Expansion of the Octet (HL) (DP IB Chemistry): Revision Note

Expansion of the octet

• Some elements in period 3 or higher can form molecules where the central atom holds more than eight electrons

• This is possible because these elements have vacant d-orbitals in their valence shell

• These orbitals can accommodate extra bonding pairs of electrons

• This phenomenon is called the expansion of the octet

  • It explains the existence of structures like:

    • PCl₅ – with five bonding pairs

    • SF₆ – with six bonding pairs

Expansion to five electron pairs

• Molecules with five electron domains around the central atom adopt a trigonal bipyramidal electron domain geometry

• Molecular geometry depends on:

  • How many domains are bonding pairs

  • How many domains are lone pairs

• Examples of chemicals where an octet is expanded to five electron pairs include:

  • Phosphorus pentachloride, PCl₅

  • Chlorine trifluoride, ClF₃

  • The triiodide ion, l₃^-

  • Sulfur tetrafluoride, SF₄

Phosphorus pentachloride, PCl₅

• The total number of valence electrons in PCl_5,is:

P + (5 x Cl) = 5 + (5 x 7) = 40

• There are 5 chlorine atoms bonded to a central phosphorus atom

  • So, there are 5 bonding pairs

• The bonding pairs account for 10 electrons

  • This leaves 40 - 10 = 30 electrons

• The remaining 30 electrons exist as 15 lone pairs

  • This means that each chlorine atom has 3 lone pairs

• Phosphorus has expanded its octet, since it has 5 pairs of electrons

• The Lewis formula for PCl₅ is: 

The octet of the central phosphorous atom has been expanded to hold 10 electrons

Sulfur tetrafluoride, SF₄

• The total number of valence electrons in SF_4,is:

S + (4 x F) = 6 + (4 x 7) = 34

• There are 4 fluorine atoms bonded to a central sulfur atom

  • So, there are 4 bonding pairs

• The bonding pairs account for 8 electrons

  • This leaves 34 - 8 = 26 electrons

• The remaining 30 electrons exist as 13 lone pairs

• Fluorine cannot expand the octet

  • So, each fluorine would accommodate 3 lone pairs

  • This accounts for 24 electrons

• This leaves 1 lone pair on the sulfur

  • Sulfur has expanded its octet, since it has 5 pairs of electrons

• The Lewis formula for SF₄ is:

The octet of the central sulfur atom has been expanded to hold 10 electrons

Triiodide ion, l₃^-

• The total number of valence electrons in I₃^-_,is:

(3 x I) + 1 electron = (3 x 7) + 1 = 22

• There are two iodine atoms bonded to a central iodine atom

  • So, there are 2 bonding pairs

• The bonding pairs account for 4 electrons

  • This leaves 22 - 4 = 18 electrons

• The remaining 18 electrons exist as 9 lone pairs

• Each outer iodine accommodates 3 lone pairs

  • This accounts for 12 electrons

• This leaves 3 lone pairs on the central iodine atom

  • The central iodine atom has expanded its octet, since it has 5 pairs of electrons

• The Lewis formula for I₃^- is:

The octet of the central iodine atom has been expanded to hold 10 electrons

Expansion to six electron pairs

• Molecules with six electron domains around the central atom adopt an octahedral electron domain geometry

• Examples of chemicals where an octet is expanded to six electron pairs include:

  • Sulfur hexafluoride, SF₆

  • Bromine pentafluoride, BrF₅

  • Xenon tetrafluoride, XeF₄

Sulfur hexafluoride, SF₆

• The total number of valence electrons in SF_6,is:

S + (6 x F) = 6 + (6 x 7) = 48

• There are 6 fluorine atoms bonded to a central sulfur atom

  • So, there are 6 bonding pairs

• The bonding pairs account for 12 electrons

  • This leaves 48 - 12 = 36 electrons

• The remaining 36 electrons exist as 18 lone pairs

  • This means that each fluorine atom has 3 lone pairs

• Sulfur has expanded its octet, since it has 6 pairs of electrons

• The Lewis formula for SF₆ is: 

The octet of the central sulfur atom has been expanded to hold 12 electrons

Bromine pentafluoride, BrF₅

• The total number of valence electrons in BrF_5,is:

Br + (5 x F) = 7 + (5 x 7) = 42

• There are 5 fluorine atoms bonded to a central bromine atom

  • So, there are 5 bonding pairs

• The bonding pairs account for 10 electrons

  • This leaves 42 - 10 = 32 electrons

• The remaining 32 electrons exist as 16 lone pairs

• Fluorine cannot expand the octet

  • So, each fluorine would accommodate 3 lone pairs

  • This accounts for 30 electrons

• This leaves 1 lone pair on the bromine

  • Bromine has expanded its octet, since it has 6 pairs of electrons

• The Lewis formula for BrF₅ is: 

The octet of the central bromine atom has been expanded to hold 12 electrons

Revisiting Valence Shell Electron Pair Repulsion Theory (VSEPR)

• The shapes and bond angles of molecules can be predicted using VSEPR theory

• This theory is based on three core rules:

  1. All bonding and lone pairs arrange themselves to be as far apart as possible

  2. Lone pairs repel more strongly than bonding pairs

  3. Multiple bonds behave like a single bonding region when predicting shape

• These principles apply to molecules with both expanded and unexpanded octets

Molecular geometry versus electron domain geometry

• In exam questions, it’s important to distinguish between:

  • Electron domain geometry

    • This is the arrangement of all electron pairs (bonding and lone) around the central atom

  • Molecular geometry

    • This is the arrangement of the atoms in space

• Electron domain geometry and molecular geometry can be the same

  • However, if lone pairs are present the geometries often differ

Electron domain and molecular geometry of water

• The Lewis formula for H₂O shows two bonding pairs and two lone pairs around the oxygen atom

  • So, there are four electron pairs

  • This makes the electron domain geometry tetrahedral

• Only the bonded atoms count for molecular shape

  • The molecular geometry is bent (also called angular, bent linear or V-shaped)

  • The bond angle is approximately 104.5°

Molecular geometry with five electron domains

• The molecular geometry of species with five electron domains depends on the number of lone and bonding pairs

• 5 bonding pairs, 0 lone pairs:

  • Electron domain geometry = trigonal bipyramidal

  • Molecular geometry = trigonal bipyramidal

  • Example = PCl₅

    • PCl₅  is a symmetrical molecule

    • This makes it nonpolar as any dipoles cancel out

• 4 bonding pairs, 1 lone pair

  • Electron domain geometry = trigonal bipyramidal

  • Molecular geometry = seesaw

  • Example = SF₄

    • SF₄ is an asymmetrical molecule

    • It has one lone pair on one side of the central axis making the overall molecule polar

• 3 bonding pairs, 2 lone pairs

  • Electron domain geometry = trigonal bipyramidal

  • Molecular geometry = T-shaped

  • Example = ClF₃

    • ClF₃ is an asymmetrical molecule

    • It has two lone pairs on one side of the central axis making the overall molecule polar

• 2 bonding pairs, 3 lone pairs

  • Electron domain geometry = trigonal bipyramidal

  • Molecular geometry = linear  

  • Example = I₃^-

Molecular geometry with six electron domains

• The molecular geometry of species with six electron domains depends on the number of lone and bonding pairs

• 6 bonding pairs, 0 lone pairs:

  • Electron domain geometry = octahedral

  • Molecular geometry = octahedral

  • Example = SF₆

    • SF₆ is a symmetrical molecule

    • This makes it nonpolar as any dipoles cancel out

• 5 bonding pairs, 1 lone pair

  • Electron domain geometry = octahedral

  • Molecular geometry = square pyramidal

  • Example = BrF₅

    • BrF₅ is an asymmetrical molecule

    • It has one lone pair at the base making the overall molecule polar

• 4 bonding pairs, 2 lone pairs

  • Electron domain geometry = octahedral

  • Molecular geometry = square planar

  • Example = XeF₄

    • XeF4 is non-polar despite having two lone pairs

    • The bonding pairs are at 90^o to the plane

    • The lone pairs are at 180^o

    • The lone pairs are arranged above and below the square plane

      • This results in an even distribution of electron cloud charge