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 P 4 S 3 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. P 4 S 3 → tetraphosphorus trisulfide Naming Covalent Compounds

Formulas and Names of Some Covalent Compounds

Write the name of each covalent compound: CO_____________________ CO 2 _____________________ PCl 3 _____________________ CCl 4 _____________________ N 2 O_____________________ Learning Check

Identify each compound as ionic or covalent and give its correct name. A. SO 3 B. MnCl 2 C. (NH 4 ) 3 PO 4 D. Cu 2 CO 3 E. N 2 O 4

Forming a H 2 Molecule Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

H 2, A Covalent Molecule In a hydrogen H 2 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 octet Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

Number of Covalent Bonds The number of covalent bonds can be determined from the 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” electrons STEP 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 H 2 O, CO 2 and (ClO 3 ) -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 Difference N-N = 0.0 Nonpolar covalent Cl-Br = 0.2 Nonpolar covalent H-Si = 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 ElectronegativityType of Bond Difference O-Cl = 0.5Polar covalent Cl-C = 0.5Polar covalent O-S = 1.0Polar 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 ElectronegativityType of Bond Difference Cl-K 3.0 – 0.8 = 2.2Ionic N-Na 3.0 – 0.9 = 2.1Ionic S-Cs2.5 – 0.7= 1.8Ionic

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-N B. N-O C. Cl-Cl D. 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: H 2 O.. H─O : H 2 O is polar │ H dipoles do not cancel

Learning Check Identify each of the following molecules as: 1) polar or 2) nonpolar. Explain. A. PBr 3 B. HBr C. Br 2 D. SiBr 4