CH 19 Chemical Bonds

Section 1: Stability in Bonding

• compound—substance formed when two or more elements combine chemically– examples: water (H2O), sodium chloride

(NaCl), sucrose (C12H22O11), carbon monoxide (CO)

• chemical formula—tells what elements a compound contains and the exact number of the atoms of each element in a unit of that compound

• subscript—the small “written below” number that gives the ratio of how many atoms of each element are in a compound

H2O

subscript

Which Elements Combine?

• Atoms combine when the compound formed is more stable than the separate atoms.

• Atoms with full outer energy levels, or outer energy levels holding eight electrons, are chemically stable.

• The gases (Noble gases) in Group 18 seldom form compounds because their outer energy levels are already full.

Section 2: Types of Bonds

• An attraction forms between atoms as the result of the gaining, losing, or sharing of electrons. This attraction pulls the atoms together forming a compound.

Ions

• ions—an electrically charged atom that is formed when the atom either gains or loses electrons

• cations—a positively charged ion formed when the atom loses an electron

Na+ Ca2+

• anions—a negatively charged ion formed when the atom gains an electron

Cl- O2-

• The periodic table can be used to determine the charge on some ions.

• The elements of groups 3-12, as well as group 14, form ions with a variety of charges.

Group Number

Ion Charge

1 1+

2 2+

13 3+

14 varies

15 3-

16 2-

17 1-

18 0

Kinds of Bonds• ionic bond—a type of bonding in which

ions are held together by the strong attraction of their opposite charges

• In an ionic bond, a transfer of electrons takes place.

• usually occurs between a metal and a nonmetal

• covalent bond—bond between atoms produced by the sharing of electrons

• usually occurs between two or more nonmetals

• molecule—neutral particle formed when two or more atoms bond covalently– examples: water (H2O), hydrogen chloride (HCl),

hydrogen (H2), oxygen (O2), and all of the halogens (group 17)

Metallic Bonding

• Occurs between metals

• “Sea of electrons”

Section 3: Writing Formulas and Naming Compounds

• The terms cation and anion are used by some in place of the term oxidation number. Oxidation number tells you how many electrons an atom has gained, lost or shared to become stable. This is what we call ion charge.

• binary compound—compound composed of two elements

NaCl MgBr2

CrissCross Method For Binary Compounds

1. Write the element symbol for the cation and the anion, the cation to the left of the anion.

Al I

2. Write each ion’s charge as a superscript.Al3+ I-

3. Crisscross the two charges, moving them diagonally downward to form subscripts.

AlI3

Examples Using Crisscross

1. calcium and fluorine• Ca2+ F- CaF2

2. magnesium and oxygen• Mg2+ O2- MgO

3. magnesium and bromine• Mg2+ Br- MgBr2

4. aluminum and fluorine• Al3+ Cl- AlCl3

5. aluminum and oxygen• Al3+ O2- Al2O3

Naming Binary Compounds

• Binary compounds are named beginning with the name of the element that formed the cation (had a + charge).

• The name of the second element, the one that formed the anion (had a – charge), is changed to end in –ide.

• examples: NaCl (sodium chloride); BaS (barium sulfide)

-ide Nomenclature for Some Common Elements

oxygen oxide

nitrogen nitride

sulfur sulfide

fluorine fluoride

chlorine chloride

bromine bromide

iodine iodide

phosphorus phosphide

hydrogen hydride

carbon carbide

Examples with Naming1. SrCl2

• strontium chloride

2. Zn3P2

• zinc phosphide

3. CaF2

• calcium fluoride

4. MgO• magnesium oxide

5. AlCl3• aluminum chloride

Special Ions

• Some elements form more than one cation or anion (or have more than one oxidation number).

• If an element has more than one, the number intended is written in parentheses after the elements.

• examples: FeCl2 iron (II) chloride CuF copper (I) fluoride

General Steps for Formula Naming of Binary Compounds

1. If the 1st element is in Groups 1, 2 or 13, no Roman numeral is needed.

2. If the 1st element (not found in Groups 1, 2, or 13) is on the reference sheet, and the name is NOT followed by a Roman numeral, then NO Roman numeral is needed. You just use the name on the sheet.

3. If the 1st element is on the reference sheet and the name IS followed by a Roman numeral, then a Roman numeral is needed. (examples: iron (III), iron (II), lead (II), lead (IV), etc.)

4. **If a Roman numeral is needed (determined by step 3), you MUST perform a backward crisscross method to determine which Roman numeral is needed.

Polyatomic Ions• group of covalently bonded atoms that act together as

one charged atom• Formulas for polyatomic ions are written the same way

as formulas for binary compounds (you just have the covalently bonded pair that acts together and has one charge).

– examples: Na2SO4 Ca(OH)2

• Names of compounds with polyatomic ions are written like those of binary compounds except the ending is not changed (you just look up the last name instead of changing the element to –ide).

– examples: Na2SO4 sodium sulfate Ca(OH)2 calcium hydroxide

Naming Binary Covalent Compounds

• Some pairs of nonmetals can form more than one compound with each other.

• For example, nitrogen and oxygen can form N2O, NO, NO2, and N2O5.

• Prefixes are used to distinguish between these covalent compounds.

# of Atoms Prefix

1 mono-

2 di-

3 tri-

4 tetra-

5 penta-

6 hexa-

7 hepta-

8 octa-

Compounds with Added Water

• hydrate—compound that has water chemically attached to its ions

• CoCl2 • 6H2O cobalt chloride hexahydrate

• anhydrous—without water