Question

For the molecule PBr₃, draw its Lewis Structure and then answer the following questions based on that structure. (The central atom in each case is the atom with the lowest electronegativity).

How many valence electrons are in the molecule? (Remember to add an electron for each negative charge or to subtract one for each positive charge).

How many lone pairs of electrons are in the central atom of the molecule?

What is the electron pair geometry of the molecule?

What is (are) the bond angle(s) at the central atom of the molecule PBr₃?

Would the molecule's structure be polar, nonpolar, or ionic?

Answer 1

Lewis Structure of PBr₃

Number of Valency Electrons in PBr₃

Number of Valency electrons in PBr₃ is 26.

Explanation:

The position of Phosphorous in the periodic table is 5th group which has the number of valence electrons as 5. Whereas for Bromine, it is placed in group 7, and it has the valence electrons of 7. Since the number of Bromine atoms in PBr₃ is 3, so the valence electrons of Bromine 7 should be multiplied by 3, so that the valence electrons for Br₃ will be 21. By adding the valence electrons for Phosphorous(5) and Br₃(21), we will get 26 electrons. In order to form a chemical bond between Phosphorous and Bromine we can use a pair of electrons between them. No. of valence electrons used for the bond formation will be 6. For obtaining the octets, it can be used in Bromines. As a result there will be 6, 8, 10, 24. Total Valence electrons is 26. By subtracting 26 -24 = 2. The two remaining valence electrons are placed on the central Phosphorous atom. Both central Phosphorous and each Bromines has 8 valence electrons each. All the valency electrons are used, and octets are filled for each atom.

Number of lone pair of electrons for the central atom of the molecule

As PBr₃ has 26 valence electrons each of the 3 Bromine atoms will use 8 valence electrons from the total 24 valence electrons in bonding, it has 3 lone pairs. The remaining 2 electrons will form a single lone pair on the P atom(26 - 24 = 2). So the central atom Phoshorous has 1 lone pair.

Electron Pair geometry of the PBr₃

According to Valence Shell Electron Pair Repulsion Theory there is a repulsion between the lone pair of electrons which surrounds the Phosphorous atom and the electron clouds on atom. They get exerted and gives the electron pair geometry as Trigonal Pyramidal Geometry.

Bond angles at the central atom of the PBr₃ molecule

Three Bromine atoms are attached to Phophorous atom with the lone pair of electrons. As it has Trigonal Pyramidal Geometry, the bond angle will be 109^o

Molecular Structure be Polar, non polar or ionic

The Presence of lone pair in the central atom, the Br atoms are not in the same plane, PBr₃ has a distorted Pyramidal geometry, as a result it is Polar