Question

why do arsenic trichloride and arsenic trifluoride have different bond angels?

Answer 1

AsCl₃

AsCl₃ is a pyramidal molecule with C_3v symmetry. The As-Cl bond is 2.161 Å and the angle Cl-As-Cl is 98° 25'±30.Arsenic trichloride contains predominantly covalent bonds, which explains its low melting point.

One arsenic trichloride molecule will have a total of 26 valence electrons - 5 from the arsenic atom and 7 from each of the three chlorine atoms.

The arsenic atom will be bonded to the three chlorine atoms through single bonds that account for 6 of the 24 valence electrons. Each of the chlorine atoms will have 3 lone pairs, which will bring the number of valence electrons used to 24

The remaining two valence electrons will be place on the arsenic atom as a lone pair.

The arsenic atom is surrounded by 4 regions of electron density, which means that its has a steric number equal to 4. According to VSEPR Theory, the molecular geometry will be trigonal pyramidal, AX3E1.

AsF₃

It has a pyramidal molecular structure in the gas phase which is also present in the solid.In the gas phase the As-F bond length is 170.6 pm and the F-As-F bond angle 96.2

In the AsF3 Lewis structure. Arsenic (As) has five valence electrons. Fluorine has seven. We have three Fluorines. For a total of 26 valence electrons. Arsenic is the least electronegative, that'll go in the center. And we'll put the Fluorine atoms around it on outside here. We'll put two electrons between atoms to form chemical bonds. We've used 6 and then we'll go around the outer atoms to fulfill their octet. 6, 8, 10 and 24. And then back to the central atom to complete the octet on the Arsenic. 26 valence electrons. Each of the Fluorines has 8 valence electrons. So it has a full outer shell and then the central Arsenic atom also has 8 valence electrons so its octet is satisfied.

Conclusion : Because of atom size difference of the chlorine and fluorine and valence electron crowding ,bond angles are different in AsF_3, AsCl_3..