chapter 8. ◦ these toy models are made from circular pieces joined together in units by sticks....

Covalent BondingChapter 8

◦ These toy models are made from circular pieces joined together in units by sticks. Atoms can also be arranged in different ways to make a variety of products.

8.1 Molecular Compounds

In nature, matter takes many forms. The noble gases, including helium and neon, are monatomic. That means they exist as single atoms.

Molecules and Molecular Compounds

◦ Some compounds are so different from ionic compounds that attractions between ions fail to explain their bonding. The atoms held together by sharing electrons are

joined by a covalent bond.

Molecules and Molecular Compounds

◦ A molecule is a neutral group of atoms joined together by covalent bonds. Air contains oxygen molecules. A diatomic molecule is a molecule consisting of

two atoms. An oxygen molecule is a diatomic molecule.

Molecules and Molecular Compounds

A compound composed of molecules is called a molecular compound. Water and carbon monoxide are molecular compounds.

Molecules and Molecular Compounds

◦ Molecular compounds tend to have relatively lower melting and boiling points than ionic compounds. Unlike ionic compounds, many molecular compounds

are liquids or gases at room temperature. Unlike ionic compounds, molecular compounds

usually contain only nonmetal atoms.

Molecules and Molecular Changes

Molecules and Molecular Compounds

Ethane, a component of natural gas, is also a molecular compound.

Molecules and Molecular Compounds

◦ A molecular formula is the chemical formula of a molecular compound.

A molecular formula shows how many atoms of each element a molecule contains.

Molecular Formulas

Molecular Formulas

Formulas of Some Molecular Compounds

Molecular Formulas

◦ The colors in this map indicate the concentrations of ozone in various parts of Earth’s atmosphere. Oxygen atoms can join in pairs to form the oxygen you breathe and can also join in groups of three oxygen atoms to form ozone.

8.2 The Nature of Covalent Bonding

8.2

◦ In covalent bonds, electron sharing usually occurs so that atoms attain the electron configurations of noble gases.

The Octet Rule in Covalent Bonding

Two atoms held together by sharing a pair of electrons are joined by a single covalent bond.

Single Covalent Bonds

8.2

◦ An electron dot structure such as H:H represents the shared pair of electrons of the covalent bond by two dots.

A structural formula represents the covalent bonds by dashes and shows the arrangement of covalently bonded atoms.

Single Covalent Bonds

◦ The halogens form single covalent bonds in their diatomic molecules. Fluorine is one example.

Single Covalent Bonds

8.2

◦ A pair of valence electrons that is not shared between atoms is called an unshared pair, also known as a lone pair or a nonbonding pair.

Single Covalent Bonds

◦ The hydrogen and oxygen atoms attain noble-gas configurations by sharing electrons.

Single Covalent Bonds

◦ The ammonia molecule has one unshared pair of electrons.

Single Covalent Bonds

◦ Methane has no unshared pairs of electrons.

Single Covalent Bonds

Pg. 220 Conceptual Problem

for Conceptual Problem 8.1

Pg. 220 Practice Problems

8.2

◦ Atoms form double or triple covalent bonds if they can attain a noble gas structure by sharing two pairs or three pairs of electrons.

Double and Triple Covalent Bonds

8.2

◦ A bond that involves two shared pairs of electrons is a double covalent bond.

◦ A bond formed by sharing three pairs of electrons is a triple covalent bond.

Double and Triple Covalent Bonds

Double and Triple Covalent Bonds

8.2

Carbon dioxide gas is soluble in water and is used to carbonate many beverages. A carbon dioxide molecule has two carbon-oxygen double bonds.

Double and Triple Covalent Bonds

◦ Carbon dioxide is an example of a triatomic molecule.

Double and Triple Covalent Bonds

8.2

In a coordinate covalent bond, the shared electron pair comes from just one of the bonding atoms.

Coordinate Covalent Bonds

A polyatomic ion, such as NH4+, is a tightly bound

group of atoms that has a positive or negative charge and behaves as a unit.

Most plants need nitrogen that is already combined in a compound to grow.

Coordinate Covalent Bonds

Coordinate Covalent Bonds

Conceptual Problem pg. 225

for Conceptual Problem 8.2Section Assessment pg. 225

◦ The energy required to break the bond between two covalently bonded atoms is known as the bond dissociation energy.

A large bond dissociation energy corresponds to a strong covalent bond.

Bond Dissociation Energies

This car is being painted by a process called electrostatic spray painting. The negatively charged droplets are attracted to the auto body. You will learn how attractive and repulsive forces influence the shapes of molecules.

8.3 Bonding Theories

8.3

◦ The valence-shell electron-pair repulsion theory, or VSEPR theory, explains the three-dimensional shape of methane.

VSEPR Theory

8.3

The hydrogens in a methane molecule are at the four corners of a geometric solid. All of the H—C—H angles are 109.5°, the tetrahedral angle.

VSEPR Theory

8.3

◦ According to VSEPR theory, the repulsion between electron pairs causes molecular shapes to adjust so that the valence-electron pairs stay as far apart as possible.

VSEPR Theory

8.3

The measured H—N—H bond angle is only 107°.

VSEPR Theory

8.3

◦ The measured bond angle in water is about 105°.

◦ The carbon dioxide molecule is linear.

VSEPR Theory

8.3

Nine Possible Molecular Shapes

VSEPR Theory

Snow covers approximately 23 percent of Earth’s surface. Each individual snowflake is formed from as many as 100 snow crystals. The polar bonds in water molecules influence the distinctive geometry of snowflakes.

8.4 Polar Bonds and Molecules

8.4

The bonding pairs of electrons in covalent bonds are pulled by the nuclei.

Bond Polarity

8.4

◦ When the atoms in a bond pull equally (as occurs when identical atoms are bonded), the bonding electrons are shared equally, and the bond is a nonpolar covalent bond.

Bond Polarity

◦ The bond in a molecule of hydrogen is nonpolar.

◦ The bond between carbon atoms in ethane is nonpolar.

Bond Polarity

8.4

◦ A polar covalent bond, known also as a polar bond, is a covalent bond between atoms in which the electrons are shared unequally.

The more electronegative atom attracts electrons more strongly and gains a partial negative charge (δ-). The less electronegative atom has a partial positive charge (δ+).

Bond Polarity

8.4

The chlorine atom attracts the electron cloud more than the hydrogen atom does.

Bond Polarity

◦ The carbon to hydrogen bonds in ethane are polar.

Bond Polarity

Bond Polarity

Pg 239

Which type of bond (nonpolar covalent, polar covalent or ionic) will form between each of the following pairs of atoms?

a. N and Hb. F and Fc. Ca and Cld. Al and Cl

Bond Polarity

Pg. 239

Identify the following bond types:

a. H and Brb. K and Clc. C and Od. Cl and Fe. Li and Of. Br and Br

Bond Polarity

◦ In a polar molecule, one end of the molecule is slightly negative and the other end is slightly positive. A molecule that has two poles is called a dipolar

molecule, or dipole.

Polar Molecules

◦ When polar molecules are placed between oppositely charged plates, they tend to become oriented with respect to the positive and negative plates. This is referred to as the “dipole moment”.

Polar Molecules

A hydrogen chloride molecule is a dipole.

Polar Molecules

Other polar molecules:◦ H2O

◦ NH3

◦ HF◦ CH3Cl

Polar Molecules

Molecules containing all nonpolar bonds, and molecules with polar bonds oriented symmetrically in space are nonpolar.

Nonpolar Molecules

When nonpolar molecules are placed between oppositely charged plates, they do not become oriented with respect to the positive and negative plates. Nonpolar molecules do not exhibit a dipole moment.

Nonpolar Molecules

Nonpolar molecules with nonpolar bonds:◦ H2

◦ N2

◦ Cl2◦ O2

◦ F2

◦ I2◦ Br2

Nonpolar Molecules

Nonpolar molecules with polar bonds:◦ CH4

◦ CO2

◦ C2H2

◦ C2H4

Nonpolar Molecules

8.4

Intermolecular forces of attraction (IMFs) are attractive forces that exist between molecules.◦ Intermolecular forces of attraction (IMFs) are

weaker than either ionic or covalent bonds. These attractions are responsible for

determining whether a molecular compound is a gas, a liquid, or a solid at a given temperature.

Attractions Between Molecules

◦ Dipole interactions occur when polar molecules are attracted to one another.

Attractions Between Molecules

8.4

◦ Dispersion forces, the weakest of all molecular interactions, are caused by the shifting motion of electrons which create temporary dipoles. The strength of dispersion forces generally increases

as the number of electrons in a molecule increases. Ex: the halogens

Attractions Between Molecules

◦ Hydrogen “bonds” are attractive forces in which a hydrogen atom covalently bonded to a very electronegative atom (N,O, or F) is also weakly bonded to another molecule’s unshared electron pair. Ex: H2O, NH3, HF

H-bonds play a significant role in the shape of DNA and protein molecules.

Attractions Between Molecules

8.4

Hydrogen Bonding in Water

Attractions Between Molecules

◦ Hydrogen bonding in water.

Attractions Between Molecules

The relatively strong attractive forces between water molecules cause the water to form small drops on a waxy surface. Without H-bonds, water would be a gas at room temperature.

Attractions Between Molecules

8.4

◦ Network solids (network crystals) are solids in which all of the atoms are covalently bonded to each other (no molecules). Melting a network solid would require breaking

covalent bonds throughout the solid. Network solids do not melt until the temperature

reaches 1000°C or higher, or they decompose without melting at all.

Intermolecular Attractions and Molecular Properties

8.4

Diamond is an example of a network solid. Diamond does not melt. It vaporizes to a gas at 3500°C or above.

Intermolecular Attractions and Molecular Properties

8.4

Silicon Carbide is a network solid. It has a melting point of about 2700°C.

Intermolecular Attractions and Molecular Properties

Intermolecular Attractions and Molecular Properties