Chemical Bonding Chapter 7. The Octet Rule Atoms tend to gain, lose, or share electrons in order to get a full set of valence electrons. “octet” – most.

Chemical Bonding

Chapter 7

The Octet Rule• Atoms tend to gain, lose, or share electrons

in order to get a full set of valence electrons.• “octet” – most atoms need 8 valence

electrons for a full set• Gaining or losing ions = ionic bonding• Sharing = covalent bonding

http://www.youtube.com/watch?v=_M9khs87xQ8&index=2&list=PL6ioyKxGFb1CnJ4tTwCcLxMCda7TXQDAV

Ionic Bonding

Properties of Ionic Compounds

• High melting points

• Brittle

• Usually salts

• Many dissolve in water– Can conduct electricity because ions separate

and are charged in the solution

Ionic Bonds

• Positively charged ion attracted to negatively charged ion– Positive ions = cations– Negative ions = anions

• Metal + nonmetal– Metals form cations– Nonmetals form anions

Types of Ions

• Monatomic = “one-atom”– H+, Ca2+, Br-, N3-

• Polyatomic = “many-atoms”– NH4

+, OH-, SO42-,

Lewis Dot Structures

• Developed by American chemist Gilbert Lewis (1875-1946)

• Valence electrons represented by dots around the element symbol– No more than two dots per side

• Can be used to show rearrangement of electrons during chemical reactions

Binary Ionic Compounds

• Contain ions of only two elements

• Formula: Cation written first, then anion– Charges of ions written as superscripts, # of

atoms in a compound written as subscripts

• Ratio written in lowest terms = empirical formula

Binary Ionic Compounds

• Draw the Lewis Dot Structures for sodium and chlorine

• Using an arrow, identify how the transfer of 1 electron can create 2 new ions

• Sodium transfers an electron to chlorine.

• Sodium becomes a positive ion with a +1 charge.

• Chlorine becomes a negative ion with a -1 charge.

Binary Ionic Compounds

Na+ + Cl- NaCl

• The total (net) charge on the compound should be zero.

• You must determine how many of each ion will need to be in the compound to balance out the charges.

The Crisscross Method for Writing Compound Formulas

• Write the ion symbols (with their charges as superscripts) for the cation and anion

• Criss-cross the two charges, moving them diagonally from one ion’s superscript to the other ion’s subscript– Drop the sign!

Crisscross Method Practice

magnesium ion and chloride ion

Mg2+ Cl-1

Mg Cl =

MgCl2

Compound Formula Practice

magnesium ion + oxide ion Mg2+ + O2-

Mg2+ + O2- MgO

calcium ion and bromide ion Ca2+ + Br-

strontium ion and nitride ion Sr2+ + N3-

Mg2O2

CaBr2

Sr3N2

Naming Ionic Compounds

• Name the cation using its element name.• Name the anion by dropping the ending of

the element name and adding –ide.

Ca3P2

calcium phosphide• If the anion is polyatomic, simply name it

using the ion’s name

Mg3(PO4)2

magnesium phosphate

Naming Ionic Compounds

• If the cation has more than one valence (it can have different charges), indicate the charge using roman numerals in parenthesis after the cation name.

FeO = iron (II) oxide

Fe2O3 = iron (III) oxide

Covalent Bonding

Covalent Bonds

• Formed by a shared pair of electrons between two atoms

• Make up molecules (which make up molecular substances)

• Between nonmetals

Formulas

• Empirical formula = lowest ratio of types of atoms in a compound

• Molecular formula = exact number of atoms of each element in a single molecule of a compound

• Structural formula = how atoms are bonded together

Formula Example: Glucose

molecular formula

C6H12O6

empirical formula

CH2O

structural formula

Lewis Dot Structures

• For molecules:– Show pairs of electrons that are shared

between atoms using 2 dots or 1 dash.– Leave electrons not involved in bonds as

dots.

Lewis Dot Structures

Draw the Lewis dot structures for:

F2

NH3

H2O

H2CO

C2H2

Exceptions to the Octet Rule

• Less than an octet– BF3

• More than an octet– SF4

• Odd number of electrons– NO

Properties of Covalent Bonds

• Polar covalent bonds– Unequal sharing b/c of electronegativity

difference– More electronegative atom gets slightly

negative charge (higher electron density)– Less electronegative atom gets slightly

positive charge (lower electron density)

• Nonpolar covalent bonds– No electronegativity difference– Share electrons equally

Properties of Covalent Bonds

• Low melting points

• Soft, flexible

• Many won’t dissolve in water– Cannot conduct electricity even if they do

dissolve (due to no charges being present)

Naming Covalent Compounds (Molecules)

• Prefixes must be added to tell the ratio of atoms in the compound.

mono- 1

di- 2

tri- 3

tetra- 4

penta- 5

hexa- 6

hepta- 7

octa- 8

nona- 9

deca- 10

Naming Covalent Compounds (Molecules)

• Most electronegative element written last in formula and name.– Drop ending of this element’s name and add –ide.

Si2Br6

disilicon hexabromide

• Don’t include mono- prefix for 1st element listed.

CF4

carbon tetrafluoride

Naming Covalent Compounds (Molecules)

• Shorten prefixes to make names easier to say.

H2O

dihydrogen monoxide

not dihydrogen monooxide

• Sometimes common names are used.O2 = oxygen

NH3 = ammonia

Hydrates and Acids

Naming Hydrates

• Hydrates are ionic compounds that absorb water into their solid structures.– Anhydrous substances are water-free

• Naming:– Name the ionic compound– Using the prefixes that you have learned,

identify the degree of hydration

MgSO4 7 H2Omagnesium sulfate heptahydrate

Naming Acids

• Acids are molecular substances that dissolve in water to produce hydrogen ions (H+). – Can separate into ions even though they are

molecular compounds

• Hydrogen is the cation in acids.

Naming Acids

• If the anion ends in –ide– Begin the name with hydro-– Add the root name of the anion, but change

the ending from –ide to –ic– Add the word acid

HBr

HCl

H2S

Hydrochloric acidHydrobromic acid

Hydrosulfuric acid

Naming Acids

• If the anion ends in –ate– Do NOT begin with hydro-– Keep the root of the anion, but change the

ending from –ate to –ic– Add the word acid

HNO3

H3PO4

HC2H3O2

nitric acid

phosphoric acid

acetic acid

Naming Acids

• If the anion ends in –ite– Do NOT begin with hydro-– Keep the root of the anion, but change the

ending from –ite to –ous– Add the word acid

HNO2

H2SO3

HClO2

nitrous acid

sulfurous acid

chlorous acid

_______ate_______ide ________ite

hydro____icacid

______icacid

______ousacid

anion

acid

add hydrogen ions (H+)

Metallic Bond, A Sea of Electrons