Covalent bonding and lewis structures
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Covalent Bonding and Lewis Structures. Ionic Bonding. Generally occurs between metals and nonmetals Can also occur with polyatomic ions. Ionic Bonding. Involves the transfer of electrons, followed by electrostatic attraction. Covalent Bonding. Generally occurs between nonmetals .

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Covalent Bonding and Lewis Structures

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Covalent bonding and lewis structures

Covalent Bonding and Lewis Structures


Ionic bonding

Ionic Bonding

  • Generally occurs between metals and nonmetals

  • Can also occur with polyatomic ions.


Ionic bonding1

Ionic Bonding

  • Involves the transfer of electrons, followed by electrostatic attraction.


Covalent bonding

Covalent Bonding

  • Generally occurs between nonmetals.

  • Involvessharingof electrons, rather than transfer.


Octet rule

Octet Rule

  • Atoms will acquire, through sharing or transfer, the electron configuration of a noble gas.

  • Most noble gases

    have 8 valence

    electrons

    • He is the exception


Valence electrons

Valence Electrons

  • The electrons in the highest occupied energy level

How can you find the number of valence electrons for a representative element?


Dot models

Dot Models

  • The number of dots is equal to the number of valence electrons.

P

Example: Phosphorus


Lewis structure for molecules

Lewis Structure for Molecules

  • Each atom in the molecule is connected by bonds.

  • Bonds are shared pairs of electrons, and they are represented by a dash.

  • Pairs that are not shared are called unshared electrons.


Covalent bonding and lewis structures

  • A single bond is created by one shared pair of electrons

  • A double bond is created by 2 shared pairs of electrons

  • A triple bond is created by 3 shared pairs of electrons.


Drawing lewis structures

Drawing Lewis Structures

Steps:

  • Count up total number of valence electrons in the molecule. Divide by 2. This is the number of pairs of electrons you must place.

  • In other words:

    # valence electrons ÷ 2 = # electron pairs


Drawing lewis structures1

Drawing Lewis Structures

2. Draw a skeleton of the molecule, joining atoms with single bonds. (1 shared pair)

  • Usually the first atom in the molecular formula is central.

  • Nature likes symmetry.

  • Hydrogen is never central!

    • Because he can’t have more than one electron pair!


Drawing lewis structures2

Drawing Lewis Structures

3. From the total pairs you counted in step 1, subtract the pairs you used in step 2. This will determine the number of pairs you have left to distribute.


Drawing lewis structures3

Drawing Lewis Structures

4. Distribute remaining pairs as unshared electrons around the atoms in the molecule. If you have too few pairs for each atom to have an octet you may need double or triple bonds somewhere in the molecule.


Exceptions to the octet rule

Exceptions to the Octet Rule

  • Atoms with less than an octet.


Exceptions to the octet rule1

Exceptions to the Octet Rule

  • Atoms with more than an octet. Elements in period 3 or higher, such as sulfur, phosphorus and bromine are capable of holding up to 6 pairs (12 e-). This is called an expanded octet and should be used when you have extra pairs that won’t fit anywhere. Note – you cannot have an expanded octet and a multiple bond in the same molecule


Exceptions to the octet rule2

Exceptions to the Octet Rule

  • Molecules with an odd number of electrons.

Total Number of Electrons = 5 + 6 = 11


Molecular shape

Molecular Shape

  • In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimizesthis repulsion.

  • We call this process Valence Shell Electron Pair Repulsion (VSEPR) theory.


Molecules with no unshared pairs around the central atom

Molecules with no unshared pairs around the central atom.

These molecules are symmetrical!!!


Molecules with unshared pairs around the central atom

Molecules with unshared pairs around the central atom.


Vsepr shapes

VSEPR Shapes

Linear Trigonal Bent Planar Triatomic

Pyramidal Tetrahedral Trigonal Bipyramidal


Vsepr bond angles

VSEPR Bond Angles

180° 120 ° <104.5 °

107.5 ° 109.5 °90 °, 120 °


Vsepr content frame

VSEPR Content Frame


Bond polarity

Bond Polarity

  • In covalent bonds, shared pairs of electrons are pulled between the nuclei of atoms sharing them.

    • Sometimes electrons are pulled equally and sometimes they are not.

  • There are two types of covalent bonds:

    • Non-polar

    • Polar


Nonpolar covalent bonds

Nonpolar Covalent Bonds

  • The electrons are shared equally.

    • All diatomic elements are nonpolar.


Polar covalent bonds

Polar Covalent Bonds

  • When the electrons are shared unequally.

    • Electronegativity is the ability to attract electrons.

    • The more electronegative atom will have a stronger attraction for the bonded electrons and will have a slightly negative charge.

    • The less electronegative atom will have a slightly positive charge.


Example hcl

Example - HCl

  • Look up the electronegativity values for hydrogen and chlorine.


Example hcl1

Example: HCl

Which is more electronegative?

  • H: 2.1

  • Cl: 3.0

    Chlorine has a slightly negative charge, while hydrogen has a slightly positive charge.


There are two ways to communicate the polarity of hcl

There are two ways to communicate the polarity of HCl:

H – Cl

H – Cl

 +

 -

The lowercase Greek letter delta shows that the atoms involved acquire only partial charges.

The arrow points to the more electronegative atom.


Example water

Example: Water

Is hydrogen or oxygen more electronegative?


Covalent bonding and lewis structures

Oxygen!!


Example water1

Example: Water

Is hydrogen or oxygen more electronegative? Oxygen!

 -

O

HH

The O-H bonds are polar.

 +

 +


The difference in electronegativities indicates the type of bond the atoms will form

The difference in electronegativities indicates the type of bond the atoms will form.

  • What type of bond will form between:

  • N and H?

  • F and F?

  • Ca and O?

  • Br and Cl?


Covalent bonding and lewis structures

3.0 – 2.1 = .9


The difference in electronegativities indicates the type of bond the atoms will form1

The difference in electronegativities indicates the type of bond the atoms will form.

  • What type of bond will form between:

  • N and H?

  • F and F?

  • Ca and O?

  • Br and Cl?

Polar!


Covalent bonding and lewis structures

4.0 – 3.0 = 1.0


The difference in electronegativities indicates the type of bond the atoms will form2

The difference in electronegativities indicates the type of bond the atoms will form.

  • What type of bond will form between:

  • N and H?

  • F and F?

  • Ca and O?

  • Br and Cl?

Polar!

Nonpolar!


Covalent bonding and lewis structures

3.5 – 1.0 = 2.5


The difference in electronegativities indicates the type of bond the atoms will form3

The difference in electronegativities indicates the type of bond the atoms will form.

  • What type of bond will form between:

  • N and H?

  • F and F?

  • Ca and O?

  • Br and Cl?

Polar!

Nonpolar!

Ionic!


Covalent bonding and lewis structures

3.0 – 2.8 = .2


The difference in electronegativities indicates the type of bond the atoms will form4

The difference in electronegativities indicates the type of bond the atoms will form.

  • What type of bond will form between:

  • N and H?

  • F and F?

  • Ca and O? Ionic!

  • Br and Cl? Nonpolar!

Polar!

Nonpolar!


Summary

Summary


Molecule polarity

Molecule Polarity

  • If a molecule has all nonpolarbonds then the molecule is nonpolar.

  • If a molecule has a polar bond then the whole molecule is usually polar, but not always.

    • If the molecular geometry is symmetrical, the bond polarities cancel, and the molecule is nonpolar.


Remember your symmetrical shapes

Remember your SYMMETRICAL shapes!

These molecules are symmetrical!!!


Example carbon dioxide

Example: carbon dioxide

CO2 has two polar bonds, but since the structure is linear (and symmetrical) the bonds cancel.

 +

 -

 -

OCO

Nonpolar molecule!


Example methane

Example: methane

CH4 has four polar bonds, but since the structure is tetrahedral (symmetrical) the bonds cancel.

 +

H

 -

 +

HCH

 +

Nonpolar molecule!

H

 +


Example water2

Example: Water

Water has a bent shape, due to unshared electrons (so NOT symmetrical)- The polarities do NOT cancel.

 -

Polar molecule

O

HH

 +

 +


Covalent bonding and lewis structures

  • In a polar molecule, one end has a positive charge and the other has a negative charge. A molecule that has poles is called a dipolar molecule or a dipole.


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