shared electrons in the water molecule more strongly. This causes the electron cloud to be shifted closer to the oxygen atom, creating partial negative charges on the oxygen and partial positive charges on the hydrogen atoms. The electron cloud repulsion between the two hydrogen atoms results in a wider bond angle than what would be expected based on a simple tetrahedral geometry (109.5°). However, the electronegativity of the oxygen atom can vary depending on the local environment, which can affect the bond angle. In certain situations, the oxygen atom can hold the shared electrons more tightly, causing a decrease in the bond angle.
2. Steric effects:
Water molecules have a bent shape due to the presence of lone pairs of electrons on the oxygen atom. These lone pairs repel the bonding pairs of electrons, resulting in a compressed bond angle. The presence of other atoms or groups near the water molecule can influence the bond angle due to steric effects. If there are bulky substituents or neighboring atoms exerting repulsive forces on the water molecule, it can lead to a decrease in the bond angle.
In summary, the decrease in the bond angle of water from 109° 28' to 104° 24' can be attributed to variations in the electronegativity of the oxygen atom and steric effects caused by neighboring atoms or groups. It's important to note that these values are approximate and can vary depending on the factors mentioned above.