Unit 7 (last one!!!!)

Chapters 8, 9.1-9.3

Chemical Bonding and Molecular Geometry

Lewis Symbols and the Octet Rule

Ionic Bonding

Covalent Bonding

Molecular Geometry

Introduction

Salt vs. Sugar

solutions conduct solutions don’t electricity conduct electricity

electrolyte non-electrolyte

ionic molecular

• Sugar and salt differ in the type of attractive forces between the atoms/ions in the compound.• Chemical bond: strong attractive force that exists between atoms (or ions) in a compound

Chemical Bonds

• Three basic types of bonds– Ionic

• Electrostatic attraction between ions (cations-anions)

– Covalent• Sharing of electrons

– Metallic• Metal atoms bonded to

several other atoms

Chemical Bonds

• Covalent Bonds: the attractive force between atoms in a molecule that results from sharing of one or more pairs of electrons– non-metals

• H2O : O

H H

• Cl2 : Cl Cl

H-O and Cl-Cl bonds result

from sharing of electrons

Remember?

• Electron configuration for sodium:

[Ne]3s1

• The 3s electron of a sodium atom is a valence electron.– electrons residing in the incomplete outer

shell of an atom• involved in chemical bonding and ion formation

Lewis Symbols

• Since valence electrons are involved in the formation of chemical bonds, it’s important to keep track of them.– electron-dot symbols or Lewis symbols

• simple way to depict valence electrons and track them during the formation of chemical bonds

To write Lewis Symbols

• You must be able to determine the number of valence electrons for the main group elements.

• For main group elements, the number of valence electrons for an element = group number of the element – N (group 5A) has 5 valence electrons– Br (group 7A) has 7 valence electrons

Lewis Symbols

• Lewis symbol has two components:– chemical symbol for the element– Plus a dot for each valence electron

• dots are placed on all 4 sides of the chemical symbol

– all four sides of the symbol are equivalent

• up to 2 dots (electrons) per side (start with single dot per side of square, then double up)

Lewis Symbol for Oxygen

Chemical symbol: O

Group number: 6A

# of valence electrons: 6

O

Lewis Symbols

Draw the Lewis symbol for silicon.

Chemical symbol: Si

Group number: 4A

# of valence electrons: 4

Si

Lewis Symbols

Draw the Lewis symbol for argon.

Chemical symbol: Ar

Group number: 8A

# of valence electrons: 8

Ar

Octet Rule

• The noble gases are particularly stable because their outer shell is full of electrons.– With the exception of He, all noble gases have 8

valence electrons.

ns2np6

• Octet Rule: Atoms tend to gain, lose, or share electrons until they are surrounded by 8 valence electrons

Octet Rule

• The octet rule can be used to predict the charge of ions formed by main group elements as well as the structure of molecular compounds.

• BUT: There are many exceptions to the octet rule.

Covalent Bonding

• Molecular compounds are held together by covalent bonds that result from the sharing of electrons.

• Simplest example of a covalent bond is

H H

Indicates 2 shared electrons = 1 bond

Covalent Bonding• When 2 H atoms approach each other, electrostatic

interactions occur between their respective electrons and their nuclei.

– The two nuclei repel each other– The two electrons repel each other– The nuclei and the electrons attract each other.

Covalent Bonding

• The attractions between the nuclei and the electrons cause the electron density to concentrate between the two nuclei.

• The atoms in H2 are held

together by the electrostatic attraction of the two nuclei for the concentration of negative charge between them.

• The shared pair of electrons between the two nuclei acts as “glue”.

Covalent Bonding

• Lewis structures (also called electron-dot structures) can be used to represent the covalent bonds that are present in a molecule.– Symbol for each atom– Bond between atoms depicted using a solid

line– Unshared electron pairs are shown around

the appropriate atom

Lewis Structures

Lewis structures are representations of molecules showing all valence electrons, bonding and nonbonding.

NOTE: Octet of electrons around each Cl atom!

Covalent Bonding

• The formation of H2:

H + H H H or H H

• The Lewis structure for HF:

H + F H F = H F

Covalent Bonding

• The bond between H and F in HF is called a single bond:– sharing of one pair of electrons

• In some molecules, atoms attain an octet of electrons by sharing more than one pair of electrons.– Double bond– Triple bond

Covalent Bonding

• Double bond: two electron pairs are shared between atoms– depicted using two lines to represent the two

shared electron pairs

O C O or O C O

Carbon dioxide

Covalent Bonding

• Triple bond: three electron pairs are shared between atoms– depicted using three lines to represent the 3

pairs of shared electrons

N + N N N or N N

Nitrogen (N2)

Covalent Bonding

• In some molecular compounds, the bonding electrons are shared equally between the atoms in the molecule:

H2 F2 N2

• Nonpolar covalent bond: bonding electrons are shared equally

Covalent Bonding• In many molecular compounds, however, one

atom attracts the bonding electrons more strongly than the other.

+ - Fluorine attracts electrons more strongly than hydrogen.H – F

+ -

The fluorine end of the molecule has greater electron density than the hydrogen end.

The H – F bond is a polar covalent bond.

Covalent Bonding

• Polar covalent bond: – a chemical bond in which the electrons are not

shared equally• one atom attracts the bonding electrons more

strongly

• The polarity of a covalent bond can be determined using the difference in electronegativity between the two atoms.

Covalent Bonding

• Electronegativity:– The ability of an atom in a molecule to attract

electrons to itself • Range: 0.7 (Cs) - 4.0 (F)

• As electronegativity increases, the attraction that an atom has for electrons increases.

Covalent Bonding• Trends to know:

– Electronegativity increases:• From left to right across

a row• From bottom to top of a

column– The four most

electronegative elements are:

• F (4.0)• O (3.5)• N (3.0)• Cl (3.0)

Covalent Bonding• Chemicals bonds exist along a continuum:

• The greater the difference in electronegativity between two atoms, the more polar their bond.

F – F en = 0 nonpolar covalent (en<0.5)

H – F en = 1.9 polar covalent (0.5<en<2.0)

Li – F en = 3.0 ionic (en>2.0)

IonicBonds

Polar CovalentBonds

Nonpolar CovalentBonds

Testing

Which of the following bonds is a polar covalent bond?

Br-Br C-H O-H Al-Cl

EN= 0 0.45 1.34 1.55

Polar covalent (0.5< en < 2.0)