Chapter 6 Molecules and Covalent Compounds

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Covalent bonds form:

• When atoms share electrons to complete octets

• Between two nonmetal atoms

• Between nonmetal atoms from Groups 4A(14), 5A(15), 6A(16), and 7A(17)

Covalent Bonds

Naming Covalent Compounds

To name covalent compounds:

• STEP 1: Name the first nonmetal as the element.

• STEP 2: Name the second nonmetal with an ide ending.

• STEP 3: Indicate the number of atoms (subscript) of each element with prefixes.

Name P4S3

1. The first nonmetal P is phosphorus.

2. The second nonmetal S is sulfide.

3. The subscript 4 of P is shown as tetra.

The subscript 3 of S is shown as tri.

P4S3 → tetraphosphorus trisulfide

Naming Covalent Compounds

Formulas and Names of Some Covalent Compounds

Write the name of each covalent compound:

CO _____________________

CO2 _____________________

PCl3 _____________________

CCl4 _____________________

N2O _____________________

Learning Check

Learning Check

Identify each compound as ionic or covalent and give its

correct name.

A. SO3

B. MnCl2

C. (NH4)3PO4

D. Cu2CO3

E. N2O4

Forming a H2 Molecule

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

H2, A Covalent Molecule

In a hydrogen H2 molecule:

• Two hydrogen atoms share electrons to form a covalent single bond.

• Each H atom acquires two (2) electrons.

• Each H becomes stable like helium (He).

Diatomic Elements

• These elements share electrons to form diatomic, covalent molecules.

Electron-Dot Formulas

Electron-dot formulas show:

• The order of bonded atoms in a covalent compound

• The bonding pairs of electrons between atoms

• The unshared (lone) valence electrons

• A central atom with an octetCopyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Number of Covalent Bonds

The number of covalent bonds can be determined fromthe number of electrons needed to complete an octet.

HONC

Electron-Dot Formulas and Models of Some Covalent Compounds

Guide to Writing Electron-Dot Formulas

STEP 1 Determine the arrangement of atoms.STEP 2 *** Do NVS method *** STEP 3 Attach the central atom to each bonded atom using the “shared” electronsSTEP 4 Add remaining electrons as lone pairs to complete octets (duet for H atoms).STEP 5 Make sure there are the correct number of

valence electrons represented in the formula

Write an electron-dot formula for H2O, CO2 and (ClO3)-1

Some Electron-Dot Formulas

Resonance structures are:

• Two or more electron-dot formulas for the same arrangement of atoms

• Related by a double-headed arrow

• Written by changing location of a double bond from the central atom to a different attached atom

• Sometimes written as a hybrid resonance structure

Resonance Structures

Carbonate has three resonance structures. If the following is one, what are the other two?

2-

:O:

C

:O: :O:

Learning Check

VSEPR

In the valence-shell electron-pair repulsion theory

(VSEPR), the electron groups around a central atom:

• Are arranged as far apart from each other as possible

• Have the least amount of repulsion of the negatively charged electrons

• Have a geometry around the central atom that determines molecular shape

Electronegativity values:• Indicate the attraction of an atom for shared

electrons • Increase from left to right going across a period on

the periodic table• Is high for the nonmetals with fluorine as the

highest • Is low for the metals

Electronegativity

Some Electronegativity Values for Group A Elements

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Low values

High values

A nonpolar covalent bond, • Occurs between nonmetals• Is an equal or almost equal sharing of electrons• Has almost no electronegativity difference (0.0 to 0.4)

Examples: Atoms Electronegativity Type of Bond

DifferenceN-N 3.0 - 3.0 = 0.0 Nonpolar covalent

Cl-Br 3.0 - 2.8 = 0.2 Nonpolar covalentH-Si 2.1 - 1.8 = 0.3 Nonpolar covalent

Nonpolar Covalent Bonds

A polar covalent bond, • Occurs between nonmetals atoms• Is an unequal sharing of electrons• Has a moderate electronegativity difference (0.5 to 1.7)

Examples: Atoms Electronegativity Type of Bond

DifferenceO-Cl 3.5 - 3.0 = 0.5 Polar covalentCl-C 3.0 - 2.5 = 0.5 Polar covalentO-S 3.5 - 2.5 = 1.0 Polar covalent

Polar Covalent Bonds

Comparing Nonpolar and Polar Covalent Bonds

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Ionic Bonds

An ionic bond, • Occurs between metal and nonmetals ions• Is a results of electron transfer• Has a large electronegativity difference (1.8 or more)

Examples: Atoms Electronegativity Type of Bond

Difference Cl-K 3.0 – 0.8 = 2.2 IonicN-Na 3.0 – 0.9 = 2.1 IonicS-Cs 2.5 – 0.7 = 1.8 Ionic

Range of Bond Types

Use the electronegativity difference to identify the type of bond between the following as:nonpolar covalent (NP), polar covalent (P), or

ionic (I).

A. K-NB. N-OC. Cl-ClD. H-Cl

Learning Check

Polar Molecules

A polar molecule, • Contains polar bonds

• Has a separation of positive and negative charge called a dipole indicated with + and

• Has dipoles that do not cancel

+ - • •

H–Cl Cl—N—Cl dipole

Cl dipoles do not

cancel

Nonpolar Molecules

A nonpolar molecule, • Contains nonpolar bonds

Cl–Cl H–H

• Or has a symmetrical arrangement of polar bonds

O=C=O Cl

Cl–C–Cl

Cl

dipoles cancel

Determining Molecular Polarity

STEP 1 Write the electron-dot formula.

STEP 2 Determine the polarity of the bonds.

STEP 3 Determine if any dipoles cancel or not.

Example: H2O

. .

H─O: H2O is polar

│

H dipoles do not cancel

Learning Check

Identify each of the following molecules as:

1) polar or 2) nonpolar. Explain.

A. PBr3

B. HBr

C. Br2

D. SiBr4