6.1 Introduction to Chemical Bonding

• Objectives – Distinguish between ionic and molecular

compounds– Define cation and anion and relate them to metal

and nonmetal

Chapter 6 Chemical Names and Formulas

Honors Chemistry 2014

Molecules and Molecular Compounds• Just a reminder before we start that only the

noble gases (He, Ne, Ar, Kr….) tend to exist as isolated atoms. Because of this, the unit for noble gases is the atom (not the molecule).

• All other elements tend to form bonds between atoms. When they do this, they are either called molecules (if they are covalent) or formula units (if they are ionic).

• This chapter teaches you the difference.

Molecules• Many atoms in nature exist in the form of molecules; atoms

bonded with other atoms. • A molecule is the smallest electrically neutral unit of a

substance that still has the properties of the substance. Molecules are made up of two or more atoms that act as a unit.

• Compounds composed of molecules are called molecular compounds.

• What do you notice about the bonds for these two molecules?

CO2H2O

Properties of Molecular Compounds• Molecular compounds (as differentiated from Ionic

compounds) tend to have:– Low melting and boiling points• Therefore many of these exist as gases or

liquids at room temperature (ex: O2 CO2 CH4 N2O)

– Typically composed of nonmetals– Example of diatomic molecule: CO– Example of triatomic molecule: H2O

Ions and Ionic Compounds

Ionic Compounds• Not all compounds are molecular compounds.• Many compounds are composed of ions.• Ions are atoms or groups of atoms that have a positive or

negative charge.• Atoms of the metallic elements, such as sodium, tend to

form ions by losing one or more electrons (they get stolen).• A cation is any atom or group of atoms that has a positive

charge. – A cation has fewer electrons than the neutral atom from

which it was formed. Example: Na has 11 e-, Na+ has 10 e-

Cation and Anion versus Neutral Atom

• Sodium cation has lost its one electron in the outer shell (its been stolen by Cl). Note outer shell has gone from 1 to 0 e-.

• Chlorine anion has stolen sodium’s electron, now its outer shell has gone from 7 to 8 electrons (full shell)

Why is the cation Na+ smaller than the neutral atom?

Because the same 11 protons have a larger pull per e- on 10 e- than they did on 11 e- and because the electrons from the outer shell have been stolen and it’s now one shell smaller.

Why is the anion Cl-

larger than the neutral atom?

The same 17 protons now have to pull inward on 18 electrons instead of just 17. They can’t pull as hard.

6.1 Ionic Naming - Cations• Compounds composed of cations (+) and anions (-) are called

ionic compounds.• Cations – Come from groups 1-12, plus “other metals” below

“stairs” in groups 13-16– For metallic elements, the name of a cation is the same as the name

of the element. For example, sodium’s cation is called “sodium ion”.– If one electron is lost, then the charge is +1 : Na+1

– If two electrons are lost, the charge is +2: Mg+2

– If three electrons are lost, the charge is +3: Al+3

– For the three cations listed above, the number of electrons lost isn’t arbitrary – it depends on the number of valence electrons (the group/column on the periodic table that the element lies in).

– More on that concept in section 6.3 …– Be aware that Na and Na+ may react very differently with other

chemicals – the atom and the ion are not the same!

6.1 Ionic Naming - Anions• Atoms of nonmetallic elements tend to form ions

by gaining one or more electrons to form an anion.• Examples: – F steals 1 electron and becomes F-1

–O steals 2 electrons and becomes O-2

– P steals 3 electrons and becomes P-3

• All anions have a name that ends in –ide– Fluorine becomes Fluoride ion–Oxygen becomes Oxide ion– Phosphorus becomes Phosphide ion

Summary of Section 6.1• Now that we have defined molecular and ionic

compounds, let’s summarize their characteristics:Characteristic Molecular Compound Ionic Compound

Representative unit Molecule (neutral charge)

Formula unit (balance of +/- charged ions to net zero)

Type of elements Nonmetallic Metallic cation with nonmetallic anion

Physical state Solid, liquid or gas Solids only

Melting point Low (usually below 300C)

High (usually above 300 C)

Section 6.2 – Representing Chemical Compounds

• A chemical formula shows the kinds and numbers of atoms in the smallest representative unit of the substance.

• A molecular formula shows the kinds and numbers of atoms present in a molecular compound (non-ionic). – A molecular formula tells you nothing about the structure of

the molecule however.– For example C2H6 (ethane) tells you there are two carbon

and six hydrogen atoms bonded together, but not the overall structure.

• A formula unit gives the lowest, whole-number ratio of ions in an ionic compound (e.g. NaCl)

The seven diatomic gases

• There are seven diatomic gases, where two identical atoms are bonded together.

• I I2 Iodine

• Bring Br2 Bromine

• Clay Cl2 Chlorine

• For F2 Fluorine

• Our O2 Oxygen

• New N2 Nitrogen

• House H2 Hydrogen

6.2 Molecular Formulas• There are several ways that a molecule can be

modeled and visualized.– Ball and stick, space filling model, 3-d bonding

6.2 Formula Units• The term formula unit is to an ionic compound as the term

molecular formula is to a molecular or covalent compound.• The formula unit NaCl for table salt means the smallest whole

number ratio of ions of Na+ and Cl- is 1 to 1, hence the formula unit Na1Cl1 which is the same thing as just writing NaCl.

That crystal lattice could really be Na50Cl50, but the equivalent lowest whole-number ratio for the formula unit is Na1Cl1 or NaCl.

6.2 Formula units• To summarize, if a compound is an ionic

compound, one unit of it is called one formula unit of the compound.

• For covalent compounds (non-ionic), such as H2O or CO2 each unit is just called a molecule.

• Remember, there is no such thing as a molecule of sodium chloride, NaCl, or magnesium chloride, MgCl2. These compounds exist as collections of positively charged and negatively charged ions arranged in repeating three dimensional patterns.

Ch. 6.2 – Ionic Naming

The Laws of Definite and Multiple Proportions

• The law of Definite Proportions states that in samples of any chemical compound, the masses of the elements are always in the same proportions.

• The law of Multiple Proportions states that whenever two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers.

Sample problem 6-2• Carbon reacts with oxygen to form two compounds.

Compound A contains 2.41 g of carbon for each 3.22 g of oxygen. Compound B contains 6.71 g carbon for each 17.9 g oxygen. What is the lowest whole-number mass ratio of carbon that combines with a given mass of oxygen?

• Compound A (This is CO)

• Compound B (This is CO2 )

• Compare the masses of carbon per gram of oxygen:

6.3 Monatomic Ions

Add – ide to anion name

• Representative ions have fixed charges that are always the same, as shown below.

6.3 Ions of metallic elements

Metallic elements can take on several different positive charges, depending on what they are bonded to, but they are always cations (+ ions)

6.3 Metal Ions • Note the use of the roman numerals and how it ties to the charge on the ion.

• Example:• Fe3+ is Iron(III) ion• So the roman numeral III

is used to indicate the charge of +3 on the ion.

• What is the charge on Tin(IV) ion?

• How about Cobalt(II) ion?

Sample problem 6-3 & 6-4• State the charge on the following ions & namea. Sulfurb. Lead, 4 e- lostc. Strontiumd. Argone. Brominef. Copper, 1 e- lost

-2+4+20 no ion-1+1

Sulfur ionLead(IV) ionStrontium ion(no ion)Bromide ionCopper (I) ion

6-3 Polyatomic Ions

Specific list of polyatomic ions you are accountable to memorize this year

-1 Ions -2 Ions -3 Ions

Name Formula Name Formula Name Formula

Acetate C2H3O2-1 Sulfite SO3

-2 Phosphate PO4-3

Hydroxide OH-1 Sulfate SO4-2

Nitrate NO3-1 Carbonate CO3

-2

Nitrite NO2-1

Bicarbonate (hydrogen carbonate)

HCO3-1

+ 1 Ions

Ammonium NH4+

Also know diatomic molecules: I2 Br2 Cl2 F2

O2 N2 H2 and also H2O, NH3 (ammonia), and CH4 (methane)

Anions

• The ions that are produced when atoms of chlorine and other halogens gain electrons are called halide ions.

• Here are some common anions:

Metal Monatomic Cations

Ion name

Lithium Li+ LithiumSodium Na+ SodiumPotassium K+ PotassiumMagnesium Mg2+ MagnesiumCalcium Ca2+ CalciumBarium Ba2+ BariumAluminum Al3+ Aluminum

Nonmetal Monatomic Anions

Ion Name

Fluorine F- FluorideChlorine Cl- ChlorideBromine Br- BromideIodine I- IodideOxygen O2- OxideSulfur S2- SulfideNitrogen N3- NitridePhosphoru

sP3- Phosphide

Recall that anions end in –ide.

Writing Ionic Compound FormulasExample: Barium nitrate

1. Write the formulas for the cation and anion, including CHARGES!

Ba2+ NO3-

2. Check to see if charges are balanced.

3. Balance charges , if necessary, using subscripts. Use parentheses if you need more than one of a polyatomic ion.

Not balanced!

( ) 2

Writing Ionic Compound FormulasExample: Ammonium sulfate

1. Write the formulas for the cation and anion, including CHARGES!

NH4+ SO4

2-

2. Check to see if charges are balanced.

3. Balance charges , if necessary, using subscripts. Use parentheses if you need more than one of a polyatomic ion.

Not balanced!

( )2

Writing Ionic Compound FormulasExample: Iron(III) chloride

1. Write the formulas for the cation and anion, including CHARGES!

Fe3+ Cl-

2. Check to see if charges are balanced.

3. Balance charges , if necessary, using subscripts. Use parentheses if you need more than one of a polyatomic ion.

Not balanced!

3

Gimmicky way to write an ionic formula (not my favorite)

Task: Write the formula for iron(III) sulfide

Caveat to this gimmick: If it comes out something like Mg2Cl4 you’ll need to reduce to lowest common denominator MgCl2

Your book uses the gimmicky way as well, so feel free to use it, as long as you don’t lose track of the meaning of what you are doing.

The goal is to make the total positive charge equal the total negative charge, which is essentially a least common multiple exercise.

Cation first, then anion

Monatomic cation = name of the elementCa2+ = calcium ion

Monatomic anion = root + -ideCl- = chloride

CaCl2 = calcium chloride

Naming Ionic Compounds

some metal forms more than one cation use Roman numeral in name

PbCl2

Pb2+ is cation

PbCl2 = lead(II) chloride

Metals with multiple oxidation states

Naming Ionic Compounds

Section 6.5 Molecular Compound NamingPrefixes used in naming Molecular Binary Compounds

mono- 1

di- 2

tri- 3

tetra- 4

penta- 5

hexa- 6

hepta- 7

octa- 8

nona- 9

deca- 10

Let’s try naming a few• PCl3

• Phosphorus trichloride• SF6

• Sulfur hexafluoride• N2O• Dinitrogen monoxide (or nitrous oxide)• P2O3

• Diphosphorus trioxideNote that the first element named is NOT a metal. If it were, it would use IONIC naming conventions. This is COVALENT naming conventions for first element = non-metal

Let’s try writing formulas for a few• Carbon tetraiodide• CI4

• Phosphorus pentafluoride• PF5

• Phosphorus tribromide• PBr3

• Dinitrogen pentoxide• N2O5

Note that once again, the first element is a NON-METAL, so COVALENT /MOLECULAR naming conventions are being used.

Compound Name Compound FormulaCarbon dioxideCarbon monoxideDiphosphorus pentoxideDinitrogen monoxideSilicon dioxideCarbon tetrabromideSulfur dioxidePhosphorus pentabromideIodine trichlorideNitrogen triiodideDinitrogen trioxide

Compound Formula Compound Name

N2O4

SO3

NO

NO2

As2O5

PCl3

CCl4

H2O

SeF6

Names of common acids

• Hydrochloric acid HCl• Sulfuric acid H2SO4

• Nitric acid HNO3

• Acetic acid (vinegar) CH3COOH

or HC2H3O2

• Phosphoric acid H3PO4

• Carbonic acid H2CO3Note that these are all COVALENT/MOLECULAR compounds, not ionic compounds.

Use this chart to go from a formulaTo a name

Use this flowchart to go from a chemical name to a chemical formula