Molecular shapes of compounds

While the hybridized orbitals were introduced, in the foregoing discussion, Valence-shell Electron-pair Repulsion (VSEPR) Model were included to suggest the shapes of various molecules. Specifically, the VSEPR model counts unshared electron pairs and the bonded atoms as the VSEPR number. A single-, double- and tripple-bond is considered as 1. After having considered the hybridized orbitals and the VSEPR model, we can not take a systematic approach to rationalize the shapes of many molecules based on the number of valence electrons. A summary in the form of a table is given here to account for the concepts of hybrid orbitals, valence bond theory, VSEPR, resonance structures, and octet rule. In this table, the geometric shapes of the molecules are described by linear, trigonal planar, tetrahedral, trigonal bypyramidal, and octahedral. The hybrid orbitals use are sp, sp2, sp3, dsp3, and d2sp3. The VSEPR number is the same for all molecules of each group. Instead of using NH3E, and OH2E2, we use :NH3, ::OH2 to emphasize the unshared (or lone) electron pairs.

To determine shape of molecule

Linear Trigonalplanar

Tetrahedral Trigonalbipyramidal

Octahedral

sp sp2 sp3 dsp3 d2sp3 BeH2

BeF2

CO2

HCNHCCH

BH3

BF3

CH2O(>C=O)>C=C<CO3

2-

benzenegraphitefullerenes•NO2

N3-

:OO2 (O3):SO2

SO3

CH4

CF4

CCl4CH3ClNH4

+

:NH3

:PF3

:SOF2

::OH2

::SF2

SiO44-

PO43-

SO42-

ClO4-

PF5

PCl5PFCl4:SF4

:TeF4

::ClF3

::BrF3

:::XeF2

:::I3-

(:::I I2-):::ICl2-

SF6

IOF5

PF6-

SiF62-

:BrF5

:IF5

::XeF4

• a lone odd electron         : a lone electron pair This table correlates a lot of interesting chemical concepts in order to understand the molecular structures of these compounds or ions. There are some intriqueing chemical relationships among the molecules in each column for you to ponder.

Only Be and C atoms are involved in linear molecules. In gas phase, BeH2 and BeF2 are stable, and these molecules do not satisfy the octet

rule. The element C makes use of sp hybridized orbitals and it has the ability to form double and triple bonds in these linear molecules.

Carbon compounds are present in trigonal planar and tetrahedral molecules, using different hybrid orbitals. The extra electron in nitrogen for its compounds in these groups appear as lone unpaired electron or lone electron pairs. More electrons in O and S lead to compounds with lone electron pairs. The five-atom anions are tetrahedral, and many resonance structures can be written for them.

Trigonal bipyramidal and octahedral molecules have 5 and 6 VSEPR pairs. When the central atoms contain more than 5 or 6 electrons, the extra electrons form lone pairs. The number of lone pairs can easily be derived using Lewis dot structures for the valence electrons.

In describing the shapes of these molecules, we often ignore the lone pairs. Thus, •NO2, N3

-, :OO2 (O3), and :SO2 are bent molecules whereas :NH3, :PF3, and :SOF2 are pyramidal. You already know that ::OH2 (water) and ::SF2 are bent molecules.

The lone electron pair takes up the equatorial location in :SF4, which has the same structure as :TeF4 described earlier. If you lay a model of this molecule on the side, it looks like a butterfly. By the same reason, ::ClF3 and ::BrF3 have a T shape, and :::XeF2, :::I3-, and :::ICl2-

are linear.

Similarly, :BrF5 and :IF5 are square pyramidal whereas ::XeF4 is square planar.