Covalent Bonding Chapter 8

Introduction The bonds that hold atoms of molecules together are fundamentally different from those of ionic compounds. The types of atoms that comprise molecules are more limited than ions. The properties of molecules are different from ionic compounds. These properties are a result of the interactions between molecules.

Molecular Compounds Section 8.1 Molecules and Molecular Compounds Molecular Formula

Different Compounds Have Different Bonding Interactions Noble gas: No interactions Ionic compound: Ionic Bonds Metal: Metallic Bonds How about the following compounds?

What type of interactions hold these compounds together?

I.) Molecules and Molecular Compounds Molecules: A neutral group of atoms joined together by covalent bonds. Molecular Compound: A compound composed of molecules.

Water is a molecular compound Made up of many molecules of water.

Covalent Bonds The sharing of electrons between two atoms. No atoms give up their electrons completely, nor does any atoms take electrons. You can think of the bond as a “tug of war” between the atoms.

The Covalent Bond We can use Lewis dot structures to show the sharing of electrons.

Molecules Are Created from Covalently Bonded Atoms

Properties of Molecular Compounds All the molecules of a given molecular compound are the same. Low melting points and boiling points Gases or liquids at room temperature Composed of atoms of two or more non-metals.

II.) Molecular Formula The chemical formula of a molecular compound. A molecular formula shows how many atoms of each element a molecule contains. Subscripts after each symbol indicates the number of atoms of the element are in the molecule. The subscripts are not necessarily the lowest whole-number ratios. Does not give any information about the molecular compounds structure.

Let’s look at some formulas for molecular compounds. NO NH3 CH4 H20 N2H2 How many atoms of each element are present in each molecular compound?

The Nature of Covalent Bonding Section 8.2 The Octet Rule in Covalent Bonding Single Covalent Bonds Double & Triple Covalent Bonds Bond Dissociation Energies

I.) The Octet Rule in Covalent Bonding Atoms will strive to obtain the electron configuration of noble gases, if they can. ns2np6 In covalent bonding, electron sharing usually occurs so that atoms attain the electron configuration of noble gases. There are exceptions, however. Metals & metalloids of Group 4A, 5A, 6A, and 7A will form covalent bonds to meet their octet.

Three Types of Covalent Bonds Single bonds, double bonds, and triple bonds. These different types of bonds are based on the number of electrons shared between to atoms. Each type of bond has different characteristics Bond length, bond strength

II.) The Single Covalent Bond A covalent bond formed when two atoms share one pair of electrons. A single bond can be represented as two dots between two atoms or a single dashed line between them. We can use a structural formula to describe the bonding between atoms.

Let’s see how hydrogen forms it’s bond. Let’s see how water forms its bonds. Let’s see how methane forms its bonds.

Sample Problem Write the structural formulas for the following molecules. F2 H2O2 Cl2

Class Problem Write the structural formulas for the following molecules. I2 PCl3 (Class Problem #5)

What happened with the electrons that did not participate in bonding? These unshared electrons are known as lone pairs and are represented as two dots around the molecular compounds. Let’s examine this with water.

III.) Double Bond and Triple Bonds Double Bond: A covalent bond formed when two atoms share two pairs of electrons. Electrons are shared so that both atoms of the double bond can obtain a noble gas configuration.

Sample Problem Try drawing the structural formulas for the following double bonded compounds. Formaldehyde (CH2O) Ethylene (C2H4)

Triple Bond: A covalent bond formed when two atoms share three pairs of electrons. Again, the sharing occurs so that both atoms can obtain an octet in their outer-most (valence) energy level.

Sample Problem Try drawing the structural formulas for the following triple bonded compounds. Hydrogen cyanide (CHN) Ethyne (C2H2)

Class Problem Write the structural formulas for the following molecules. SO2 NO3 HCN (Class Problem #6)

Coordinate Covalent Bonds A type of covalent bond in which one atom contributes both bonding electrons. The shared electron pair comes from one of the bonding atoms. Can show the donated pair in structural formula as arrows that point from the atom donating the pair to the atom receiving the pair. Once formed the this type of covalent bond is like any other.

Sample Problem Draw the structural formula for the following compounds. Carbon monoxide (CO) Ammonium ion (NH4+)

Polyatomic Ions A tightly bound group of atoms that has a positive or negative charge and behaves as a unit. The bonds holding polyatomic ions together are covalent and coordinate covalent bonds. Electrons are gained or lost to give the polyatomic ions their charges.

Class Problem Write the structural formulas for the following ion: SO32- (Class Problem #7)

Examining Differences in the Bonds

IV.) Bond Dissociation Energy The energy required to break the bond between two covalently bonded atom.

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

Resonance A resonance structure occurs when it is possible to draw or more valid electron dot structures that have the same number of electron pairs for a molecule or ion. The actual bonding in such a compound is a hybrid, or a mixture, of the extremes represented by the resonance structures.

Let’s draw the structure for ozone (O3)

Exceptions to the Octet Rule For some molecules and ions it is not possible to draw the structures to satisfy the octet rule. The octet rule cannot be satisfied for molecules and ions whose total number of valence electrons is an odd number. These molecules will have either more than or less than the required octet; many will have unpaired electrons.

Exceptions (cont.) Some molecules with an even number of valence electrons will also violate the octet rule. Some atoms expand the octet to include 10 or 12 electrons in their valence shell.

Let’s look at some exceptions. Nitrogen dioxide (NO2) Boron trifluoride (BF3) Phosphorus pentachloride (PCl5)

Polar Bonds and Molecules Section 8.4 Bond Polarity Polar Molecules Attractions Between Molecules

I.) Bond Polarity Covalent bonds differ in the way the electrons are shared. The character of the bonds depends on the kind and number of atoms joined together. The type of covalent bonding helps to determine the properties of a molecule. Covalent bonds can be polar or nonpolar.

Comparing Bond Types (Nonpolar)

Nonpolar Covalent Bonds Atoms of equal “strength” pull on the electrons. Identical atoms share electrons equally. H2, O2, N2 are examples.

Polar Covalent Bonds A covalent bond in which the electrons are not shared equally. The “stronger” atom will pull more of the electrons to itself.

Atom’s Strength = Electronegativity The ability of an atom of an element to attract electrons when the atom is in a compound. (Chapter 6)

Periodic Trend in Electronegativity

Electronegativity Differences & Bond Type Electronegativity Difference Range Most Probable Type of Bond Example 0.0 – 0.3 Nonpolar covalent H-H (0.0) 0.4 – 0.9 Moderately Polar Covalent H-Cl (0.9) 1.0 – 1.9 Very Polar Covalent H-F (1.9) ≥ 2.0 Ionic Na+Cl- (2.1)

Use the table to determine the type of bond each pair of atom will form. H, N K, Cl C, Cl Cl,Cl

Sample Problem Draw the electron dot structure for each molecule. Identify polar covalent bonds by assigning slightly positive and slightly negative symbols to the appropriate atoms. HOOH BrCl HBr H2O

Class Problem Identify the bonds between atoms of each pair of elements as nonpolar covalent, moderately polar covalent, very polar, or ionic. H and Br K and Cl C and O Br and Br (Class Problem #8) p.239 #30

II.) Polar Molecules One end is slightly negative, the other slightly positive. Dipole: A molecule that has two poles.

III.) Attraction Between Molecules These are the intermolecular forces. Intermolecular forces are weaker than ionic or covalent bonds. These interactions determines the physical state of molecular compounds at room temperature. Three types: Dipole interactions Dispersion forces Hydrogen bonds

Comparing Intermolecular and Intramolecular Forces

Dipole Interactions These forces occur between polar molecules. The negative ends are attracted to the positive ends. These forces are similar to but much weaker than ionic interactions

Dispersion Forces Weakest of all interactions Occurs between nonpolar molecules. Induced charges hold these molecules together. Strength of this interaction increases with increasing # of electrons.

Hydrogen Bonds These are interactions in which a hydrogen covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom.

H-bonds are about 5% the strength of a covalent bond. These bonds are extremely important in determining the properties of water and many biological molecules.

Covalent Bonding Chapter 8 The End