Bonding, Molecular Shape & Structure

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Total Number of valence electrons = 5 + (3 x 7) = 26

Total Number of valence electrons = 4 + (2 x 6) = 16

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E.g. 2; CHBr3

Total Number of valence electrons = 4 + 1 + (3 x 7) = 26

Exceptions to the Octet Rule in Covalent Bonding

Molecules with an odd number of electrons

Other Natural Radicals, which do not obey Lewis Structures

(e.g. O2)

Molecules in which an atom has less than an octet

3. Molecules in which an atom has more than an octet

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1. Odd Number of Electrons

NO

Number of valence electrons = 11

NO2

Number of valence electrons = 17

O2

Resonance occurs when more than one valid Lewis structure can be written for a particular molecule (i.e. rearrange electrons)

Molecules and atoms which are neutral (contain no formal charge) and with an unpaired electron are called Radicals

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2. Less than an Octet

Includes Lewis acids such as halides of B, Al and compounds of Be

BCl3

Group 3A atom only has six electrons around it

However, Lewis acids “accept” a pair of electrons readily from Lewis bases to establish a stable octet

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AlX3

Aluminium chloride is an ionic solid in which Al3+ is surrounded by six Cl-.

However, it sublimes at 192 °C to vapour Al2Cl6 molecules

B2H6

A Lewis structure cannot be written for diborane.

This is explained by a three-centre bond – single electron is delocalized over a B-H-B

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Octet Rule Always Applies to the Second Period = n2 ; number of orbitals

2s, 2px, 2py, 2pz ---orbitals cannot hold more than two electrons

Ne [He]; 2s2, 2px2, 2py2, 2pz2

n = 2

n = 3

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Third Period ; n2 = 32 = 9 orbitals

Ar [Ne]; 3s2, 3px2, 3py2, 3pz2 3d0 3d0 3d0 3d0 3d0

n = 3

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3. More than an Octet

PCl5

Elements from the third Period and beyond, have ns, np and unfilled nd orbitals which can be used in bonding

P : (Ne) 3s2 3p3 3d0

Number of valence electrons = 5 + (5 x 7) = 40

10 electrons around the phosphorus

SF4

S : (Ne) 3s2 3p4 3d0

Number of valence electrons = 6 + (4 x 7) = 34

The Larger the central atom, the more atoms you can bond to it – usually small atoms such as F, Cl and O allow central atoms such as P and S to expand their valency.

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