chapter 8 – covalent bonding mr. samaniego lawndale high school the unspoken hero: “covalent...

Chapter 8 – Covalent Bonding

Mr. Samaniego

Lawndale High School

The unspoken

hero: “Covalent

Bond”

Review of Chapter 7

In Chapter 7, we learned about electrons being transferred (“given up” or “stolen away”)

This type of “tug of war” between a METAL and NONMETAL is called an IONIC BOND, which results in a SALT being formed

Chapter 8.1 – Molecular Compounds

In this chapter, you will learn about another type of bond in which electrons are shared

Covalent Bonds are atoms held together by SHARING electrons between NONMETALS

Salt versus MoleculesA metal cation and nonmetal anion are joined together by an ionic bond called SALTA group of atoms joined together by a

covalent bond is called a MOLECULEA Compound is a group of two or more

elements bonded together (Ionic or Covalent).

Monatomic vs. Diatomic Molecules

Most molecules can be monatomic or diatomic

Diatomic Molecule is a molecule consisting of two atoms

There are 7 diatomic molecules (SUPER 7) – N2, O2, F2, Cl2, Br2, I2, H2

You can also remember them as: H2O2F2Br2I2N2Cl2

Properties of Molecular Compounds

Lower Melting Points than Ionic Compounds (which means that they are weaker than ionic)

Liquids or gases at room temperature

Molecular Formulas

The Molecular Formula is the formula of a molecular compound

It shows how many atoms of each element a molecule contains

ExampleH2O contains 3 atoms (2 atoms of H, 1 atom of O)

C2H6 contains 8 atoms (2 atoms of C, 6 atoms of H)

Practice

How many atoms total and of each do the following molecular compounds contain?

1. H2

2. CO

3. CO2

4. NH3

5. C2H6O

Practice: True or False

1. All molecular compounds are composed of atoms of two or more elements.

2. All compounds are molecules.

3. Molecular compounds are composed of two or more nonmetals.

4. Atoms in molecular compounds exchange electrons.

5. Molecular compounds have higher melting and boiling points than ionic compounds.

Ionic versus Covalent

IONIC COVALENT

Bonded Name Salt Molecule

Bonding Type Transfer e- Share e-

Types of Elements Metal & Nonmetal Nonmetals

Physical State Solid Solid, Liquid, or Gas

Melting Point High (above 300ºC) Low (below 300 ºC)

Solubility Dissolves in Water Varies

Conductivity Good Poor

Chapter 8.2 – Covalent Bonding

Remember that ionic compounds transfer electrons in order to attain a noble gas electron configuration

Covalent compounds form by sharing electrons to attain a noble gas electron configuration

Regardless of the type of bond, the Octet Rule still must be obeyed (8 valence electrons)

Single Covalent Bond

A Single Covalent Bond consists of two atoms held together by sharing 1 pair of electrons (2 e-)

Electron Dot Structure

Shared versus Unshared Electrons

A Shared Pair is a pair of valence electrons that is shared between atoms

An Unshared Pair is a pair of valence electrons that is not shared between atoms

Practice Lewis Dot Structures

Chemical Formula

# of Valence

Electrons

Single Line Bond

Structure

# of Remaining Electrons

Lewis Dot Structure

Octet Check All Atoms=8Hydrogen=2

F2

H2O

NH3

CH4

Double Covalent Bonds

A Double Covalent Bond is a bond that involves 2 shared pairs of electrons (4 e-)

Sometimes atoms attain noble gas configuration by sharing 2 or 3 pairs of electrons

Triple Covalent Bond

A Triple Covalent Bond is a bond that involves 3 shared pairs of electrons (6 e-)

Covalent Bonds

Practice Lewis Dot Structure

Chemical Formula

# of Valence

Electrons

Single Line Bond

Structure

# of Remaining Electrons

Lewis Dot Structure

Octet Check All Atoms=8Hydrogen=2

O2

CO2

N2

HCN

Bond Dissociation Energy

Bond Dissociation Energy is the energy required to break a bond between two atoms

A large bond dissociation energy corresponds to a strong bond which makes it unreactive

Carbon has strong bonds, which makes carbon compounds stable and unreactive

Chapter 8.3 - Bonding Theories

A Molecular Orbital is an orbital that applies to the entire molecule, instead of just one atom

So far, the orbitals we have been discussing are atomic orbitals (s, p, d, f) for each atom

When two atoms combine, their atomic orbitals overlap and they make molecular orbitals

Molecular Orbitals

Just as atomic orbitals belong to a particular atom, a molecular orbital belongs to molecules as a wholeEach orbital is filled with 2 electrons

A Bonding Orbital is an orbital that can be occupied by two electrons of a covalent bond (it’s the space in between the two atoms)

There are 2 types of bonding orbitals: sigma and pi

Sigma Bond ()

A Sigma Bond is when 2 atomic orbitals combine to form a molecular orbital that is symmetrical around the axis

S orbitals overlapping P orbitals overlapping end-to-end

Pi Bond ()

Pi bonding electrons are likely to be found in a sausage-shape above and below the axis

Pi bonds are weaker than sigma bonds Pi bonds are weaker than sigma bonds because they overlap lessbecause they overlap less

P orbitals overlapping side-by-side

VSEPR Theory

VSEPR Theory predicts the 3D shape of molecules

According to VSEPR, the repulsion of electrons causes the shape of the molecule to adjust so that the electrons are far apart

A Few VSEPR Shapes

Nine possible molecular shapes

VSEPR Theory

Unshared pairs of electrons are very Unshared pairs of electrons are very important in predicting the shapes of important in predicting the shapes of moleculesmolecules

Each bond (single, double, or triple) or Each bond (single, double, or triple) or unshared pair is considered a steric numberunshared pair is considered a steric number

VSEPR can only be used with the central atomVSEPR can only be used with the central atom

Unshared pairs of electrons are very Unshared pairs of electrons are very important in predicting the shapes of important in predicting the shapes of moleculesmolecules

Use the steric number to predict the Use the steric number to predict the molecular geometrymolecular geometry

Practice

Methane (CHMethane (CH44) – tetrahedral) – tetrahedral

Ammonia (NHAmmonia (NH33) – pyramidal) – pyramidal

Water (HWater (H22O) – bentO) – bent

Carbon Dioxide (COCarbon Dioxide (CO22) - linear) - linear

Hybrid Orbitals

VSEPR is good at describing the molecular shapes, but not the types of bonds formed

In hybridization, several atomic orbitals In hybridization, several atomic orbitals mix to form hybrid orbitalsmix to form hybrid orbitals

Orbital hybridization provides information about both molecular bonding and molecular shape

Bond Hybridization

Hybridization Involving Single Bonds – sp3 orbital Ethane (C2H6)

Hybridization Involving Double Bonds – sp2 orbital Ethene (C2H4)

Hybridization Involving Triple Bonds – sp orbital Ethyne (C2H2)

Chapter 8.4 – Polar Bonds and Molecules

There are two types of covalent bonds Nonpolar Covalent Bonds (share equally) Polar Covalent Bonds (share unequally)

Polar Covalent

• A Polar Covalent Bond is unequal sharing of electrons between two atoms (HCl)

• In a polar covalent bond, one atom typically has a negative charge, and the other atom has a positive charge

Nonpolar Covalent Bond

• A Nonpolar Covalent Bond is equal sharing of electrons between two atoms (Cl2, N2, O2)

Classification of Bonds

You can determine the type of bond between two atoms by calculating the difference in electronegativity values between the elements

Type of Bond Electronegativity Difference

Nonpolar Covalent 0 0.4

Polar Covalent 0.5 1.9

Ionic 2.0 4.0

Practice

What type of bond is HCl? (H = 2.1, Cl = 3.1)

Your Turn To PracticeYour Turn To Practice N(3.0) and H(2.1)N(3.0) and H(2.1)

H(2.1) and H(2.1)H(2.1) and H(2.1)

Ca(1.0) and Cl(3.0)Ca(1.0) and Cl(3.0)

Al(1.5) and Cl(3.0)Al(1.5) and Cl(3.0)

Mg(1.2) and O(3.5)Mg(1.2) and O(3.5)

H(2.1) and F(4.0)H(2.1) and F(4.0)

Difference = 3.1 – 2.1 = 1.0Difference = 3.1 – 2.1 = 1.0Therefore it is polar covalent bond.Therefore it is polar covalent bond.

Dipole

• No bond is purely ionic or covalent … they have a little bit of both characters

When there is unequal sharing of electrons a dipole exists Dipole is a molecule Dipole is a molecule that has two poles or that has two poles or regions with opposite regions with opposite chargeschargesA dipole is represented A dipole is represented by a dipole arrow by a dipole arrow pointing towards the pointing towards the more negative endmore negative end

Practice Drawing Dipoles

P- BrP = 2.1Br = 2.8

P –Br P –Br + + --

Practice H(2.1) – S(2.5) F(4.0) - C(2.5) C(2.5) - Si(1.8) N(3.0) – O(3.5)

Attractions Between Molecules

Intermolecular attractions are weaker than ionic, covalent, and metallic bonds

Besides ionic, metallic, and covalent bonds, there are also attractions between molecules

There are 2 main types of attractions between molecules: Van der Waals and Hydrogen

Van der Waals Forces

Van der Waals forces consists of the two weak attractions between molecules

1. dipole 1. dipole interactions – interactions – polar molecules polar molecules attracted to one attracted to one anotheranother

2. dispersion 2. dispersion forces – forces – caused by caused by the motion the motion of electrons of electrons (weakest of (weakest of all forces)all forces)

Hydrogen Bond

Hydrogen Bonds are forces where a hydrogen atom is weakly attracted to an unshared electron pair of another atom

Hydrogen Bond

This other atom may be in the same This other atom may be in the same molecule or in a nearby molecule, but molecule or in a nearby molecule, but always has to include hydrogenalways has to include hydrogen

Hydrogen Bonds have about 5% of the Hydrogen Bonds have about 5% of the strength of an average covalent bondstrength of an average covalent bond

Hydrogen Bond is the strongest of all Hydrogen Bond is the strongest of all intermolecular forcesintermolecular forces

Intermolecular Attractions

A Network Solid contains atoms that are A Network Solid contains atoms that are all covalently bonded to each otherall covalently bonded to each other

A few solids that consist of molecules do A few solids that consist of molecules do not melt until the temperature reaches not melt until the temperature reaches 10001000ººC or higher called network solids C or higher called network solids (Example: diamond, silicon carbide)(Example: diamond, silicon carbide)

• Melting a network solid would require breaking bonds throughout the solid (which is difficult to do)

Classwork

Chapter 8 Assessment Page 247

#’s 39-41, 43-46, 51, 53, 54, 57-59, 61, 65, 68, 83, 85, 86, 89, 96, 99, 100