Chemical Bonding & Structure - Ms. Suchy's science site€¦ · Further aspect of covalent bonding...

Chemical Bonding & Structure

Further aspects of covalent bonding and structureHybridization

Ms. Thompson - HL ChemistryWooster High School

Saturday, August 1, 15

Topic 14.1Further aspect of covalent bonding and

structure• Covalent bonds result from the overlap of atomic orbitals. A sigma

bond (σ) is formed by the direct head-on/end-to-end overlap of atomic orbitals, resulting in electron density concentrated between the nuclei of the bonding atoms. A pi (π) bond is formed by the sideways overlap of atomic orbitals, resulting in electronic density above and below the plane of the nuclei of the bonding atoms.

• Formal charge (FC) can be used to decide which Lewis (electron dot) structure it Is preferred from several. The FC is the charge an atom would have if all atoms in the molecule had the same electronegativity. FC =(Number of valence electrons) - 1/2(Number of bonding electrons) - (Number of non-bonding electrons). The Lewis (electron dot) structure with the atoms having FC values closest to zero is preferred.

structure• Exceptions to the octet rule include some species having

incomplete octets and expanded octets. • Delocalization involves electrons that are shared by more than

two at times in a molecule or ion as opposed to being localized between a pair of atoms.

• Resonance involves two or more Lewis (electron dot) structures to represent a particular molecule or ion. A resonance structure is one of two or more alternative Lewis (electron dot) structures for a molecule or ion that cannot be described fully with one Lewis(electron dot) structure alone.

Further aspect of covalent bonding and structure

Nature of science

• Principle of Occam’s razor — bonding theories have been modified overtime. Newer theories need to remain as simple as possible while maximizing explanatory power, for example the idea of formal charge.

Formal charge

• Sometimes different Lewis dot structures can be drawn for the a molecular species or polyatomic ion that obey the octet rule

• To determine which Lewis structure to use - must rely on formal charge (FC) of the atoms present in the molecule or ion.

FC = (# of valence electrons) - 1/2 (# of bonding electrons) - (# of non-bonding electrons)

• If there are more than one Lewis structures available, the most reasonable one will be:• the one with FC difference (∆FC=FCmax-FCmin)

closest to zero• the one that has negative charges located on most

electronegative atomSaturday, August 1, 15

Formal charge

• For example, tetrachloromethane, CCl4:

FC (C) = 4 - 1/2(8) - 0 = 0FC (Cl) = 7 - 1/2(2) - 6 = 0

• For example, carbonate anion, CO32-:FC (C) = 4 - 1/2(8) - 0 = 0FC (OA) = 6 - 1/2(4) - 4 = 0FC (OB) = 6 - 1/2(2) - 6 = -1

BBSaturday, August 1, 15

Formal charge

• For example, boron trifluoride, BF3:

FC (B) = 3 - 1/2(6) - 0 = 0FC (F) = 7 - 1/2(2) - 6 = 0∆FC = 0

FC (B) = 3 - 1/2(8) - 0 = -1FC (FA) = 7 - 1/2(2) - 6 = 0FC (FB) = 7 - 1/2(4) - 4 = +1∆FC = FCmax - FCmin

∆FC = (+1) - (-1) = +2

For structure a: For structure b:

Different interpretations of charge

• Oxidation state, formal charge, ionic charge, partial charge, total charge, etc... Let’s examine HF molecule•Oxidation state: H: +1 F: -1• Formal Charges: FC(H)=0, FC(F)=0, ∆FC=0•Partial Charges: Electronegativity for H = 2.2 and F = 4.0

so partial charge on F due to the pulling of electrons•Total (Net) Charges: Would be 0 as it is a neutral

molecule

Molecular geometries based on five and six electron domains

• Use same method to deduce molecular geometry for 5 & 6 electron domains as you would 2, 3, & 4 electron domains (topic 4)

• For five electron domains, lone pairs (non-bonding pairs) occupy the equatorial positions.• Based on terms of repulsion, order of interactions:

• LP|LP > LP|BP > BP|BP

Number of electron domains

Electron domain geometry Molecular geometry Notes

5 trigonal bipyramidal

tetrahedral (AB4)

5 BP’sexample: PF5

see-saw

AB4E4 BP’s and 1 LPexample: SF4

5 trigonal bipyramidalurl

T-shaped

3 BP’s and 2 LP’sexample: ClF3

Number of electron domains

Electron domain geometry Molecular geometry Notes

linear

2 BP’s and 3 LP’sexample: I3-

6 octahedral

octahedral

6 BP’sexample: SF6

6 octahedral

square-based pyramidal

AB5E5 BP’s and 1 LPexample: BrF5

6 octahedral

square planar

4 BP’s and 2 LP’sexample: XeF4

Overlap of atomic orbitals: Sigma and pi bonding

• A single covalent bond consists of two electrons shared between two atoms A and B• A single bond is a sigma bond (σ)

• A double covalent bond consists of four electrons, two pairs, shared between two atoms A and B. • A double bond is a sigma bond (σ) plus a pi bond (π)

• A triple covalent bond consists of six electrons, two pairs, shared between two atoms A and B.• A triple bond is a sigma bond (σ) plus two pi bonds (π)

A B––σ

––––A Bσ + π

––––––A Bσ + 2π

BA•• BA•••• BA

••

••••

Overlap of atomic orbitals: Sigma and pi bonding

• Lewis structure is a simple model showing how the valence electrons are distributed in a molecule or polyatomic ion. • We will learn a more in depth theory based on quantum

mechanics molecular orbital theory (MOT)• Helps to better visualize sigma and pi bonds and see the difference

• For atomic orbitals to overlap, must be close in energy and have identical symmetry. Three possible outcomes:• bonding orbital: sigma (σ) or pi (π) bond• anti-bonding orbital: sigma star (σ*) or pi star (π*) orbital• non-bonding situation

• Order of bond lengths: A–B > A=B > A-B• Order of bond strength: A-B > A=B > A–B

Description of a sigma and pi bond

• In the formation of a sigma bond there is a direct head-on overlap of the atomic orbitals along the internuclear axis and the electron density is located along this axis.

• In the formation of a pi bond there is a sideways overlap of the atomic orbitals and the electron density is located above and below the internuclear axis

Combination of atomic orbitals Molecular orbitals formed Type

σ bonding

s + sσ*

anti-bonding

s + px

σ bonding

s + pxσ*

anti-bonding

s + py non-bonding

s + pz non-bonding

px + px

σ bonding

px + pxσ*

anti-bonding

+Saturday, August 1, 15

Combination of atomic orbitals Molecular orbitals formed Type

py + py

π bonding

py + py

π* anti-bonding

pz + pz

πbonding

pz + pz

π* anti-bonding

px + py non-bonding

px + pz non-bonding

py + pz non-bonding

Delocalization and resonance

• A molecule or polyatomic ion can have multiple Lewis structures - identical arrangement of atoms but different arrangement of electrons.

• These are known as resonance forms as they contribute to the electronic structure called a resonance hybrid.

Delocalization and resonance

• Can also be represented with a dashed curve or two dashed lines as below

• This shows delocalization which is a quantum mechanical concept used to describe the pi bonding in a conjugated system

• A conjugated system is a molecular entity whose structure can be represented as a system of alternating single and multiple bonds. Not localized between two atoms but instead each link has a fractional double bond character or bond order.• It is the interaction between:

• One p orbital with another across an intervening sigma bond.

• A double bond and a p orbital containing a lone pair of electrons

Practice Problem... I Do ...

Consider the following species:a) BrF3b) IF5

For each species, deduce:(i) the electron domain geometry(ii) the molecular geometry(iii) the approximate bond angle(s)(iv) a valid Lewis (electron dot) structure

Practice Problem(i) the electron domain geometry

For Br:number of valence electrons = 7number of sigma bonds = 3total number of valence electrons = 10Number of electron domains = 5

From table 1, electron domain geometry: trigonal bypyramidal

(ii) the molecular geometry

This is an example of a AB3E2 system, so the two lone pairs are located in the equatorial positions:

molecular geometry: t-shaped

Practice ProblemBrF3

(iii) the approximate bond angle(s)

Bond angles: less than 90º The Br-F-Br experimental value is 86.2º but this cannot be predicted from the VSEPR theory

(iv) a valid Lewis (electron dot) structure

To draw the Lewis (electron dot) structure, complete the octets of fluorine atoms:

Practice Problem(i) the electron domain geometry

For I:number of valence electrons = 7number of sigma bonds = 5total number of valence electrons = 12number of electron domains = 6

From table 1, electron domain geometry: trigonal bypyramidal

(ii) the molecular geometry

This is an example of a AB5E system, so the one lone pair is located in any of the six equivalent positions:

molecular geometry: square-based pyramidal

Practice ProblemIF5

(iii) the approximate bond angle(s)

Bond angles: six F-I-F bond angles are less than 90º. Each one of these F-I-F bond angles is 80.9º

Practice ProblemIF5

(iv) a valid Lewis (electron dot) structure

To draw the Lewis (electron dot) structure, complete the octets of fluorine atoms:

Practice ProblemIF5

Practice Problem... We Do ...

Consider the following species:a) [ICl2]-

b) SOF4

Practice Problem... You Do ...

Consider the following species:a) [ICl4]-

An environmental perspective: Catalysis of ozone depletion

•Ozone, O3, is a v-shaped (bent) molecule with a bond angle of 116.8º and its two O-O bond lengths are equal (128pm). Two contributing resonance forms are below:

• Calculate bond order:total number of O-O bonding pairs = 3 = 1.5total number of O-O positions 2

• A high bond order indicates more attraction between electrons. A higher bond order also means that the atoms are held together more tightly. With a lower bond order, there is less attraction between electrons and this causes the atoms to be held together more loosely. Bond order also indicates the stability of the bond. The higher the bond order, the more electrons holding the atoms together, and therefore the greater the stability.

• In topic 6 you learned about how the ozone layer (located in stratosphere) absorbs over 95% of harmful UV radiation hvO3(g) --> O2(g) + O•(g)

O2(g) + O•(g) --> O3(g) + heat

There is a net energy conversion from UV radiation to heat energy. Ozone depletion leads to increased skin cancer (melanoma) and cataracts from more UV radiation hitting Earth’s surface.

• The bonds in ozone can be broken by UV radiation (hv). THe bond order for ozone is 1.5 and for the O=O bond it’s 2 - which is much stronger and harder to break and requires higher energy (shorter wavelength) to break.

• Energy, E, of a photon of light is inversely proportional to the wavelength, λ, so the greater the energy, the shorter the wavelength and vice versa.

E = hv = hc/λWhere; h = Planck’s constant 6.63 x 10-34 J s v = frequency of the radiationc = speed of light = 3.00 x 108 m s-1 λ = wavelength of radiation

With a partner, work on the following problem:

The average bond enthalpy in ozone is 362 kJ mol-1. Using the relationships given in section 1 and the bond enthalpy data given in section 11 of the data booklet, calculate the maximum wavelength,

in nm, of the UV radiation required to break the O=O double bond in oxygen and the O–O bond in ozone.

Practice Problem

Practice Problem... You Do ...

Work within a group of 3-4 and answer the following question:

Deduce the molecular polarities of the following:• SF6

• cisplatin, Pt(NH3)2Cl2• transplatin, Pt(NH3)2Cl2

structure➡Covalent bonds result from the overlap of atomic orbitals. A sigma

bond (σ) is formed by the direct head-on/end-to-end overlap of atomic orbitals, resulting in electron density concentrated between the nuclei of the bonding atoms. A pi (π) bond is formed by the sideways overlap of atomic orbitals, resulting in electronic density above and below the plane of the nuclei of the bonding atoms.

➡Formal charge (FC) can be used to decide which Lewis (electron dot) structure it Is preferred from several. The FC is the charge an atom would have if all atoms in the molecule had the same electronegativity. FC =(Number of valence electrons) - 1/2(Number of bonding electrons) - (Number of non-bonding electrons). The Lewis (electron dot) structure with the atoms having FC values closest to zero is preferred.

structure➡Exceptions to the octet rule include some species having

incomplete octets and expanded octets. ➡Delocalization involves electrons that are shared by more than

two at times in a molecule or ion as opposed to being localized between a pair of atoms.

➡Resonance involves two or more Lewis (electron dot) structures to represent a particular molecule or ion. A resonance structure is one of two or more alternative Lewis (electron dot) structures for a molecule or ion that cannot be described fully with one Lewis(electron dot) structure alone.

Chemical Bonding & Structure - Ms. Suchy's science site€¦ · Further aspect of covalent bonding...

Documents

Transcript of Chemical Bonding & Structure - Ms. Suchy's science site€¦ · Further aspect of covalent bonding...

Additional Aspects of Molecular Bonding & Structure Chapters 8 and 9 BLB 12 th.

How Unusual is the Milky Way’s Assembly History? · The Milky Way (MW) is sometimes regarded as a template for studies of the structure and evolution of ˘L? spiral galaxies. Yet,

Ligações Metálicas Bond between atoms with similar low electronegativities (ΔX ~0). Metallic bonding is essentially covalent bonding with delocalized electrons.

Atomic structure and bonding

Rotation of a Rigid BodyRotation of a Rigid Body · For Rigid Body sometimes it is convenient to describe the rotation aboutFor Rigid Body sometimes it is convenient to describe the

ATOMIC STRUCTURE & CHEMICAL BONDING - … · Web viewName Reactions Aldol Condensation: condensation between two molecule of an aldehyde or a ketone having atleast one α-hydrogen

Bonding of the Polymer Polyetheretherketone (PEEK) to Human

CHAPTER X A New Non-covalent Bonding Mode in ...

Part One Structure and Bonding · 2018. 3. 22. · 6 1 Bonding in Coordination Compounds Figure 1.2 Representation of the d orbitals and point charges in an O h crystal ﬁ eld. Figure

Unit 6B: Atomic Structure and Bonding Theory

IBDP HL bonding

Tuning magnetic anisotropy by the π-bonding features of ...

acids Bonding Introduction nd · Bonding π-acids Metal Carbonyl Compounds ... compounds in many important chemical processes that include catalysis, sensors and others. Metal Carbonyl

Chapter 07 Atomic Structure and Periodicity Notes (answers) Chemistry (Old)/Chapter 07... · · 2012-02-24Copyrighted by Gabriel Tang B.Ed., B.Sc. Page 51. Unit 2: Chemical Bonding

11.3: A Resonance Picture of Bonding in Benzene

Sub-nanometer Scale Atomic Bonding Characterization via HAADF STEM

Open-Shell Lanthanide(II+) or -(III+) Complexes Bearing ... · Silylene Ligands: Synthesis, Structure, and Bonding Analysis ... Metalorganics and Inorganic Materials, ... out salt

Part One Structure and Bonding - Wiley-VCH...6 1 Bonding in Coordination Compounds Figure 1.2 Representation of the d orbitals and point charges in an O h crystal ﬁ eld.Figure 1.3

CHM4311$ Coordinaon$and$back$and$pi5 bonding$ · 2013-01-07 · 2/1/12 1 CHM4311$ Coordinaon$and$back$and$pi5 bonding$ Nature$of$coordinave$bond$ Back5bonding$ Bonding$of$metals$to$π5systems$

Hydrogen Bonding versus π–π Stacking …Delicate intermolecular interactions, like hydrogen bonding (HB) and π-type interactions that rarely occur simultaneously in traditional