GS-2 Physical Sciences Question 40

[priya.bork7031]

can someone explain #40 to me? I was under the impression that phosphorous is too big to make good pi bonds, and that when it binds to itself, it can one weak pi, but not two, Why must the answer be C?

40) At very high temperatures, P4 molecules decompose to give rise to P2 molecules. What type of bonding would you expect in these molecules?

1. Ionic with P3+ cations and P3- anions[x]
2. Covalent with no sigma bonds, 3 pi bonds and 2 lone pairs of electrons [x]
3. Covalent with 1 sigma bonds, 2 pi bonds and 2 lone pairs of electrons[x]
4. Covalent with 2 sigma bonds, 1 pi bonds and 2 lone pairs of electrons[x]

INCORRECT:
Your Answer: D
Correct Answer: C
Explanation

CHM 2.3, 3.2

Phosphorus is in Group V and therefore requires three more electrons to approach the more stable noble gas electronic configuration. If these are shared with another phosphorus atom, three covalent bonds exist between the two atoms. Multiple bonds usually contain one sigma bond, the other bonds being pi bonds. Each atom will also possess a lone pair of electrons. {Notice this is the same as for N2 as nitrogen is in the same group as phosphorus in the periodic table; when you see something you are unfamiliar with, always look for similarities with something you know well}
Answer choice A. is incorrect because if phosphorus loses three electrons, it will have a total of 12 electrons, which does not correspond to a stable noble gas-like configuration.

[y2boodz6039]

hello,

i'm not sure if my reasoning is 100% correct but it led me to the right answer... i did this through process of elimination.

choice a: this doesn't sound right as you would expect covalent bonding between these phosphorus molecules, also the structure of P4 molecules in the 1st question clarifies this

choice b: for any molecule, you cant have pi bonds with NO sigma bonds.

choice d: for P2 molecules, how would you have 2 sigma bonds plus a pi bond? this didn't sound correct to me either.

thus, i was left with choice c, 1 sigma + 2 pi bonds, which sounds like a triple bond to me, which does make sense since phosphorus has 3 valence electrons and this fulfils the octet of phosphorus.

hope this helped.

[param_8794381]

I doubt if this answer is correct... Phosphorus is sp2 hybridized here, so three bonds should result from this... 2 sigma and 1 molecular orbital having 1 lone pair of electron (it is wrong to state that it has 2 lone pair anyways.. since pair is 2)... the remaining p orbital should form the pi bond... so only 1 pi bond...

[walkerahl5194]

yeah, i dont think this answer is correct either.

Let's start with the basics:

1.A sigma bond is just a single bond.

2.When you have a double bond, you have 1 sigma bond and 1 pi bond.

3.When you have triple bond, you have 1 sigma bond and 2 pi bonds.


Thus, according to the reasoning above, answer C is incorrect because two pi bonds should have at least 2 sigma bonds.

[mohsinalam5969]

For P2:
One of the 3s electron of Phosphorus(P) will undergo excitation and this electron will combine with 1 3p orbital to form sp hybridisation. One of the sp hybrid orbitals overlaps head-on with that of the adjacent P atom to give one P-P sigma bond. The two other unhybridised 3p orbitals will overlap side-on to from 2 pi bonds which lie at right angles to each other. This will form the triple bonds of P, which will give out 488 kJ mol-1 of energy.
For P4:
The Phosphorus atom has 3 electrons in each of its 3p orbitals and each of the 3p orbital will overlap head-on with the 3p orbitals of the other 3 phosphorus atoms to form 3 P-P sigma bonds. This formation of compound will give out 804 kJ mol-1 of energy, which is more than the 488 kJ mol-1 of energy that P2 gives out. As P2 gave out less energy, it has a weaker bond and is less stable than P4. Therefore, P will combine to form P4 instead of P2 as P4 is more stable