Shapes of molecules

Xenon forms a number of covalently bonded compounds with fluorine. Suggest a shape for `XeF_4`. Explain why you chose this shape.

The central atom is xenon which has eight valence electrons and each fluorine atom has 7 valence electrons. In `XeF₄`, xenon forms four single bonds with four fluorine atoms. After forming these bonds, xenon has 2 lone pairs of electrons left over. Therefore, the total number of electron pairs around the xenon atom is 6. Normally, the electron pair geometry with 6 electron pairs is octahedral, which is symmetrical, nevertheless, the molecule's form is impacted by the presence of the two lone pairs. In order to reduce repulsion with other electron pairs, the lone pairs will occupy two opposite locations in the octahedral geometry. Consequently, the molecule `XeF₄` has a square planar form.

The structure of xenon trioxide is shown below.

i. By referring to electron pairs, explain why xenon trioxide has this shape.

ii. Draw the structure of xenon trioxide to show the partial charges on the atoms and the direction of the dipole in the molecule.

i. `XeO_3` has one xenon atom and three oxygen atoms. In the Lewis structure, xenon forms double bonds with each of the three oxygen atoms. Additionally, xenon has one lone pair of electrons. With four regions of electron density around the xenon atom, the electron geometry is tetrahedral. This means that the regions of electron density arrange themselves in a way to minimize repulsion. Nevertheless, the presence of the lone pair alters the shape of the molecule due to the lone pair exerting greater repulsion compared to bond pairs. The molecular geometry is pyramidal, which can be visualized as similar to a triangular pyramid with the three oxygen atoms forming the base and the xenon atom positioned at the top.

ii.

i. Draw a diagram of an ammonia molecule, showing its shape. Show any lone pairs of electrons.

ii. State the bond angle H-N-H in this molecule

i.

ii. NH₃ has one nitrogen atom and three hydrogen atoms. Nitrogen forms single bonds with each of the three hydrogen atoms. Additionally, nitrogen has one lone pair of electrons. With four regions of electron density around the nitrogen atom, the electron geometry is tetrahedral. This means that the regions of electron density arrange themselves in a way to minimize repulsion. However, the tetrahedral arrangement of the electron pairs around the central atom becomes distorted. Ammonia has three bonding pairs of electrons and one lone pair. As lone pair–bond pair repulsion is greater than bond pair–bond pair repulsion, the bonding pairs of electrons are pushed closer together. This gives the ammonia molecule a triangular pyramidal shape. The H-N-H bond angle is about 107°.

The shape of iodine trichloride, ICl₃, is shown below.

i. Describe the shape of this molecule.

ii. Explain why the ICl₃ molecule has this shape.

iii. Suggest a value for the Cl-I-Cl bond angle.

i. The electron pair geometry of this molecule is trigonal bipyramidal, while the shape of the structure is T-shaped.

ii. ICl₃ contains one iodine atom and three chlorine atoms. In ICl₃, iodine is the central atom and forms single bonds with three chlorine atoms. Additionally, iodine has two lone pairs of electrons. Therefore, the total number of electron domains (bonding pairs + lone pairs) around the iodine atom is five. The arrangement of these five electron domains is classified as trigonal bipyramidal geometry. This is because with five electron pairs (3 bonding and 2 lone pairs), the optimal arrangement minimizes repulsion between them by adopting this shape. One chlorine atom and the two lone pairs are axially positioned, while two of the chlorine atoms are equatorial in a trigonal bipyramidal configuration. Because of the lone pairs and how they are arranged, ICl₃ will have a T-shaped molecular structure 

iii. The molecule is T-shaped, and the bond angle is 90^o.

Tetrachloromethane, CCl₄, is a non-polar molecule.

i. Draw a diagram to show the shape of this molecule.

ii. Explain why this molecule is non-polar.

i.

ii. Due to its symmetrical tetrahedral shape, the individual dipole moments of the C-Cl bonds cancel each other out, resulting in a nonpolar molecule.

Hydrogen sulfide, H₂S, is a covalent compound. Draw a diagram of a hydrogen sulfide molecule to show its shape. Show on your diagram:

i. The value of the H-S-H bond angle

ii. The partial charges on each atom as δ+ or δ–

iii. An arrow showing the exact direction of the dipole in the molecule as a whole.

i. The central sulfur (`S`) atom in hydrogen sulfide (`H₂S`) experiences `sp³` hybridization. One s orbital and three p orbitals from sulfur are mixed during this hybridization process to create four comparable `sp³` hybrid orbitals. Two of these orbitals have lone pairs of electrons, and the other two are employed to create S-H bonds with the hydrogen (`H`) atoms.

These lone pairs cause the molecular geometry of `H₂S`which is bent to be distorted, affecting the bond angle and exerting repulsion. Because of the increased repulsion from the lone pairs, the H-S-H bond angle in `H₂S` is about 92^o, which is less than the ideal tetrahedral angle of 109.5^o.

ii.

iii.

Since sulfur is more electronegative than hydrogen, the hydrogen atoms' dipole moment goes in the direction of sulfur, making `H₂S`a polar molecule as well. An arrow can be used to illustrate this; it begins with a single hydrogen atom and points in the direction of sulfur, signifying the dipole's orientation because of the difference in electronegativity between S and H. The entire molecular structure is polar due to its net dipole moment.

Explain why each bond angle in BH₃ is 120^o.

Boron has 3 bonding pairs of electrons, which are formed with three hydrogen atoms leading this molecule to have a planar shape. The sum of the angles is 360^o. However, all bonding pairs of electrons in BH₃ repel each other equally making each bond angle have 120^o.

What is the correct bond angle in a trigonal planar molecule?

A. 107^o

B. 120^o

C. 180^o

D. 109.5^o

The answer is B.

A is incorrect because this angle is for a pyramidal molecule such as NH₃

C is incorrect because this angle is for a linear molecule such as CO₂

D is incorrect because this angle is a tetrahedral molecule such as NH₄

Which one of these species has a bond angle of 120^o?

A. `H_3O^+`

B. `TlBr""_3^(2-`

C. `BCl_3`

D. `NH_3`

The answer is C.

Boron has 3 valence electrons and the total number of valence electrons of 3 Cl is 21. Thus, there are 24 electrons overall, which means 12 pairs of electrons, leading this molecule to have 3 bonding pairs and no lone pairs. Therefore, BCl₃ is trigonal planar with a bond angle of 120^o

A, B and D have a trigonal pyramidal shape with a bond angle of 107^o.

Which of these molecule is not polar?

A. CO₂

B. HCl

C. H₂O

D. NH₃

The answer is A.

CO₂ is linear, which means symmetrical, leading the dipole moments to cancel each other out. Therefore, this molecule is non-polar overall.

B is incorrect because of the difference in electronegativity between 2 atoms

C is incorrect because it is non-linear making the dipole moments not cancel each other out. 

D is incorrect because it is pyramidal making the dipole moments not cancel each other out.