Tutor Homework Molecular Geometry Definition

STEPS INVOLVED

There are three basic steps to determining the molecular shape of a molecule:

  1. Write the Lewis dot structure of the molecule. That gives you the steric number (SN) — the number of bond pairs and lone pairs around the central atom.

  2. Use the SN and VSEPR theory to determine the electron pair geometry of the molecule.

  3. Use the VSEPR shape to determine the angles between the bonding pairs.

VSEPR PRINCIPLES:

  1. The repulsion between valence electron pairs in the outer shell of the central atom determines the shape of the molecule. You must determine the steric number (SN) — the number of bonding pairs and lone pairs about the central atom.

  2. Lone pairs repel more than bond bonding pairs.

A. SN = 2

What is the shape of ?

The Lewis dot structure for is

The central atom has two bond pairs in its outer shell (SN = 2).

Repulsion between these two pairs causes the atoms to be as far apart as possible.

The shape of the molecule is linear, and the bond angle is 180°.

B. SN = 3

There are two possibilities.

i. — Three bonding pairs

What is the shape of ?

The Lewis dot structure is

The atom has three bond pairs in its outer shell.

Minimizing the repulsion causes the atoms to form an equilateral triangle about the atom, as shown below.

The shape of the molecule is trigonal planar.

All the atoms are in the same plane, and the bond angles are all 120°.

ii. — two bond pairs and one lone pair

What is the shape of ?

The Lewis dot structure is

The central atom, , has three groups bonded to it, two oxygen atoms and a lone pair.

The electron pair geometry of is trigonal planar.

It would be drawn as

The molecular shape of is not trigonal planar.

In determining the molecular shape, we consider only the positions of the atoms, not the lone pairs.

Hence, the molecular shape of is bent and is represented as

The lone pair of electrons occupies a relatively large volume, since they are held by only one atom.

They compress the bond angle between the oxygens and sulfur to about 119.5°.

C. SN = 4

There are four possibilities.

i. — four bond pairs

What is the shape of ?

The Lewis dot structure is

The shape of this molecule, however, is not planar, as you might think from the way we draw this dot structure.

The four bond pairs are arranged about the atom, pointing toward the corners of a regular tetrahedron.

This shape minimizes the repulsion between the bond pairs.

The 109.5° angle is the same for all bond angles and is called the tetrahedral bond angle.

The shape of the molecule is tetrahedral.

ii. — three bond pairs and one lone pair.

What is the molecular geometry of ?

The Lewis dot structure of is

The central atom, , has four groups bonded to it: three hydrogen atoms and a lone pair.

The electron pair geometry of is tetrahedral.

It is drawn as shown below:

Remember that, in determining the molecular shape, we consider only the positions of the atoms, not the lone pairs.

If we look only at the atoms, we see a short, rather distorted tetrahedron.

This is called a pyramid.

The pyramid has a triangular base.

Hence the shape is trigonal pyramidal.

The greater repulsion of the lone pair causes the atoms in to be bent closer together than the normal tetrahedral angle of 109.5°.

In the observed bond angle is 107.3°.

iii. — two bond pairs and two lone pairs

What is the molecular geometry of ?

The Lewis dot structure of is

The central atom, , has four groups bonded to it, two hydrogen atoms and two lone pairs.

The electron pair geometry of is tetrahedral.

It is drawn as shown below:

The shape is called bent.

The bond angle is less than that in , partly because of the greater repulsions caused by two lone pairs.

In water, the observed bond angle is 104.5°.

All bond angles in molecules are significantly less than 109.5°.

D. SN = 5

There are four possibilities.

i. — five bond pairs

What is the molecular shape of ?

The Lewis dot structure of is

Atoms past in the Periodic Table can “expand their octet” and have more than eight valence electrons.

Here, the atom has ten valence electrons.

If you view the atom at the centre of a sphere like the earth, you have one atom at the North Pole, one atom at the South Pole, and three atoms spread evenly around the equator.

Note that the atoms occupy two types of positions.

The two atoms that are on a straight line that passes through the nucleus are said to occupy axial positions.

The other three atoms are in equatorial positions.

If you join the atoms by straight lines, the diagram looks like two trigonal pyramids joined together at the base.

The molecule has a trigonal bipyramidal shape.

There are two different bond angles in the molecule.

The axial atoms are at angles of 90° to the equatorial atoms, while the equatorial atoms are at angles of 120° to each other.

ii. — Four bond pairs and one lone pair.

What is the molecular shape of ?

The Lewis dot structure of is

The five electron pairs assume a trigonal bipyramidal geometry.

The lone pair occupies an equatorial position, because that gets it furthest away from the other electron pairs.

If you turn the molecule on its side, it looks like a see-saw, with the axial atoms at the ends, and the equatorial atoms acting as the pivot.

The shape is called a see-saw.

The axial bond angle is 173.1° rather than 180°, because of the lone pair of electrons in the equatorial plane.

The equatorial bond angle is compressed to 101.6° from its normal value of 120°.

iii. — tree bonding pairs and two lone pairs

What is the molecular shape of ?

The Lewis dot structure of is

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The five electron pairs assume a trigonal bipyramidal geometry.

The two lone pairs occupy the equatorial positions in order to minimize repulsions.

The bonding pairs form a T-shape, and that is the shape of the molecule.

Actually, the molecule has a “distorted-T” shape because the two lone pairs reduce the bond angle from 90° to 86.98°, and the bond angle across the top of the T is reduced from 180° to 173.96°.

iv. — two bonding pairs and three lone pairs

What is the molecular shape of ?

The Lewis dot structure of is

The five electron pairs assume a trigonal bipyramidal geometry.

The three lone pairs occupy the three equatorial positions, so what you have is a xenon atom with three lone pairs pointing toward the corners of an equilateral triangle, with one atom below the triangle and another above.

The result is a linear shape for the molecule, and the bond angle is 180°.

E. SN = 6

There are five possibilities.

i. — six bonding pairs

What is the molecular shape of ?

The Lewis dot structure of is

The six bonding pairs arrange themselves with four equatorial bond pairs and one more pair at each of the polar locations.

The shape is called octahedral.

Every bond angle is 90°.

ii. — five bonding pairs and one lone pair

What is the structure of ?

The Lewis dot structure of is

The electron pairs arrange themselves at the corners of an octahedron with a lone pair occupying one of these positions.

Four of the atoms are at the corners of a square, and one is directly above the atom.

Lines connecting the atoms form a pyramid with a square base, so the shape is square pyramidal.

The angles between neighbouring atoms are 90°.

iii. — four bond pairs and two lone pairs

What is the molecular shape of ?

The Lewis dot structure is

The electron pairs arrange themselves at the corners of an octahedron.

The four bonding pairs point toward the corners of a square, and the lone pairs occupy the axial positions above and below the plane of the square.

Since the four atoms are at the corners of a square, the molecular shape is square planar.

The bond angles between neighbouring atoms are 90°.

iv. — three bonding pairs and three lone pairs

There are no molecules that belong to the system.

However, the electron geometry is octahedral, with the three lone pairs occupying equatorial positions.

The remaining bond pairs are arranged in a T-shape with bond angles of 90°.

v. — two bonding pairs and four lone pairs

No stable molecules are known.

However, we predict the molecular shape to be linear with bond angles of 180°.

SUMMARY

To predict the shape of a molecule:

  1. Write the Lewis dot structure for the molecule.

  2. Determine the steric number of the central atom.

  3. Decide on the electron pair orientation based on the steric number.

  4. Consider the placement of lone pairs and any distortions from "regular" shapes.

  5. Name the shape based on the location of atoms attached to the central atom

The table below summarizes all the molecular shapes.

You must be aware with the 18th group of the long form of the periodic table. The 18th group is placed at the right most side of the periodic table. The elements of 18th group are known as rare elements or noble elements or noble gases. There is weak van der Waal interactions exist between these elements. Therefore these elements exist in gaseous state only. The valence shell electronic configuration of 18th group is $ns^2$, $np^6$. In other words, they have a completely filled valence shell. That is the reason; the ionisation potential of the Noble gases very high as they have no tendency to lose electrons like metals. 

Similarly, the electronegativity of these elements can be considered as zero because they do not attract any external electron. We can say that they cannot form any type of chemical bond with similar or different elements to form compounds. These elements are monatomic in nature and cannot bind with other elements. Only xenon and krypton can form some halides and oxyhalides. This is because the ionisation potential of xenon and oxygen is very close to each other. Hence it can also form compounds like xenon. Today we have several well known halides of xenon like $XeF_2$, $XeF_4$, $XeF_6$, and $XeOF_4$ etc. Similarly krypton can also form some compounds.

What is the Molecular Geometry of XeF2?

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Two factors are necessary to consider the chemical reaction of this type. They are ionization energy and bond energy. If the strength of the bond is more than the ionization energy required the compound formed will be stable. The ionization energies of He,Ne,Ar and Kr are very high. 

The ionization energy of Xe → Xe+, which is 1170 kJ/mol is almost similar to the ionization energy of Oxygen when it forms O2 → O2+ ion. This means Xenon can form compounds almost with the same ease as the formation of compounds of oxygen. 

When proportionate quantities of Xe and F2 that is 1: 5 respectively, are heated at 400ºC a colorless volatile solid of XeF2 is formed. Since it is a solid it has a crystalline structure and bonds to form a crystalline lattice. The explanation satisfying the laws of crystals and the stability thus obtained by the molecule is called the molecular geometry.

  • There are 22 electrons in the molecule that are considered as valence electrons. These are arranged in trigonal bipyramid molecular geometry.
  • The three non bonded pair of electrons prefer the equatorial position.
  • The bonded pair of electrons take the apical position of the trigonal bipyramid and hence the molecule is linear.

XeF2 Shape

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The overall 22 valence electrons in the molecule with 3 pairs of electrons present on each fluorine atom forms the octet structure between the bonded pair between them and central Xenon atom. This result in a total of 8 pairs. with the remaining three lone pairs on Xenon atom completes the 11 pairs. 


These gives rise to trigonal bipyramid molecular geometry. The non bonded pair of electrons amounting to three give rise to equatorial position. The electron bonded pairs take the apex position in this trigonal bipyramid giving rise to linear molecule.

The molecule is categorized as linear as the bonded pairs float around the central Xe atom except the lone pairs. When studied under spectroscope these lone pairs resemble a cloud.

XeF2 Structure

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There are 22 electrons in the molecule that are valence electrons. The 3 pairs of electrons are there on each fluorine atom as shown in the figure, which form the octet structure along with the bonded pair between them and central Xe atom. Thus there are a total of 8 pairs. The remaining 3 lone pairs on Xe atom as shown in the figure completes the 11 pairs. 




These are arranged in trigonal bi pyramid molecular geometry. The three non bonded pair of electrons prefer the equatorial position. The bonded pair of electrons take the apical position of the trigonal bi pyramid and hence the molecule is linear. 



The molecule is termed as linear because the bonded pairs cannot move around the central Xenon atom but the lone pairs can move. In spectroscope these lone pairs appear like a cloud.



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