Section 2.3—Chemical Formulas

We need to be able to read the formulas for chemicals in the antacids!

Objective: Explain and use nomenclature rules of writing ionic and covalent chemical formulas

Binary Ionic compounds

Recall these Definitions:

Binary Ionic Compound- compound containing two elements—one metal and one non-metal – bonded through an ionic bond.

+Cation

+Cation

-Anion

-Anion Ionic Compound

Ionic bond- bond formed by attraction between + and - ions

Ionic Charges

Think about it: If ionic bonds are the attractive force between a cation (+ charge) and an anion (- charge) AND that an ionic compound contains a metal and a nonmetal, it must also be true that the metals must be charged and the nonmetals must be charged!

HOW CAN WE DETERMINE THAT CHARGE??

Charges for these elements will be GIVEN by a roman numeral following the cation’s nameex. iron (III) oxide: Fe is +3

Variable Positive Charges

Writing Formulas for Binary Ionic Compounds

To write these formulas:Write the symbol & charge of the first

element (the metal, cation)Write the symbol & charge of the second

element (the non-metal, anion)Add more of the cations and/or anions in

order to have a neutral compoundUse subscripts to show how many of each

type of ion is there.

Example #1

Sodium chloride

Example #1

Sodium chloride

Cation

Anion

Na+1

Cl-1

NaCl

Example #1

Sodium chloride

Cation

Anion

Na+1

Cl-1

NaCl

Na+1 Cl-1

+1 + -1 = 0

The compound is neutral…no subscripts are needed.

Example #2

Calcium bromide

Example #2

Calcium bromide

Cation

Anion

Ca+2

Br-1

Example #2

Calcium bromide

Cation

Anion

Ca+2

Br-1

CaBr2

Ca+2 Br-1

+2 + -1 = +1

Ca+2 Br-1 Br-1

The subscript “2” is used to show that 2 anions are needed.

+2 + -1 + -1 = 0

Let’s Practice

Example:Write the following chemical formulas

Cesium chloride

Potassium oxide

Aluminum sulfide

Calcium bromide

Let’s Practice

CsCl

K2O

Al2S3

CaBr2

Example:Write the following chemical formulas

Cesium chloride

Potassium oxide

Aluminum sulfide

Calcium bromide

Shortcut

Criss-cross the NUMBER of the charge to obtain the subscripts. Reduce if necessary.

Example1: Al+3 S-2 Al2S3

Example 2: Ca+2 O-2 Ca2O2 CaO

Polyatomic Ionic Compounds

Recall these Definitions

Polyatomic Ion- a group of atoms that are bonded together and have an overall charge

+Cation

+Cation

Polyatomic Ionic Compound- compound containing at least one polyatomic ion

-Polyatomic

Anion

Polyatomic IonicCompound

To write these formulas:Write the symbol & charge of the cation &

anion; if either is a polyatomic ion, look up its charge.

Add additional cations or anions in order to have a neutral compound (OR use the criss-cross shortcut.)

Use subscripts to show the number of ionsWhen using subscripts with a

polyatomic ion, you MUST put the polyatomic ion in parenthesis.

Identifying & Naming Polyatomic Ionic

Example #3

Sodium carbonate

Example #3

Sodium carbonate

Cation

PolyatomicAnion

Na+1

CO3-2

Example #3

Sodium carbonate

Cation

PolyatomicAnion

Na+1

CO3-2

Na2CO3

Na+ CO32-

+1 + -2 = -1

Na+ Na+ CO32-

The subscript “2” is used to show that 2 cations are needed.

+1 + 1 + -2 = 0

Example #4

Magnesium nitrate

Example #4

Magnesium nitrate

Cation

PolyatomicAnion

Mg+2

NO3-1

Example #4

Magnesium nitrate

Cation

PolyatomicAnion

Mg+2

NO3-1

Mg(NO3)2Parenthesis are used to show 2 anion groups are needed.

Mg+2 NO3-

+2 + -1 = +1

Mg+2 NO3- NO3

-

The subscript “2” is used to show that 2 anions are needed.

+2 + -1 + -1 = 0

Let’s Practice

Example:Write the following chemical formulas

Sodium nitrate

Calcium chlorate

Aluminum sulfite

Calcium hydroxide

Ammonium Phosphate

Let’s Practice

NaNO3

Ca(ClO3)2

Al2(SO3)3

Ca(OH)2

(NH4)3PO4

Example:Write the following chemical formulas

Sodium nitrate

Calcium chlorate

Aluminum sulfite

Calcium hydroxide

Ammonium Phosphate

Binary Covalent Compounds

Definition

Binary Covalent Compound compound made from two non-metals that share electrons

Nonmetal

Nonmetal

Nonmetal

Nonmetal Covalent compound

Covalent bond atoms share electrons

How we named them:Prefixes were used to indicate the number of

atoms of each elementExample: N5O pentanitrogen monoxide

Identifying & Naming Binary Covalent

Writing Formulas

To write these formulas:Write the symbols of the first and second

elementTranslate the covalent prefixes (assume the

first element is “1” if there’s no prefix) into subscripts to show number of atoms.

Atoms DO NOT form charges when bonding covalently…you DO NOT need to worry about charges with this type!

Example #7

Dinitrogen Tetraoxide

Example #7

Dinitrogen Tetraoxide

N

O

“Di-” = 2

“Tetra-” = 4

N2O4

Example #8

Silicon dioxide

Example #8

Silicon dioxide

Si

O

“Mono-” is not written for the first element

“Di-” = 2

SiO2

Let’s Practice

Example:Write the following chemical formulas

Carbon monoxide

Nitrogen dioxide

Diphosphorus pentaoxide

Let’s Practice

CO

NO2

P2O5

Example:Write the following chemical formulas

Carbon monoxide

Nitrogen dioxide

Diphosphorus pentaoxide

Diatomic Elements

•Some elements are so chemically reactive that they cannot and do not exist in nature as single atoms.•If they do not find an atom of another element to bond to, they will bond to an atom of their same kind.•The formula for these such elements ALWAYS has a “2” subscript.

•These elements are: hydrogen, H2 chlorine, Cl2 nitrogen, N2 bromine, Br2

oxygen, O2 iodine, I2 fluorine, F2

Common Names & Formulas

•There are a few compounds that are known by a common name, rather than a scientific one.

•H2O is known as water.•NH3 is known as ammonia.

(Note: do not confuse this with the ion ammonium, NH4

+)