Bond angle 109^o 28' means Regular Tetrahedral geometry and hybridization is sp³.

Steric Number (SN) for sp³ hybridization is 4.

In sp³ molecules can have different shapes which is decided by SN number

(1) $$\,\,\,\,$$ 4 bond pair in a molecule then angle between bonds 109^o 28^'

(2) $$\,\,\,\,$$ 3 bond pair and 1 lone pair in a molecule then angle between bonds 107^o.

(3) $$\,\,\,\,$$ 2 bond pair and 2 lone pair in a molecule then angle between bonds 104.5^o

(4) $$\,\,\,\,$$ 1 bond pair and 3 lone pair in a molecule then bond angle is undefined as there is only one bond.

(1) $$\,\,\,\,$$ In NH₃, 3 Bond pair(BP) +1 lone pair (LP) present so angle between bond 107^o.

AIEEE 2002 Chemistry - Chemical Bonding & Molecular Structure Question 243 English Explanation 1

AIEEE 2002 Chemistry - Chemical Bonding & Molecular Structure Question 243 English Explanation 1

(2) $$\,\,\,\,$$ BF$$_4^ - $$, 4 bond pair present so angle is 109^o 28^'.

AIEEE 2002 Chemistry - Chemical Bonding & Molecular Structure Question 243 English Explanation 2

(3) In NH$$_4^ + $$, 4 bond pair present so angle between bond is 109^o 28^'

AIEEE 2002 Chemistry - Chemical Bonding & Molecular Structure Question 243 English Explanation 3

AIEEE 2002 Chemistry - Chemical Bonding & Molecular Structure Question 243 English Explanation 3

(4) $$\,\,\,\,$$ BF₃ has sp² hybridization. So bond angle is 120^o.

AIEEE 2002 Chemistry - Chemical Bonding & Molecular Structure Question 243 English Explanation 4

AIEEE 2002 Chemistry - Chemical Bonding & Molecular Structure Question 243 English Explanation 4

(5) $$\,\,\,\,$$ In NH$$_2^ - $$, 2 bond pair and 2 lone pair present, so bond angle is 104.5^o

AIEEE 2002 Chemistry - Chemical Bonding & Molecular Structure Question 243 English Explanation 5

AIEEE 2002 Chemistry - Chemical Bonding & Molecular Structure Question 243 English Explanation 5

So, $$NH{}^ + $$ and BF$$_4^ - $$ has bond angle 109^o 28^'.