Question

Examine the bond lengths and bond angles for three analogous molecules to the bottom: ammonia, NH3 101.7 pm H 107.8 phosphine, PH3 1.42 A and arsine, AsH:3 1.519A 91.8° 1) Do the bond angles and lengths follow predict ions based on VSEPR theory? Explain. 2) Using the informat ion at the top as a guide, what effects would replacing the phosphor i (plural for phosphorus) with arsenics have on the overa structure of DNA? 3) How are the molecular representation (the structures the class built), and the biological representation alike, and how are they different? Why would an author or scient ist prefer to use one over the other?

Answer 1

Looking at the three hydrides drawn above, we can say that,

1) Yes the bond angles and bond lengths follow the predictions of VSEPR. As the electronegativity goes down in the order N > P > As and the size increases in the same order N > P > As, the valence electrons are more loosely held while going left to right in the order. Thus, the bond length increases from N > P > As in the same order. As the size increases for N to As, the distance between the H's decreases as expected.

2) Replacing P with As would decrease the strain in the DNA. This is because the As is bigger in size and hence DNA would have more flexibility.

3) Molecular representation with As being the replacement element in place of P may favour the DNA structure, however, biological significance suggests, As is poisionous and unlike P which is used in a number of processes in the body, As is not preferred replacement in biological systems.