chem bond ppt

Chapter 5Chemical Bonding

The Covalent Bond

Material from karentimberlake.com and H. Stephen Stoker

Covalent or Ionic ???

Electronegativity – the attraction that an atom has for the electrons that it is sharing w/ another atom

Scale devised by Nobel Prize winner Linus Pauling

Based on scale with fluorine assigned 4.0, the highest value


Forming Chemical Bonds

• According to the Lewis model– an atom may lose or gain enough electrons to

acquire a filled valence shell and become an ion. An ionic bondionic bond is the result of the force of attraction between a cation and an anion.

– an atom may share electrons with one or more other atoms to acquire a filled valence shell. A covalentcovalent bondbond is the result of the force of attraction between two atoms that share one or more pairs of electrons.


Covalent Bonds

A chemical bond in which 2 atoms share a single of electron to form one bond

Examples F and H


Covalent Bonds

Two nonmetal atoms form a covalent bond because they have less energy

after they bonded

H + H H : H = HH = H2

hydrogen molecule


Double Covalent Bond

2 pairs of electrons are shared between 2 atoms

Example O2

O + O O::O

double bond


Triple Covalent Bond

3 pairs of electrons are shared between 2 atoms

Example N2

N + N N:::N

triple bond


Diatomic Elements

Elements that are naturally in molecules with 2 atoms each.

HONClBrIF (pneumonic) Existing as diatomic molecule yields a stable octet Gases that exist as diatomic molecules are H2,

F2, N2, O2, Cl2, Br2, I2

Examples Fluorine & Bromine


Learning Check

Use the name of the element to name

the following diatomic molecules.

H2 hydrogen

N2 nitrogen

Cl2 _______________

O2 _______________

I2 _______________


Solution

Use the name of the element to

name the following diatomic

molecules.

H2 hydrogen

N2 nitrogen

Cl2 chlorine

O2 oxygen

I2 iodine


Lewis Structures

Other molecules having single covalent bonds

H2O

The hydrogens share their electrons w/ oxygen so that O has 8 e- and each H has 2 e-


Covalent Bonds in NH3

Bonding pairs

H

H : N : H

Lone pair of electrons

(unshared pair)


Drawing Lewis Structures

1. Determine the number of valence electrons in the molecule

2. Decide on the arrangement of atoms in the molecule

3. Connect the atoms by single bonds

4. Show bonding electrons as a single line; show nonbonding electrons as a pair of Lewis dots

5. In a single bondsingle bond, atoms share one pair of electrons; in a double bonddouble bond, they share two pairs, and in a triple bondtriple bond they share three pairs.


Coordinate Covalent Bond

Bond in which only one atom donates electrons to form the bond

Sometimes an arrow is used to designate the coordinate covalent bond


Chemical Bonding: The Covalent Bond Model cont’d



← Fig. 5.10

Linus Pauling received the Nobel Prize in chemistry in 1954 for his work on the nature of the chemical bond.

© Bettman/CORBIS


Electronegativity

The attraction of an atom for electrons is called its electronegativity.

Fluorine has the greatest electronegativity.The metals have low electronegativities.



→ Fig. 5.11

Abbreviated periodic table showing Pauling electronegativity values for selected representative elements.


Electronegativity

• Electronegativity:Electronegativity: a measure an atom’s attraction for the electrons it shares in a chemical bond with another atom– on the Pauling scale, fluorine, the most

electronegative element is assigned a value of 4.0, and all other elements are assigned values relative to fluorine

- -

Electronegativity increases


Bond Polarity: Nonpolar

Nonpolar covalent bond Electrons are shared between atoms with

the same electronegativity values. Difference = 0 Examples:

N2 Br2


Bond Polarity: Polar

Polar covalent bond Electrons are shared between different

nonmetal atoms Examples:O-Cl O-S N-Cl


Chemical Bonding: The Covalent Bond Model

← Fig. 5.12

(a) In the nonpolar covalent bond present, there is a symmetrical distribution of electron density. (b) In the polar covalent bond present, electron density is displaced because of its electronegativity.


Learning Check

Identify the type of bond between the following atoms A. K-N1) nonpolar 2) polar 3) ionicB. N-O1) nonpolar 2) polar 3) ionic

C. Cl-Cl

1) nonpolar 2) polar 3) ionic


Solution

A. K-N 3) ionic

B. N-O 2) polar, covalent

C. Cl-Cl 1) nonpolar, covalent


To decide whether a bond is covalent or ionic find the difference in electronegativities

< 2.0 covalent

> 2.0 ionic

Try KF, MgS, Cl2

Covalent or Ionic ???


Character of Bonds

Chemist find it better to express chemical bonds as % ionic and % covalent

Relates electronegativity to ionic and covalent percentages

Table 7.2 (p 159) in text

Try KF, MgS, Cl2


Molecular Polarity

Just like bonds molecules can have polarity Look at

Molecule geometry atoms (how atoms are arranged in space)

Bond polarity



Fig. 5.13

(a) Methane is a nonpolar tetrahedral molecule. (b) Methyl chloride is a polar tetrahedral molecule.


Electronegativity


Writing Formulas

Nonmetal/Nonmetal In covalent bonds, the element with the

lowest electronegativity is written first



→ Table 5.1



→ Table 5.2


Naming of 2 Nonmetals

1. Name each element2. End the last element in –ide3. Add prefixes to show more than 1 atomPrefixesmon 1 hexa 6di 2 hepta 7 tri 3 octa 8tetra 4 nona 9pent 5 deca 10


Learning Check

Fill in the blanks to complete the following names of covalent compounds.

CO carbon ______oxide

CO2 carbon _______________

PCl3 phosphorus

_______chloride

CCl4 carbon ________chloride

N2O _____nitrogen

_____oxide© Karen Timberlake


Solution

CO carbon monoxide

CO2 carbon dioxide

PCl3 phosphorus

trichloride

CCl4 carbon tetrachloride

N2O dinitrogen monoxide

© Karen Timberlake


Learning Check

A. P2O5

B. Cl2O7

C. Cl2


Learning Check

• Examples– draw a Lewis structure for hydrogen

peroxide, H2O2

– draw a Lewis structure for methanol, CH3OH

– draw a Lewis structure for acetic acid, CH3COOH


Lewis Structures

Carbonic acidFormaldehydeAcetyleneEthylene

Hydrogen chlorideMethaneAmmoniaWater

H

H N H C H H ClH

HC C

HC C HH

HC

HHO

H

H2O (8) NH3 (8) CH4 (8) HCl (8)

C2H4 (12) C2H2 (10) CH2O (12) H2CO3 (24)

H

HHO

H

O OC HH

O


3-D Characteristics of Molecules

Atoms and molecules have 3 dimensions Shapes of molecules lead to additional properties

of covalent compounds Polar covalent Bonding

When electrons are not shared equally between two atoms Bond that is certain % ionic

Nonpolar covalent Bonding Electrons are shared equally Diatomic atoms


More Electron Pairs

Electron Angle Bonded Name of

Pairs Atoms Shape

4 109° 4 tetrahedral

4 109° 3 pyramidal

4 109° 2 bent, angular


Shapes of Molecules

Number of electron pairs 2

(= negative charge clouds)

Number of bonded atoms 2

Angle 180°

Name of shape linear


Electron Shape with 3 Pairs


SO2

S has 2 bonded atoms , 1 lone pair (electron cloud)

120°, angular

.. .. .. :O:: S:O: S

.. O O

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