UNIT: Nomenclature

Objectives: Lesson 2 of 3• You will learn what a Covalent Compound is• You will learn how to write the formulas and names for Covalent

Compounds• You will learn and hopefully memorize the most common

polyatomic ions• You will also learn how to write formulas and names of polyatomic

compounds

TOPIC: Covalent and Polyatomic Compounds

QuickwriteIn 1-2 sentences answer one of the questions below:• Consider the Compound CO2, how is this compound

different from NaCl (hint: look at their location on the periodic table)???

• Why do you think we use prefixes such as “tri-” on words like triceratops, tricycle or tripod?

Covalent Compounds• Earlier we learned about

Ionic Compounds which form between cations (usually metals) and anions (usually a non-metals)

• Covalent Compounds form when two or more non-metal elements share electrons

• Recall that Hydrogen is classified as a non-metal on the periodic table

• The simplest covalent compound is Hydrogen gas or H2

• Notice, that the electrons are not transferred, BUT, instead, they are shared between the two atoms

• As a result, Covalent Compounds DO NOT have charges like Ionic Compounds

• Therefore, we use a completely different naming system when writing their chemical formulas and names

+-+ -

H2 - Covalent Compound

Cations(usually metals)

Anions

(usually non-

metals)

What is a Covalent Compound?• A compound formed between two non-metals

that involves the sharing of electrons• Example: CO, Carbon Monoxide

Naming Covalent Compounds:• When naming covalent compounds there are certain steps you must follow• These steps will help you throughout this unit and the rest of the course:

1. The first element in the formula is named first, and the full element name is used

2. The 2nd element is named as though it were an anion (add –ide)

3. Prefixes (mono- 1, di- 2, tri- 3) are used to denote the numbers of atoms

4. The prefix mono- is never used for naming the first element

Prefixes

di- (two) CO2 - carbon dioxide

tri- (three) N2O3 - dinitrogen trioxide

tetra- (four) CCl4 - carbon tetrachloride

Prefix Example mono- (one) CO - carbon monoxide

penta- (five) N2O5 - dinitrogen pentoxide

hexa- (six) UF6 - uranium hexafluoride

Naming Covalent Compounds:• Write the name for the covalent compound CO -

Carbon Monoxide

1. The first element in the formula is named first, and the full element name is usedFor Example: we just say “Carbon” for Carbon Monoxide

2. The 2nd element is named as though it were an anionFor Example: the Oxygen in CO is oxide

3. Prefixes are used to denote the numbers of atoms-For Example: Since there is 1 oxygen atom we would use the prefix mono- for Monoxide

4. The prefix mono- is never used for naming the first elementFor Example: CO is called Carbon Monoxide NOT Monocarbon Monoxide

Naming Covalent Compounds:• Write the name for the covalent compound SO3

Sulfur Trioxide1. The first element in the formula is named first, and the full element name is used

For Example: we just say “Sulfur” for Sulfur Trioxide

2. The 2nd element is named as though it were an anionFor Example: the Oxygen in SO3 is oxide

3. Prefixes are used to denote the numbers of atoms-For Example: Since there are 3 oxygen atoms we would use the prefix tri- for Trioxide

4. The prefix mono- is never used for naming the first elementFor Example: SO3 is called Sulfur Trioxide NOT Monosulfur Trioxide

Naming Covalent Compounds:• Write the name for the covalent compound

N2O5 - Dinitrogen Pentaoxide

1. The first element in the formula is named first, and the full element name is usedFor Example: we just say “Nitrogen” for Dinitrogen Pentaoxide

2. The 2nd element is named as though it were an anionFor Example: the Oxygen in N2O5 is oxide

3. Prefixes are used to denote the numbers of atoms-For Example: there are 5 oxygen atoms so we would use the prefix penta- for Pentaoxide

4. The prefix mono- is never used for naming the first elementFor Example: Since their are 2 nitrogen atoms, we use the prefix di- for Dinitrogen

What are the Steps for Naming Covalent Compounds?

1. The first element in the formula is named first, and the full element name is used2. The 2nd element is named as though it were an anion

3. Prefixes are used to denote the numbers of atoms- mono- 1, di- 2, tri- 3, tetra- 4, penta- 5, hexa- 6

4. The prefix mono- is never used for naming the first element

Practice:Write the names for the following compounds below:1) SF6

2) PCl5

3) N2O4

4) CCl4

5) CO2

6) NO2

Answer: Sulfur Hexafluoride

Answer: Phosphorous Pentachloride

Answer: Dinitrogen Tetraoxide

Answer: Carbon Tetrachloride

Answer: Carbon Dioxide

Answer: Nitrogen Dioxide

Practice:Write the formulas for the following compounds below:

1) Dinitrogen Dioxide

2) Sulfur Trioxide

3) Diphosphorous Pentaoxide

4) Nitrogen Monoxide

5) Carbon Tetrafluoride

6) Silcon Dioxide

Answer: N2O2

Answer: SO3

Answer: P2O5

Answer: NO

Answer: CF4

Answer: SiO2

Polyatomic Ions• Polyatomic Ions are a group of two or more atoms that have

a particular charge to them• An example would include the ammonium ion NH4

+1

• It is composed of 1 nitrogen atom and 4 hydrogen atoms which give it a +1 charge

• These ions are assigned special names that you must memorize!

[ ]+

Polyatomic Ions• Common Polyatomic Ions:

Name Formula Name Formula

nitrate NO3-1 chromate CrO4

-2

chlorate ClO3-1 phospite PO3

-3

sulfate SO4-2 phosphate PO4

-3

carbonate CO3-2 acetate C2H3O2

-1

hydroxide OH -1 cyanide CN-1

ammonium NH4+1 sulfite SO3

-2

What are Polyatomic Ions? • A group of two or more atoms that have a

particular charge to them• No naming rules exist, you must memorize

their names and charge• Polyatomic Ions

you must memorize:

Name Formula Name Formula

nitrate NO3-1 chromate CrO4

-2

chlorate ClO3-1 phospite PO3

-3

sulfate SO4-2 phosphate PO4

-3

carbonate CO3-2 acetate C2H3O2

-1

hydroxide OH -1 cyanide CN-1

ammonium NH4+1 sulfite SO3

-2

Practice:Write the formula and charge for each polyatomic ion below:

1) hydroxide

2) nitrate

3) phosphate

4) sulfate

Answer: OH-

Answer: NO3-

Answer: PO4-3

Answer: SO4-2

Ionic Compounds with Polyatomic Ions• Naming ionic compounds that contain polyatomic ions

is very similar to naming ionic compounds• For example, the compound NaOH contains the cation,

Sodium (Na+1) and the polyatomic ion hydroxide (OH-1)• Remember, when Ionic compounds come together,

their charges must always be neutral or ZERO• Just like ionic compounds, we write the cation first and

the anion second; giving us the compound Sodium Hydroxide--NaOH

Ionic Compounds with Polyatomic Ions• Let us look at the compound ammonium nitrate

NH4NO3 (a very explosive chemical)

• Ammonium Nitrate is made up of 2 polyatomic ions Ammonium (NH4

+ ) & Nitrate (NO3

-)• When two polyatomic ions come together, a

neutral compound NH4NO3 forms between them• Notice, the cation is named first, and anion second

Ionic Compounds with Polyatomic Ions• Consider Silver (I) Nitrate: AgNO3

• The Silver (Ag+1) cation comes first, and the polyatomic anion Nitrate (NO3

-1) comes second• Just like in ionic compounds, the charges need to be neutral• The Silver cation Ag+ with a plus one charge is balanced by the

negatively charged polyatomic ion NO3-1 with a negative one

charge• It is important to realize that because Silver (Ag) is a transition

metal it takes on multiple charges, therefore we still use roman numerals to illustrate charge

• Therefore we write: Silver (I) Nitrate

Ionic Compounds with Polyatomic Ions• Consider one more compound: Mg(OH)2

• The Mg+2 cation comes first, and the negative polyatomic OH- anion comes second

• Just like in ionic compounds, the charges need to be neutral• Mg+2 needs be balanced by 2 negatively charged polyatomic

ions OH-

• In the case of Mg(OH)2 when more than one polyatomic ion exists, we put PARENTHESIS around them

• By putting parenthesis around (OH)2 and a subscript of 2, this tells us we have 2 oxygen atoms and 2 hydrogen atoms

( )

What are Ionic Compounds with Polyatomic Ions?

• Neutrally charged Compounds that contain a polyatomic ion

• When more than one polyatomic ion exists in a compound we put parenthesis around them

• Ex: Mg(OH)2 & AgNO3

Practice:Write the names for the following compounds below:

1) NaOH

2) Ba(OH)2

3) NH4Cl

4) AgNO3

5) CaCO3

6) Mg(NO3)2

Answer: Sodium Hydroxide

Answer: Ammonium Chloride

Answer: Barium Hydroxide

Answer: Silver (I) Nitrate

Answer: Calcium Carbonate

Answer: Magnesium Nitrate

For assistance, look back at the rules for naming Ionic Compounds

Practice:Write the formulas for the following compounds below:

1) Calcium Hydroxide

2) Magnesium Sulfate

3) Aluminum Phosphate

4) Ammonium Hydroxide

5) Iron (II) Sulfate

6) Calcium Phosphate

Answer: Ca(OH)2

Answer: MgSO4

Answer: AlPO4

Answer: NH4OH

Answer: FeSO4

Answer: Ca3(PO4)2

For assistance, look back at the rules for naming Ionic Compounds

Summarize:(you can always write you own)

• Explain the difference between a Ionic Compound and a Covalent Compound:

• What numbers do the following prefixes represent: tetra-, tri-, penta- , hexa-

• Write down the polyatomic ions you need to memorize, arrange them in order according to their charge:

• Explain how we name polyatomic compounds:• Explain how we write formulas for polyatomic compounds:• What do we do if more than one polyatomic ion is present in a

compound?