Question

Why bond angle in amine is less than 109.5 degree ?

Answer 1

• Ammonia has four groups of electrons and so it has the same electronic geometry as other molecules like CH₄.
• In case of NH₃ however, one group has an unshared pair of electrons, and as per VSEPR the four electron groups around a central atom will be directed towards the corners of tetrahedron to give maximum separation.
• N has tetrahedral electronic geometry, which refers to the geometric arrangement of electron groups around central atom.
• The molecular geometry however excludes the lone pairs on the central atom and describes only the arrangement of atoms around the central atom.
• Ammonia (NH₃) is an example of molecule in which the central atom has both shared and unshared electron pairs. The shape of ammonia molecule (NH₃) is trigonal pyramid.
• There are three bonding pairs of electrons and one non bonding pair. The bond angle in a molecule of ammonia are 107^o, a value very close to tetrahedral angle (109^o.5^').
• We can write a general tetrahedral structure for the electron pairs of ammonia by placing the non bonding pair at one corner.
• A tetrahedral arrangement of the electron pairs explains the trigonal pyramidal arrangement of the four atoms.
• The bond angles are 107^o (not 109^o.5^') because the non bonding pair occupies more space than the bonding pairs.