Chapter 6

Ionic and Molecular Compounds

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Readiness

Key Math Skills:

• Using Positive and Negative Numbers in Calculations (1.4B)
• Solving Equations (1.4D)

Core Chemistry Skills:

• Writing Electron Arrangements (4.6)
• Drawing Electron-Dot Symbols (4.7)

Chemical Compounds

• Divided into two broad classes: ionic compounds and molecular compounds.
• Ionic compounds:
  • High melting points (500^{\circ}C - 2000^{\circ}C)
  • Good conductors of electricity in the molten (liquid) state or in solution.
• Molecular compounds:
  • Lower melting points
  • Many are gases or liquids at room temperature.
• Some element combinations produce ionic compounds, while others form molecular compounds.

6.1 Ions: Transfer of Electrons

• Atoms form positive ions by losing electrons and negative ions by gaining electrons.
• Ionic bonds result from strong attractive forces between positive and negative ions.
• Learning Goal: Write symbols for simple ions of representative elements.

Ionic and Covalent Bonds

• Chemical bonds form when atoms lose, gain, or share valence electrons to achieve an octet (8 valence electrons).
• Ionic bonds: Atoms of one element lose valence electrons, and atoms of another element gain them.
• Covalent bonds: Nonmetal atoms share electrons to attain a noble gas arrangement.

Positive Ions: Metals Lose Electrons

Metals—Group 1A (1), Group 2A (2), and Group 3A (13)—

• have low ionization energies.
• Readily lose one or more valence electrons to form positive ions.
• Lose electrons until they have the same number of valence electrons as the nearest noble gas (usually eight).

Positive Ions: Loss of Electrons

Sodium atoms (Group 1A/1) are neutral with 11 electrons and 11 protons.

• They lose one electron to achieve the same number of valence electrons as neon, attaining a filled energy level.
• They form an ion with 10 electrons and 11 protons, resulting in an ionic charge of 1+ (Na^+

Positive Ions: Loss of Electrons

Magnesium atoms (Group 2A/2) are neutral with 12 electrons and 12 protons.

• They lose two electrons to achieve the same number of valence electrons as neon, attaining a filled energy level.
• They form an ion with 10 electrons and 12 protons, resulting in an ionic charge of 2+ (Mg^{2+}
• Core Chemistry Skill: Writing Positive and Negative Ions

Negative Ions: Nonmetals Gain Electrons

Nonmetals—Group 5A (15), Group 6A (16), and Group 7A (17)—

• have high ionization energies.
• Readily gain one or more valence electrons to form negative ions in ionic compounds.
• Gain electrons until they have the same number of valence electrons as the nearest noble gas (usually eight).

Negative Ions: Gain of Electrons

Chlorine atoms (Group 7A/17) are neutral with 17 electrons and 17 protons.

• They gain one electron to achieve the same number of valence electrons as argon, attaining a filled energy level.
• They form an ion with 18 electrons and 17 protons, resulting in an ionic charge of 1− (Cl^−).
• Core Chemistry Skill: Writing Positive and Negative Ions

Ionic Charges from Group Numbers

• The periodic table group numbers help determine the charges for representative element ions.

Chemistry Link to Health

• Ions are vital for regulating body functions.

Study Check

Write the formula and symbol of an ion with 16 protons and 18 electrons.

Solution

The element with 16 protons is sulfur (S). An ion of sulfur with 18 electrons has a charge of 2−. The sulfide ion is S^{2-}.

6.2 Writing Formulas for Ionic Compounds

• Ionic compounds contain positive and negative ions held together by strong electrical attractions between oppositely charged ions.
• Learning Goal: Use charge balance to write correct formulas for ionic compounds.

Properties of Ionic Compounds

Ionic compounds:

• Consist of positive and negative ions
• Have ionic bonds, which are attractions between positively and negatively charged ions
• Have high melting points
• Are solid at room temperature

NaCl, an Ionic Compound

Sodium chloride (NaCl), commonly known as table salt shows the arrangement of Na^+ and Cl^− ions in NaCl crystals.

Formulas of Ionic Compounds

The chemical formula:

• Represents symbols and subscripts in the lowest whole-number ratio of atoms or ions
• Ensures that the sum of ion charges equals zero, achieving charge balance (total positive charge = total negative charge)

Subscripts in Formulas

• Core Chemistry Skill: Writing Ionic Formulas

Writing Ionic Formulas from Ion Charges

To balance ionic charge in an ionic compound:

• Total positive charge = total negative charge.

Study Check

Write the ionic formula of the compound formed with Ba^{2+} and Cl^− ions.

Solution

1. Write the symbols of the ions: Ba^{2+} and Cl^−
2. Balance the charges: 1(2+) + 2(1−) = 0
3. Write the cation first and the anion second, resulting in the formula BaCl_2.

Study Check

Select the correct formula for each ionic compound:

• A. Na^+ and O^{2−}: 1) NaO 2) Na2O 3) NaO2
• B. Al^{3+} and Cl^−: 1) AlCl3 2) AlCl 3) Al3Cl
• C. Mg^{2+} and N^{3−}: 1) MgN 2) Mg2N3 3) Mg3N2

Solution

• A. Na^+ and O^{2−}: 2) Na_2O (Check: 2Na^+ + O^{2−} = 2(1+) + 1(2−) = 0)
• B. Al^{3+} and Cl^−: 1) AlCl_3 (Check: Al^{3+} + 3Cl^− = (3+) + 3(1−) = 0)
• C. Mg^{2+} and N^{3−}: 3) Mg3N2 (Check: 3Mg^{2+} + 2N^{3−} = 2(3+) + 2(3−) = 0)

6.3 Naming Ionic Compounds

Iodized salt contains KI (potassium iodide) to prevent iodine deficiency.

• Learning Goal: Given the formula of an ionic compound, write the correct name; given the name, write the correct formula.

Naming Ionic Compounds

When naming an ionic compound:

• The name of the metal is written first and is the same as the element name.
• The name of the nonmetal consists of the first syllable of its name + "ide" ending and is written second.
• A space separates the names of the metal and nonmetal ion.
• Core Chemistry Skill: Naming Ionic Compounds

Guide to Naming Ionic Compounds

Naming Ionic Compounds: K2O

Name the ionic compound K_2O.

1. Identify the cation and anion:
   The cation, K^+, is from Group 1A(1), and the anion, O^{2−}, is from Group 6A(16).
2. Name the cation by its element name:
   The cation, K^+, is potassium.
3. Name the anion by using the first syllable of its element name followed by "ide". The anion is oxide.
4. Write the cation name first, followed by the anion name: K_2O is potassium oxide.

Study Check

Write formulas and names for compounds of the following ions: Br^−, S^{2−}, N^{3−} with Na^+, Al^{3+}.

Solution

Metals with Variable Charge

Transition metals (excluding Zn^{2+}, Cd^{2+}, Ag^+) form two or more positive ions (cations).

• A Roman numeral indicates the ion charge in parentheses after the metal name.
  • Cu^{2+}: copper(II)
  • Cu^+: copper(I)
  • Fe^{2+}: iron(II)
  • Fe^{3+}: iron(III)
  • Pb^{2+}: lead(II)
  • Pb^{4+}: lead(IV)
  • Cr^{3+}: chromium(III)
  • Cr^{6+}: chromium(VI)

Determination of Variable Charge

• Use the charge on the anion and charge balance to calculate the charge on the metal ion.
• Example: MnF_2
  • Mn charge + 2 F− charge = 0
  • ? + 2(1−) = 0
  • 2+ + 2− = 0
  • Manganese(II) fluoride

Ion Charges: Periodic Table

Guide to Naming Ionic Compounds with Variable Charge Metals

Naming Ionic Compounds with Variable Charge Metals: FeCl2

Name the ionic compound FeCl_2.

1. Determine the charge of the cation from the anion:
   Analyze the Problem:

2. Name the cation by its element name and use a Roman numeral in parentheses for the charge: iron(II)
3. Name the anion by using the first syllable of its element name followed by ide: chloride
4. Write the cation name first and the anion name second: iron(II) chloride

Study Check

Name the following ionic compound containing a variable charge metal: MnO_2

Solution

1. Determine the charge of the cation from the anion:
   Analyze the Problem:

2. Name the cation by its element name and use a Roman numeral in parentheses for the charge: manganese(IV)
3. Name the anion by using the first syllable of its element name followed by ide: oxide
4. Write the cation name first and the anion name second: manganese(IV) oxide

Writing Formulas from the Name of an Ionic Compound

The formula for an ionic compound is written from:

• The first part of the name, which describes the metal ion.
• The second part of the name, which specifies the nonmetal ion.
• Subscripts are added to balance the charge.

Guide to Writing Formulas from the Name of an Ionic Compound

Writing Formulas from the Name of an Ionic Compound: Iron(III) Chloride

Write the formula for iron(III) chloride.

1. Identify the cation and anion:
   Analyze the Problem:

2. Balance the charges:
   Fe^{3+} Cl^− Cl^− Cl^− 1(3+) + 3(1−) = 0

3. Write the formula, cation first, using subscripts from the charge balance: FeCl_3

Study Check

Write chemical formulas for:

• A. nickel(II) sulfide
• B. zinc chloride
• C. iron(III) oxide

Solution

1. Identify the cation and anion:
   • A. nickel(II) sulfide: The Roman numeral (II) indicates the charge on the nickel ion is 2+, Ni^{2+}.
   • B. zinc chloride: Zinc is one of the transition metals with a fixed charge of 2+, Zn^{2+}.
   • C. iron(III) oxide: The Roman numeral (III) indicates the charge on the iron ion is 3+, Fe^{3+}.
2. Balance the charges:
   • A. nickel(II) sulfide: Ni^{2+} S^{2−} 1(2+) + 1(2−) = 0
   • B. zinc chloride: Zn^{2+} Cl^− Cl^− 1(2+) + 2(1−) = 0
   • C. iron(III) oxide: Fe^{3+} O^{2−} Fe^{3+} O^{2−} O^{2−} 2(3+) + 3(2−) = 0
3. Write the formula, cation first, using subscripts from the charge balance:
   • A. nickel(II) sulfide: NiS
   • B. zinc chloride: ZnCl_2
   • C. iron(III) oxide: Fe2O3

6.4 Polyatomic Ions

Sodium chlorite, NaClO_2, is used in processing and bleaching pulp from wood fibers and recycled cardboard.

• Learning Goal: Write the name and formula for an ionic compound containing a polyatomic ion.

Polyatomic Ions

Polyatomic ions:

• Are a group of atoms with an overall charge.
• Often consist of a nonmetal (phosphorus, sulfur, carbon, or nitrogen) and oxygen.
• Usually have a 1−, 2−, or 3− charge.
• Have a negative charge, except for NH_4^+, ammonium, which has a positive charge.

Writing Formulas for Compounds Containing Polyatomic Ions

When writing formulas for ionic compounds containing polyatomic ions, use the same rules for charge balance as simple ionic compounds.

• Magnesium nitrate example:
  • Magnesium ion: Mg^{2+}
  • Nitrate ion: NO_3^−
  • Mg^{2+} + NO3^− + NO3^− (2+) + 2(1−) = 0 Mg^{2+} + 2 NO3^− = Mg(NO3)_2
  • Parentheses are placed around the polyatomic ion, and the subscript is placed outside.

Guide to Writing Formulas with Polyatomic Ions

Writing Formulas with Polyatomic Ions: Aluminum Bicarbonate

Write the formula for aluminum bicarbonate.

1. Identify the cation and polyatomic ion (anion):
   • Cation: Al^{3+}
   • Polyatomic ion: HCO_3^−
2. Balance the charges:
   • Al^{3+} , HCO3^−, HCO3^−, HCO_3^− 1(3+) + 3(1−) = 0
3. Write the formula, cation first, using the subscripts from charge balance: Al(HCO3)3

Naming Compounds with Polyatomic Ions

When naming ionic compounds containing polyatomic ions:

• Write the positive ion (usually a metal) first.
• Write the name of the polyatomic ion second.
• Recognizing polyatomic ions in a chemical formula helps to name the compound correctly.
• As with other ionic compounds, no prefixes are used.

Guide to Naming Compounds with Polyatomic Ions

Flow Chart: Naming Ionic Compounds

Study Check

Name the following ionic compounds:

• A. Ca(NO3)2
• B. FePO_4

Solution

Study Check

Name the following ionic compounds:

• A. CaSO_4
• B. MgSO_4
• C. Pb3(PO4)_2

Solution

6.5 Molecular Compounds: Sharing Electrons

The number of covalent bonds a nonmetal atom forms usually equals the number of electrons it needs to achieve a stable electron arrangement.

• Learning Goal: Given the formula of a molecular compound, write its correct name; given the name, write its formula.

Covalent Bonds

Molecular compounds form when:

• Atoms of two or more nonmetals share electrons and form a covalent bond.
• Valence electrons are shared by nonmetal atoms to achieve stability.
• A molecule forms when two or more atoms share electrons.

Formation of a Hydrogen Molecule

A hydrogen molecule (H_2):

• Forms as atoms move closer, and the positive charge of the nucleus attracts the electron of the other atom.
• Has a shared pair of electrons that form a covalent bond, giving each atom a noble gas arrangement of He to each H atom.
• Is more stable than individual H atoms.

Electron-Dot Formulas: Molecular Compounds

To draw the electron-dot formula for a fluorine molecule (F_2):

• Start with the electron-dot symbols for each fluorine atom.
• Each fluorine atom shares one electron to form a covalent bond, giving each fluorine an octet.

Diatomic Molecules

Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine exist as diatomic molecules.

• Seven diatomic molecules: H2, N2, O2, F2, Cl2, Br2, and I_2.

Sharing Electrons between Atoms of Different Elements

The number of electrons a nonmetal atom shares and the number of covalent bonds it forms usually equals the number of electrons it needs to achieve a noble gas arrangement.

• Core Chemistry Skill: Drawing Electron-Dot Formulas

Drawing Electron-Dot Formulas

To draw the electron-dot formula for methane (CH_4):

• Start by drawing the electron-dot symbols for carbon and hydrogen.
• Carbon needs four hydrogen atoms to complete the noble gas configurations for carbon and hydrogen.

Exceptions to the Octet Rule

Octet rule exceptions:

• Hydrogen requires only two electrons for a noble gas arrangement.
• Nonmetals P, S, Cl, Br, and I can form compounds with 10 or 12 valence electrons.
• The S atom has an octet in many compounds, but in SF_6, there are 12 valence electrons (six bonds to the sulfur atom).

Double and Triple Covalent Bonds

A double bond:

• Occurs when atoms share two pairs of electrons.
• Forms when there are not enough electrons to complete octets.

A triple bond:

• Occurs when atoms share three pairs of electrons.
• Forms when there are not enough electrons to complete octets.

Guide to Drawing Electron-Dot Formulas

Electron-Dot Formulas with Multiple Bonds: CO2

Draw the electron-dot formula for carbon dioxide (CO_2) where C is the central atom.

1. Determine the arrangement of atoms: O C O
2. Determine the total number of valence electrons.
3. Attach each bonded atom to the central atom with a pair of electrons.
4. Place the remaining electrons using single or multiple bonds to complete octets.

Forming Triple Bonds: N2

In a nitrogen molecule (N_2):

• Each N atom shares three electrons to gain an octet.
• The multiple bond formed is a triple bond.
• The name is the same as that of the element.

Names and Formulas of Molecular Compounds

When naming covalent compounds:

• The first nonmetal in the formula is named by its element name.
• The second nonmetal is named using the first syllable of its name followed by "ide."
• If a subscript is used, a prefix is used in front of its name.
• When vowels "o" and "o" or "a" and "o" appear together, the first vowel is omitted.

Prefixes Used in Naming Molecular Compounds

• Core Chemistry Skill: Writing the Names and Formulas for Molecular Compounds

Guide to Naming Molecular Compounds

Naming Molecular Compounds: NCl3

Name the covalent compound NCl_3.

1. Name the first nonmetal by its element name:
   The first nonmetal (N) is nitrogen.

2. Name the second element by using the first syllable of its name followed by "ide."
   The second nonmetal (Cl) is chloride.
3. Add prefixes to indicate the number of atoms.
   Because there is one nitrogen atom, no prefix is needed. The subscript 3 for the Cl atoms is shown as the prefix "tri".

• NCl_3 is nitrogen trichloride.

Study Check

Write the name of each covalent compound:

• A. CO
• B. NO_2
• C. PF_3
• D. CCl_4

Solution

Write the name of each covalent compound.

1. Name the first nonmetal by its element name:
   • A. CO: The first nonmetal (C) is carbon.
   • B. NO_2: The first nonmetal (N) is nitrogen.
   • C. PF_3: The first nonmetal (P) is phosphorus.
   • D. CCl_4: The first nonmetal (C) is carbon.
2. Name the second element by using the first syllable of its name followed by "ide".
   • A. CO: The second nonmetal (O) is oxide.
   • B. NO_2: The second nonmetal (O) is oxide.
   • C. PF_3: The second nonmetal (F) is fluoride.
   • D. CCl_4: The second nonmetal (Cl) is chloride.
3. Add prefixes to indicate the number of atoms.
   • A. CO: Carbon monoxide
   • B. NO_2: Nitrogen dioxide
   • C. PF_3: Phosphorus trifluoride
   • D. CCl_4: Carbon tetrachloride

Writing Formulas from Names of Molecular Compounds

Study Check

Write the correct formula for each:

• A. phosphorus pentachloride
• B. dinitrogen trioxide
• C. sulfur hexafluoride

Solution

1. Write the symbols in the order of the elements in the name:
   • A. phosphorus pentachloride: P Cl
   • B. dinitrogen trioxide: N O
   • C. sulfur hexafluoride: S F
2. Write any prefixes as subscripts:
   • A. phosphorus pentachloride: PCl_5
   • B. dinitrogen trioxide: N2O3
   • C. sulfur hexafluoride: SF_6

Summary—Naming Simple Compounds

Compounds: Ionic or Covalent?

A compound is usually:

• Ionic if the first element in the formula or the name is a metal or the polyatomic ion NH_4^+.
  • Example: In K_2O, K is a metal; thus, it is ionic: potassium oxide.
• Covalent if the first element in the formula is a nonmetal.
  • Example: In N_2O, N is a nonmetal; thus, it is covalent: dinitrogen oxide.

6.6 Electronegativity and Bond Polarity

Electronegativity values of representative elements in Group 1A(1) to Group 7A(17).

• Learning Goal: Use electronegativity to determine bond polarity.

Electronegativity

Electronegativity indicates the attraction of an atom for shared electrons in a bond.

Electronegativity:

• Increases from left to right across a period on the periodic table.
• Increases from bottom to top of the periodic table.
• Is high for nonmetals, with fluorine having the highest value.
• Is low for metals.
• Core Chemistry Skill: Using Electronegativity

Nonpolar and Polar Bonds

In the nonpolar covalent bond of H_2, electrons are shared equally. In the polar covalent bond of HCl, electrons are shared unequally.

Nonpolar Covalent Bonds

A nonpolar covalent bond:

• Occurs between nonmetals.
• Involves equal or almost equal sharing of electrons.
• Has a small electronegativity difference.
  Electronegativity examples:
• N—N : 3.0 - 3.0 = 0.0 (Nonpolar covalent)
• Cl—Br: 3.0 - 2.8 = 0.2 (Nonpolar covalent)
• H—Si: 2.1 - 1.8 = 0.3 (Nonpolar covalent)

Polar Covalent Bonds

A polar covalent bond:

• Occurs between nonmetal atoms.
• Involves unequal sharing of electrons.
• Has a moderate electronegativity difference.
  • Electronegativity examples:
    • O—Cl: 3.5 - 3.0 = 0.5 (Polar covalent)
    • Cl—C: 3.0 - 2.5 = 0.5 (Polar covalent)
    • O—S: 3.5 - 2.5 = 1.0 (Polar covalent)

Dipoles and Bond Polarity

A polar covalent bond becomes more polar as the electronegativity difference increases.

• The separation of charges in a polar bond is a dipole.
• Use delta notation (\delta^+ or \delta^−) to indicate partial charges.
• Use an arrow pointing from the positive to the negative end of the dipole.

Ionic Bonds

An ionic bond:

• Occurs between metal and nonmetal ions.
• Results from electron transfer.
• Has a large electronegativity difference (1.8 or more).
  • Electronegativity examples:
    • Cl—K: 3.0 – 0.8 = 2.2 (Ionic)
    • N—Na: 3.0 – 0.9 = 2.1 (Ionic)
    • S—Cs: 2.5 – 0.7 = 1.8 (Ionic)

Variations in Bonding

The electronegativity difference between two atoms predicts the type of chemical bond:

• 0-0.4: Nonpolar covalent.
• 0.5-1.8: Polar covalent.
• Greater than 1.8: Ionic.

Electronegativity and Bond Types

Electronegativity and Predicting Bond Type

Study Check

Use electronegativity difference to identify the bond type (NP, P, or I):

• A. K—N
• B. N—O
• C. Cl—Cl
• D. H—Cl

Solution

Use electronegativity difference to identify the bond type (NP, P, or I):

6.7 Shapes and Polarity of Molecules

Learning Goal: Use electronegativity to determine bond polarity.

• In methane (CH_4), the central C atom is bonded to four H atoms. Tetrahedral geometry minimizes repulsion, with bond angles of 109°.

VSEPR Theory

The valence shell electron-pair repulsion (VSEPR) theory:

• Describes the orientation of electron groups around the central atom.
• States that electron groups arrange as far apart as possible around the central atom.
• States that the specific molecular shape is determined by the number of atoms attached to the central atom.
• Core Chemistry Skill: Predicting Shape

Central Atoms with Two Electron Groups

In CO_2:

• Two electron groups are positioned around the central carbon atom.
• Repulsion is minimized by placing the two groups on opposite sides of the carbon, resulting in a linear arrangement with bond angles of 180°.
• The shape (two electron groups around the central atom) is linear.

Central Atoms with Three Electron Groups

In formaldehyde (H_2CO):

• Three electron groups surround the central carbon atom.
• Repulsion is minimized by placing the three groups as far apart as possible, with bond angles of 120°.
• Shape (three electron groups around the central atom) is trigonal planar.

Central Atoms with Three Electron Groups

In sulfur dioxide (SO_2):

• Three electron groups surround the central S atom:
  1. A single-bonded O atom
  2. A double-bonded O atom
  3. A lone pair of electrons
• Repulsion is minimized by positioning the three groups as far apart as possible.
• With two bonds and a lone pair on the central atom, the shape is bent.

Central Atoms with Four Electron Groups

In methane (CH_4):

• Four electron groups (attached to H atoms) surround the central carbon atom.
• Repulsion is minimized by placing the four groups at the corners of a tetrahedron with bond angles of 109°.
• With four bonds on the central atom, the shape is tetrahedral.

Central Atoms with Four Electron Groups

In ammonia (NH_3):

• Three bonding groups (to H atoms) and one lone pair surround the central N atom.
• Repulsion is minimized by placing the four groups at the corners of a tetrahedron with bond angles of 109°.
• With three bonds and a lone pair