Compounds With Planar Shape

Featured in this post are compounds having planar shape other than trigonal planar.

The following compounds are all planar, with the last two having more than one central atom.

xenon tetrafluoride, XeF4

benzene molecule, C6H6

CH2=NH

Geometry of Xenon Tetrafluoride, XeF₄

The Lewis structure of xenon tetrafluoride, XeF4, below shows that the compound's central atom has four bonding electron pairs and two lone electron pairs.

According to the VSEPR theory, the arrangement that affords the maximum distance between each of the mentioned electron pairs is a square bipyramidal.





The axial positions of the two lone electron pairs offer the least net repulsion than it would have been if one or both of these non-bonding electron pairs were in the equatorial positions.



In the illustration shown above, a square bipyramidal outline is superimposed on the xenon tetrafluoride molecule.

• shape of xenon tetrafluoride: square planar
• F-Xe-F bond angle: 90°

Geometry of Benzene, C₆H₆

The resonance structures of benzene are shown below; the third structure is a suggested representation of the delocalization state of the pi electrons.





Each of the six carbon atoms can be taken as a central atom about which the geometry is trigonal planar.





The result is a hexagonal planar molecule with one carbon atom and one hydrogen atom jutting out at each of the six vertices.

Geometry of CH₂=NH

The CH2=NH molecule has two central atoms, carbon and nitrogen (see the Lewis structure below).





About each center, the geometry is trigonal planar.





However, the shape about the nitrogen atom is angular due to the presence of a lone electron pair.



All of the atoms lie in one plane.

Compounds With Bent Shape

The following chemical species have bent geometry:

water molecule, H2O

nitrogen dioxide molecule, NO2

nitrite ion, NO2-

sulfur dioxide molecule, SO2

The illustrations and discussion below show how the presence of non-bonding electron pair(s) on a molecule's central atom affects the geometry of the molecule and the bond angle of its atoms.

Geometry of Water, H₂O

As shown in its Lewis structure below, the central atom of water has four pairs of electrons: two bonding pairs and two non-bonding or lone pairs.

These pairs of electrons are presumed to take a tetrahedral arrangement in space.





Here, the water molecule is depicted as being superimposed in a tetrahedral outline.





The lone pairs of electrons are relatively closer to the nucleus of the central atom and they tend to crowd the two bonding pairs together so that the H-O-H bond angle is less than the ideal tetrahedral angle of 109.5°.

• shape of water: bent or angular
• H-O-H bond angle: 104.5°

Geometry of Nitrogen Dioxide, NO₂

The central atom of nitrogen dioxide, nitrogen, has two sigma bond electron pairs and an unpaired non-bonding electron as shown in its Lewis electron dot structure below.





In order for this group of electrons to be farthest from each other, it is predicted that they take a trigonal planar arrangement.





Shown in the next illustration is the nitrogen dioxide molecule superimposed on a trigonal planar outline.





The observed O-N-O bond angle of 134.1° in nitrogen dioxide molecule, being closer to trigonal angle of 120°, confirms this prediction.

The opening out of the O-N-O bond angle is due to the less crowding affected by the half filled non-bonding orbital of the central atom.

This half-filled orbital accounts for the nitrogen dioxide (NO₂) being paramagnetic.

• shape of nitrogen dioxide: bent or angular
• O-N-O bond angle: 134.1°

Geometry of Nitrite Ion, NO₂^-

An addition of an electron to the central atom of nitrogen dioxide (NO₂) creates the nitrite anion, NO₂^-.





In the above diagram, the resonance structures of nitrite ion is shown. The oxygen atoms in red partial shades are atoms with negative formal charge.

As is the case with the nitrogen dioxide, the arrangement of the two sigma bond electron pairs and the lone electron pair of nitrite ion's central atom lies on a trigonal plane.

The only difference is the observed O-N-O bond angle of 115° for the nitrite ion. This bond angle now being much closer but less than the trigonal angle of 120° is attributed to the filling up of the non-bonding orbital of the nitrite ion's central atom.

The lone electron pair is relatively closer to the central atom and occupies more space than the adjacent bonding electron pairs do.

• shape of nitrite ion: bent or angular
• O-N-O bond angle: 115°

Geometry of Sulfur Dioxide, SO₂

Referring to the contributing Lewis structures of sulfur dioxide below, there is a -1 formal charge on one of the oxygen atoms and +1 formal charge on the sulfur atom which cancel each other out, leaving the molecule with zero net charge.

Given the two sigma bond electron pairs and one lone electron pair on the central atom of sulfur dioxide, as shown in the figure above, SO₂ is predicted to have a trigonal planar geometry.

A trigonal planar outline is shown below superimposed on the sulfur dioxide molecule.

Given this geometry, it is expected that SO₂ molecule has an angular shape. This is supported by the fact that sulfur dioxide has a dipole moment.

The O-S-O bond angle is expected to be somewhat less than the trigonal angle of 120° due to the presence of a lone electron pair on the central atom.

• shape of sulfur dioxide: bent or angular
• O-S-O bond angle: less than 120°

Molecular Geometry: Molecules With Trigonal Planar Shapes

Molecules with trigonal planar shape are characterized by their central atoms having 3 bonding electron pairs.

These electron pairs refer to those ones involved in sigma bonds only.

Electron pairs from pi bonds of double and triple bonds do not count.

In this particular coordination geometry, each electron pair is at an angle of 120° from any of the other two electron pairs.

The illustrations of the geometry and the Lewis structures given below are for the following molecules:

BF3

BCl3

SO3

H2C=O

Geometry of BF₃

• shape of molecule: trigonal planar
• F-B-F bond angle: 120°

A trigonal plane is superimposed on the BF₃ molecule.

Lewis structures of BF₃ and BCl₃

Due to its 3 valence electrons, boron does not follow the octet rule and thus forms electron-deficient compounds such as BF₃ ( boron trifluoride ) and BCl₃ ( boron trichloride ).

The characteristic reaction of the above compounds is to accept/share an electron pair from other atoms or compounds.

The reaction is called Lewis acid-base reaction.

A Lewis acid accepts a pair of electrons; a Lewis base donates a pair of electrons.



Geometry of BCl₃

Geometry of SO₃

Lewis structures of SO₃ and H₂C=O

Shown above is the Lewis structures of SO₃ ( sulfur trioxide ) and H₂C=O ( formaldehyde ) molecules.



Geometry of H₂C=O

Geometries of Molecules and Ions

The shapes of compounds, either molecules or polyatomic ions, are very important in helping us understand better their reactions.

Fortunately, the geometries of most molecules and ions can be predicted quite reliably even by considering only their electron-electron pair interactions.

The idea is that the repulsive forces that exist between bonding and non-bonding pairs of electrons of a molecule or an ion cause those pairs of electrons to adapt certain spatial arrangement that allows minimum repulsion.

The spatial arrangement of the electron pairs of a molecule depends on its number of atoms and the number of valence electrons of its central atom.

So, in order to predict the geometry of a molecule or an ion, one needs to know its number of bonding and non-bonding pairs of electrons by determining the following:

• total number of atoms in the molecule or ion
• number of valence electrons of the molecule's central atom
• the Lewis structure of the molecule or ion

In the following illustrations, all of the possible coordination geometries for different compounds are depicted using ball-and-stick models.

In each illustration, information such as number of electron pairs of the compound and the number of its atoms are given.

Linear Geometry

• number of electron pairs: 2
• coordination geometry: linear
• number of atoms: 3 ( 1 central atom, 2 bonding atoms)

Trigonal Planar Geometry

• number of electron pairs: 3
• coordination geometry: trigonal planar
• number of atoms: 4 ( 1 central atom, 3 bonding atoms)

Tetrahedral Geometry

• number of electron pairs: 4
• coordination geometry: tetrahedral
• number of atoms: 4-5 ( 1 central atom, 3-4 bonding atoms)

Trigonal Bipyramidal Geometry

• number of electron pairs: 5
• coordination geometry: trigonal bipyramidal
• number of atoms: 6 ( 1 central atom, 5 bonding atoms)

Octahedral Geometry

• number of electron pairs: 6
• coordination geometry: octahedral
• number of atoms: 7 ( 1 central atom, 6 bonding atoms)

Pentagonal Bipyramidal Geometry

• number of electron pairs: 7
• coordination geometry: pentagonal bipyramidal
• number of atoms: 8 ( 1 central atom, 7 bonding atoms)