Resonance Structures Resonance structures are  two or more electron-dot formulas for the same arrangement of atoms.  related by a double-headed arrow ( ).  written by changing the location of a double bond between the central atom and a different attached atom.

Writing Resonance Structures for SO2

Learning Check FNO2, a rocket propellant, has two resonance structures. One is shown below. What is the other resonance structure?

Solution FNO2, a rocket propellant, has two resonance structures. One is shown below. What is the other resonance structure?

Names of Covalent Compounds When naming a covalent compound,  the first nonmetal in the formula is named by its element name, and  the second nonmetal is named using the first syllable of its element name, followed by ide. Prefixes are used  to indicate the number of atoms present for each element in the compound, and  because two nonmetals can form two or more different compounds

Prefixes Used in Naming Covalent Compounds

Some Covalent Compounds

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Guide to Naming Covalent Compounds with Two Nonmetals

Naming Covalent Compounds, SO2 Analyze the Problem. Symbols of Elements Name Subscripts

S

O

sulfur

oxygen

1

2

Prefix

(none) understood

di

Step 1 Name the first nonmetal by its element name. In SO2, the first nonmetal (S) is sulfur.

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Naming Covalent Compounds, SO2 Step 2 Name the second nonmetal by using the first syllable of its name followed by ide. The second nonmetal (O) is named oxide. Step 3 Add prefixes to indicate the number of atoms (subscripts). Because there is one sulfur atom, no prefix is needed. The subscript two for the oxygen atoms is written as the prefix di . The name of SO2 is sulfur dioxide.

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Learning Check Name the covalent compound P2O5.

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Solution Name the covalent compound P2O5. Analyze the Problem. Symbols of Elements

P

O

Name Subscripts Prefix

phosphorus

oxygen

2

5

di

penta

Step 1 Name the first nonmetal by its element name. In P2O5, the first nonmetal (P) is phosphorus.

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Solution Step 2 Name the second nonmetal by using the first syllable of its name followed by ide. The second nonmetal (O) is named oxide. Step 3 Add prefixes to indicate the number of atoms (subscripts). The subscript for the two P atoms is di; the subscript for the five oxygen atoms is penta. The name of P2O5 is phosphorus pentoxide. When the vowels o and o, or a and o appear together (pentaoxide), the first vowel is omitted.

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Learning Check Select the correct name for each compound. 1. SiCl4 A. silicon chloride B. tetrasilicon chloride C. silicon tetrachloride

#2 Name Cl2O7 A. dichlorine heptoxide B. dichlorine oxide C. chlorine heptoxide

Writing Formulas of Covalent Compounds Write the formula for sulfur hexafluoride. Analyze the Problem. Name

sulfur

hexafluoride

Symbols of Elements Subscripts

S

F

1

6 (from hexa)

Step 1 Write the symbols in order of the elements in the name. SF Step 2 Write any prefixes as subscripts. Prefix hexa = 6 Formula: SF6

Learning Check Write the correct formula for dinitrogen trioxide.

Learning Check Name each of the following compounds. 1. Ca3(PO4)2 2. FeBr3

3. SCl2 4. Cl2O

Solution Name each of the following compounds. 1. Ca3(PO4)2

Ca2+, PO43−

calcium phosphate

2. FeBr3

Fe3+, Br −

iron(III) bromide

3. SCl2

1 S, 2 Cl

sulfur dichloride

4. Cl2O

2 Cl, 1 O

dichlorine oxide

Learning Check Write the formulas for the following compounds. 1. calcium nitrate 2. boron trifluoride

3. aluminum carbonate 4. dinitrogen tetroxide 5. copper(I) phosphate

Solution Write the formulas for the following compounds. 1. calcium nitrate

Ca2+, NO3−

Ca(NO3)2

2. boron trifluoride

1 B, 3 F

BF3

3. aluminum carbonate

Al3+, CO32−

Al2(CO3) 3

4. dinitrogen tetroxide

2 N, 4 O

N2O4

5. copper(I) phosphate

Cu+, PO43−

Cu3PO4

Electronegativity Electronegativity  is a measure of an atom’s ability to attract bonding electrons.  increases from left to right, going across a period on the periodic table.  decreases going down a group on the periodic table.  is high for the nonmetals, with fluorine as the highest.  is low for the metals and transition metals.

Bonding and Electronegativity The difference in the electronegativity of two atoms can be used to predict the type of bond that forms. Bonds are classified as:  nonpolar covalent  polar covalent  ionic

Some Electronegativity Values

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Nonpolar Covalent Bonds A nonpolar covalent bond  occurs between nonmetals.  has an equal or almost equal sharing of electrons.  has almost no electronegativity difference (0.0 to 0.4). Examples: Atoms Electronegativity Difference

Type of Bond

Polar Covalent Bonds A polar covalent bond  occurs between nonmetal atoms that do not share electrons equally.  has a moderate electronegativity difference (0.5 to 1.7). Examples: Atoms Electronegativity Type of Bond Difference

Ionic Bonds An ionic bond  occurs between metal and nonmetal ions.  is a result of electron transfer.  has a large electronegativity difference (1.8 or more). Examples: Atoms Electronegativity Type of Bond Difference

Electronegativity and Bond Types

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Predicting Bond Types

Learning Check Use the electronegativity difference to identify the type of bond between the following atoms as nonpolar covalent (NP), polar covalent (P), or ionic (I). A. K–N 2.2 B. N–O 0.5 C. Cl–Cl 0.0 D. B–Cl 1.0

Break Time!!!

Valence-Shell Electron-Pair Repulsion Theory (VSEPR) In the valence-shell electron-pair repulsion (VSEPR) theory, the electron groups around a central atom  are arranged as far apart from each other as possible.  have the least amount of electron-electron repulsion.  are used to predict the molecular shape.

Shapes of Molecules The three-dimensional shape of a molecule can be predicted by  the number of bonding groups and lone-pair electrons around the central atom  VSEPR theory (valence-shell-electron-pair repulsion).

Two Electron Groups In a molecule of BeCl2,  there are two electron groups bonded to the central atom, Be (Be is an exception to the octet rule).

 to minimize repulsion, the arrangement of two electron groups is 180, or opposite each other.  the shape of the molecule is linear.

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Three Electron Groups In a molecule of BF3,  three electron groups are bonded to the central atom B (B is an exception to the octet rule).

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Three Electron Groups with a Lone Pair In a molecule of SO2,  S has 3 electron groups; 2 electron groups bonded to O atoms and one lone pair.

 repulsion is minimized with the electron groups at angles of 120, a trigonal planar arrangement.  the shape is determined by the two O atoms bonded to S, giving SO2 a bent (~120) shape. © 2013 Pearson Education, Inc.

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Four Electron Groups In a molecule of CH4,  there are four electron groups around C.  repulsion is minimized by placing four electron groups at angles of 109, which is a tetrahedral arrangement.  the four bonded atoms form a tetrahedral shape.

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Four Electron Groups with a Lone Pair In a molecule of NH3,  three electron groups bond to H atoms, and the fourth one is a lone (nonbonding) pair.  repulsion is minimized with 4 electron groups in a tetrahedral arrangement.  the three bonded atoms form a pyramidal (~109) shape.

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Four Electron Groups with Two Lone Pairs In a molecule of H2O,  two electron groups are bonded to H atoms and two are lone pairs (4 electron groups).  four electron groups minimize repulsion in a tetrahedral arrangement.  the shape with two bonded atoms is bent (~109).

Molecular Shapes for 2 and 3 Bonded Atoms

Molecular Shapes for 4 Bonded Atoms

Guide to Predicting Molecular Shape (VSEPR Theory)

Predicting the Molecular Shape of H2Se Step 1 Draw the electron-dot formula. In the electrondot formula for H2Se, there are four electron groups, including two lone pairs of electrons around Se.

Predicting the Molecular Shape of H2Se Step 2 Arrange the electron groups around the central atom to minimize repulsion. The four electron groups around Se would have a tetrahedral arrangement. Step 3 Use the atoms bonded to the central atom to determine the molecular shape. Two bonded atoms give H2Se a bent shape with a bond angle of ~109.

Learning Check What is the shape of a molecule of NF3?

Solution What is the shape of a molecule of NF3? B. trigonal pyramidal Step 1 Draw the electron-dot formula. In the electron-dot structure for NF3, there are three electron groups and a lone pair.

F

N F

F

Solution What is the shape of a molecule of NF3? B. trigonal pyramidal Step 2 Arrange the electron groups around the central atom to minimize repulsion. The four electron groups around N would have a tetrahedral arrangement. Step 3 Use the atoms bonded to the central atom to determine the molecular shape. Three bonding groups and a lone pair give NF3 a trigonal pyramidal shape with a bond angle of 109.

Next Week • Clicker Quiz on Chapter 5 • Chapter 6 • Group Assignment Ch5/6