Chapter 5“Formula Writing”

By:Jelyn A. Godoy

H2O

Objective

Apply the steps in writing formula.

Identify the rules in writing formula.

Identify formula , subscripts , coefficients and parenthesis.

Use subscripts , coefficients and parenthesis in writing formula.

Formulas

Formulas are combinations of symbols that represent a compound. A formula indicates which elements are involved and the number of atoms of each element contained in the compound.

In writing formulas, we use subscripts, coefficients, and parentheses in addition to the symbols of the elements.

Subscripts indicate the number of atoms of an element, as in H2 where two is the subscript meaning two hydrogen atoms. If there is no subscript with a symbol, it is assumed there is only one atom of that element.

H2 Subscript

Coefficients, numbers in front of the formula, indicate the number of molecules of compound, as in 4HCl where four is the coefficient indicating four molecules of HCl.

Parentheses are used to separate a radical from the rest of the formula when it would be confusing.Example: Fe2 (CO3)3

coefficients 4HCl

Steps in Formula Writing

1.Determine the symbols for the elements in a compound.

2. Determine the valence of each of the atoms.

Predicting Oxidation StateGroup 1AGroup 1A:: Lose 1 electron to form Lose 1 electron to form 1+1+ ions ions

HH11++ LiLi11++ NaNa11++ KK11++ RbRb11++

Predicting Oxidation StateGroup 2AGroup 2A:: Loses 2 electrons to form Loses 2 electrons to form 2+2+ ions ions

BeBe2+2+ MgMg2+2+ CaCa2+2+ SrSr2+2+ BaBa2+2+

Predicting Oxidation StateGroup 3AGroup 3A:: Loses 3 Loses 3

electrons to form electrons to form 3+3+ ions ions

BB3+3+ AlAl3+3+ GaGa3+3+

Predicting Oxidation StateGroup 4AGroup 4A:: Do they Do they loselose 4 electrons or 4 electrons or gaingain 4 electrons? 4 electrons?

Neither! Neither! Group 4A Group 4A elements rarely form elements rarely form ions ions (they tend to share)(they tend to share)

Predicting Oxidation StateGroup 5AGroup 5A:: Gains 3 Gains 3 electrons to form electrons to form 3-3- ions ions

NN3-3-

PP3-3-

AsAs3-3-

Nitride

Phosphide

Arsenide

Predicting Oxidation StateGroup 6AGroup 6A:: Gains 2 Gains 2 electrons to form electrons to form 2-2- ions ions

OO2-2-

SS2-2-

SeSe2-2-

Oxide

Sulfide

Selenide

Predicting Ionic ChargesGroup 7AGroup 7A:: Gains 1 Gains 1 electron to form electron to form 1-1- ions ions

FF1-1-

ClCl1-1-

BrBr1-1-Fluoride

Chloride

Bromide

II1-1- Iodide

Predicting Oxidation StateGroup 8AGroup 8A:: Stable Stable noble gases noble gases do notdo not form ions!form ions!

Predicting Oxidation StateGroup B elementsGroup B elements:: Many Many transitiontransition elements elements

have have more than onemore than one possible oxidation state. possible oxidation state.Iron (II) = Fe2+

Iron (III) = Fe3+

Note the use of Roman numerals to show charges

Predicting Ionic Charges Some of the Some of the post-transitionpost-transition elements also elements also have have more than onemore than one possible oxidation state. possible oxidation state.Tin (II) = Sn2+ Lead (II) = Pb2+

Tin (IV) = Sn4+ Lead (IV) = Pb 4+

Predicting Oxidation StateGroup B elementsGroup B elements:: Some Some transitiontransition elements elements have have only oneonly one possible oxidation state, such possible oxidation state, such as these three:as these three:

Zinc = Zn2+Silver = Ag1+ Cadmium = Cd2+

3. Write the positive element's symbol first, followed by that of the negative element.

Example:

Al+3 S-2 --- Al2S3

Fe+3 Cl-1 --- FeCl3 Cu+1 O-2 --- Cu2O

4. Balance the positive and negative charges. Try to balance the following

Fe+3 Cl-1

Al+3S-2

Cu+1 O-2

Ca+2N-3

RULES IN WRITING CHEMICAL FORMULA

Rule 1: CRISS-CROSS rule1. Write the correct symbols of the

elements. 2. Above each symbol, write the

correct valence or oxidation numbers. Ex. Al+3 O-2

3. To obtain the proper subscript, criss-cross the valence or oxidation number and drop the algebraic sign. Ex. Al+3O-2 became Al2O3

The positive ioncharge without itssign becomes thesubscript of the

negative ion

The negative ion charge without itssign becomes the

subscript of thepositive ion

1 (+3) + 3(-1) = 0

+1 -1

+1 -2

+1 -1

+2 -1

Rule 2: When the subscript is number 1, subscript is not written.

Example:Sodium Chloride- Na+1 Cl+1 --- NaCl

Sodium hydroxide

NaOH

Sodium Chloride chloride

NaCl

Potassium Hydroxide hydroxide

KOH

Potassium chloride

KCl

Rule 3: When the oxidation # of both elements are numerically equal but grater than 1, the subscript are not also written.Example:Calcium Oxide~ Ca+2O-2 ----- CaO

Calcium oxide

CaO

Iron(II) Oxide oxide

FeO

Rule 4: All radicals take more than once( the subscript is 2 or more) must be enclosed in Parenthesis().Example:

Ammonium Sulfate~~ NH4+1 SO4

-2-----

(NH4)2 SO4

Ammonium Oxide

ONH 24)(

Iron(lll) Sulfate Sulphate

342 )(SOFe

Iron(lll) Carbonate Carbonate

332 )(COFe

Ammonium Sulfate Sulphate

424)( SONH

Ammonium Carbonate

324)( CONH

“Learn from yesterday, live for to day, hope for

tomorrow. The important thing is not to stop

questioning”

Thank you!GOD

Bless!!!!!!!