Lesson 5-3 Writing Chemical Formulas



=


=		=

3D"sho2.jpg 3D"Chemistry2.jpg
Lesson=20 5-3

Writing = Chemical=20 Formulas

3D"labtech1.gif

Need More = Help?

A = chemical=20 formula is a combination of elemental symbols and = subscript=20 numbers that is used to show the composition of a = compound. =20 Depending of the type of compound that the formula represents, = the=20 information that it provides will vary slightly. Before we = go=20 about learning how to write chemical formulas, it is important = that you=20 clearly understand the difference between molecular=20 compounds and ionic=20 compounds.

Ionic=20 compounds are composed of charged ions that are held = together=20 by electrostatic forces. A typical type of ionic compound, = called=20 a binary compound because it is made up of = two=20 elements, will be composed of metallic positive = ions=20 (cations) and nonmetal negative ions=20 (anions). Another type of ionic = compound, called=20 a ternary compound as it contain three=20 elements, is composed of monatomic ions and polyatomic=20 ions. When dealing with ionic formulas it is very = important to=20 remember that the formula does not show how the compound = actually exists=20 in nature. It only shows the ratio by which the individual = ions=20 combine. For example, the ionic formula for calcium = chloride is=20 CaCl₂. Since calcium chloride is an ionic compound, = this=20 formula does not mean that there are actually two chlorine atoms = floating around attached to one calcium atom. Ionic = compounds are=20 actually continuous, lacking the discrete units that make up a = sample of=20 a molecular substance. Rather, the formula shows = that a=20 sample of calcium chloride contains twice as many chlorine atoms = as=20 calcium atoms. Remember that ionic compounds are = not=20 molecules, so the formula CaCl₂ is said to = represent=20 one formula unit of calcium=20 chloride.

= Molecular=20 compounds are held together by covalent bonds, or = shared pairs=20 of electrons. Molecular formulas do show these molecules = as they=20 actually exist as discrete units in nature. When we say = that the=20 molecular formula of water is H₂O, we can see that = the=20 molecules of water are made up of three atoms, two hydrogen = atoms are=20 covalently bonded to each oxygen atom. A special type of = chemical=20 formula, called an empirical formula, shows the = composition of a molecule not as it actually exists, but in a = simple=20 whole number ratio. The difference between empirical and=20 molecular formulas will be explained in = lesson=20 5-5.

This lesson = will=20 concentrate on writing simple chemical formulas when given a = formula=20 name. In learning how to write chemical formulas, you will = make=20 use of the oxidation numbers that you learned about in lesson=20 5-2. For your convenience, print out the tables from = lesson=20 5-2 before you continue with this lesson, as they will be = referred to=20 from time to time.

Writing Ionic=20 Formulas

I. Binary=20 Compounds - Binary compounds are compounds that are = composed of=20 only two elements. When you write the formulas for binary=20 compounds, they will consist of two elemental symbols, and they = may also=20 have one or two subscript numbers, if the elements don't combine = in a=20 one to one ratio. You are probably familiar with the = formula NaCl=20 for table salt. This formula shows no subscripts because = one ion=20 of Na will be present for each ion of Cl, in any sample of table = salt.

You will be given the name of a binary = compound=20 and you will be expected to be able to write the proper formula = for the=20 compound. There will be two sources of information for = writing the=20 correct formula. The compounds name will give you the = elements=20 that make up the compound. The oxidation numbers of the = ions=20 involved will show you the ratio by which they combine. = Let's go=20 through an example;

Example 1. Write the = correct=20 formula for Barium Fluoride.

Step one - Write the symbols for the = elements in=20 the compound. If you need to review the elemental symbols, = see = lesson=20 5-1. Note that the ending "ide" is used for fluoride = to show=20 that it is a negative ion of fluorine.

Barium =3D Ba=20 =              = Fluoride =3D F

Step two - Look up the = oxidation=20 numbers of the elements involved (in table 5-2b or some similar = table),=20 and write them as superscripts to the right of the elemental=20 symbols. Note that when no number accompanies a charge = symbol, as=20 in the case of fluoride below, they charge value is understood = to be=20 "1".

Barium =3D = Ba^{2+=20
=}            = Fluoride =3D F^{^{- =}}

Step three - Use the = correct=20 combination of ions to produce a compound with a net charge of=20 zero. In=20 this case, (2+) + 2(-1) =3D 0. So, two fluoride ions will = cancel out=20 one barium ion. Since it would take two fluoride ions = (each with=20 a charge of negative one) to cancel out one barium ion (with a = charge of=20 plus two) we use a subscript of two after the symbol for = fluorine to=20 show the ratio.

BaF₂=20

If this seems confusing = to you, it=20 will get simpler over time.

Example 2. Write the = proper=20 formula for the ionic compound lithium bromide.

Step one - Write the = symbols for the=20 elements in the compound. = Note that the ending "ide" is used for bromide = to show=20 that it is a negative ion of bromine.

Lithium =3D Li=20         Bromide =3D=20 Br

Step two - Look up the = oxidation=20 numbers of the elements involved (in table 5-2b or some similar = table),=20 and write them as superscripts to the right of the elemental=20 symbols. Note that when no number accompanies a charge = symbol, as=20 in the case of fluoride below, they charge value is understood = to be=20 "1".

Lithium =3D = Li⁺=20         Bromide =3D=20 Br^{^-}

Step three - Use the = correct=20 combination of ions to produce a compound with a net charge of=20 zero. In=20 this case, (+1) + (-1) =3D 0. so, one lithium ion will cancel = out the=20 charge of one bromide ion. This means that the two = elements will=20 combine in a one to one ratio, and know subscripts will be=20 needed.

LiBr

II. Ternary Compounds = - Ternary=20 compounds are composed of three different elements. The = most=20 common types of ternary compounds consist of a metallic cation = (positive=20 ion) and a polyatomic anion (negative ion). The only = common=20 polyatomic ion with a positive charge is the ammonium ion. = At any=20 rate, To write these formulas you will want to have reference = tables=20 with the information provided on tables 5-2b and = 5-2d.

Example 1. Write the proper = chemical=20 formula for potassium hydroxide.

Step one - Write the symbols for the = monatomic=20 and polyatomic ions in the compound. You will find the symbol potassium on table = 5-2b. =20 Hydroxide is a polyatomic ion, which will be found on table = 5-2d. =20 Eventually you will recognize the name of a polyatomic ion, but = for now=20 if you can't find an ion on one table, look on the = other.

Potassium =3D K=20      Hydroxide =3D OH

Step two - Look up the = oxidation=20 numbers of the ions involved (in table 5-2b and 5-2d, or some = similar=20 tables), and write them as superscripts to the right of the = elemental=20 symbols.

Potassium =3D = K⁺⁼²⁰    Hydroxide =3D = OH^{^-} =20

Step three - Use the = correct=20 combination of ions to produce a compound with a net charge of=20 zero. Parenthesis=20 must be used if you need more than one of a polyatomic = ion. In this = case, (+1) +=20 (-1) =3D 0. So, only one of each ion is used. No = subscripts=20 are necessary. If you needed more than one hydroxide ion, = it would=20 be put in parenthesis with the subscript on the = outside.

KOH

Note the importance of = upper and=20 lower case

Example 2. Show the = correct=20 formula for Calcium Nitrate.

Step one - Write the = symbols for the=20 monatomic and polyatomic ions in the compound.

Calcium =3D Ca=20       Nitrate =3D NO₃=20

Step two - Look up the = oxidation=20 numbers of the ions involved (in table 5-2b and 5-2d, or some = similar=20 tables), and write them as superscripts to the right of the = elemental=20 symbols.

Calcium =3D = Ca²⁺=20       Nitrate =3D=20 NO₃^{^-}

Step three - Use the = correct=20 combination of ions to produce a compound with a net charge of=20 zero. Parenthesis must be used if you need = more than one=20 of a polyatomic ion. In this=20 case (+2) + 2(-1) =3D 0. We need to show two nitrate ions = in our=20 formula. The subscript is put on the outside of the = parenthesis to=20 show that the entire polyatomic ion is doubled.

Ca(NO₃)₂

The=20 correct use of parenthesis will seem hard at first, but you must = master=20 this skill with practice!

III. The Stock = System=20 - Some elements, like iron and lead, have more than one = oxidation=20 number. If you were given a compound name like lead = chloride, you=20 would not know if you should used an oxidation number of +2 or = +4 for=20 the lead. The stock system is used to specify which form = of an=20 element, that shows multiple oxidation numbers, is used in a = particular=20 compound. A roman numeral is shown after the name of the = positive=20 ion (cation) to indicate the oxidation number of the positive=20 ion.

Example 1. Show the = correct=20 formula for lead(IV) nitrate.

Step one - Write the = symbols for=20 the ions in the compound.

Lead =3D Pb=20           Nitrate = =3D=20 NO₃

Step two - Look up the = oxidation=20 number of the negative ion involved (in table 5-2b and 5-2d, or = some=20 similar tables). The positive ion will have a positive = oxidation=20 number equal to the roman numeral. Write the numbers as=20 superscripts to the right of the elemental symbols.

Lead =3D = Pb⁴⁺⁼²⁰         Nitrate = =3D=20 NO₃^{^-}

Step three - Use the = correct=20 combination of ions to produce a compound with a net charge of=20 zero. Parenthesis must be used if you need = more than one=20 of a polyatomic ion.

Pb(NO₃)₄ =   =20

Example 2. Show the = correct=20 formula for Copper(II) Fluoride

Step one - Write the = symbols for=20 the ions in the compound.

Copper =3D Cu=20       Fluoride =3D = F

Step two - Look up the = oxidation=20 number of the negative ion involved (in table 5-2b and 5-2d, or = some=20 similar tables). The positive ion will have a positive = oxidation=20 number equal to the roman numeral. Write the numbers as=20 superscripts to the right of the elemental symbols.

Copper =3D = Cu²⁺=20       Fluoride =3D = F^{^-}=20

Step three - Use the = correct=20 combination of ions to produce a compound with a net charge of=20 zero. Parenthesis=20 must be used if you need more than one of a polyatomic = ion.

CuF₂₌₂₀

Writing = Molecular=20 Formulas

I. Binary = Molecular Compounds=20 - The standard method for naming binary molecular = compounds has=20 changed over the years. Currently, the stock=20 system is commonly used for naming molecular = compounds. =20 Names like "carbon dioxide", "carbon monoxide", and "dinitrogen=20 pentoxide" are really remnants of an older system that used = prefixes to=20 identify the number of elements involved. When you are = writing the=20 formula for a molecular compound using the stock system, you = will not=20 really notice any difference from the methods described above, = until you=20 study bonding. You should be aware that you are not = dealing with=20 ions when you are working with molecular formulas, rather you = are=20 looking up what might be called the apparent = charge on=20 each atom.

Example 1. Write the = correct=20 formula for nitrogen(IV) oxide.

Step one - Write the = symbols for the=20 elements involved.

Nitrogen =3D=20 N     Oxide =3D O

Step two - Use the roman = numeral as=20 the apparent charge of the first element. Find the = apparent chart=20 of the second element by looking on reference tables such as=20 5-2a.

Nitrogen =3D = N⁴⁺=20      Oxide =3D=20 O^{^2-}

Step three - Determine the = ratio by=20 which the elements will bond to show a net charge of zero. = Use=20 subscripts to indicate the number of atoms of each element=20 present. In this case, (+4) + = 2(-2) =3D=20 0.

NO₂

Example 2. Write the = correct=20 formula for nitrogen(III) oxide.

Step one - Write the = symbols for the=20 elements involved.

Nitrogen =3D=20 N     Oxide =3D O

Step two - Use the roman = numeral as=20 the apparent charge of the first element. Find the = apparent charge=20 of the second element by looking on reference tables such as=20 5-2a.

Nitrogen =3D = N³⁺=20      Oxide =3D=20 O^{^2-}

Step three - Determine the = ratio by=20 which the elements will bond to show a net charge of zero. = Use=20 subscripts to indicate the number of atoms of each element=20 present. In this case, 2(+3) = + 3(-2) =3D=20 0.

N₂O₃

II. Other = Molecular=20 Formulas - There are other types of molecular formulas, = besides=20 binary, which you will eventually be required to write. = These=20 lessons will be presented at other = times.

Writing = Chemical=20 Formulas Quizzes

Review=20 5-3c - Writing Formulas Review By Andrew Avallone, Class of=20 2003

Review=20 5-3d - Writing Formulas Review By Andrew Avallone, Class of=20 2003

Writing = Chemical=20 Formulas Worksheets

Worksheet=20 5-3a - Writing Chemical Formulas for Binary Ionic=20 Compounds

Worksheet=20 5-3b - Writing Chemical Formulas for Ternary Ionic=20 Compounds

Worksheet=20 5-3c - Writing Chemical Formulas with the Stock=20 System

Worksheet=20 5-3d - Writing Chemical Formulas with the Stock=20 System

Please forward all questions, comments and criticisms to Gregory L. = Curran.
=A9=20 Copyright 2004 Fordham Preparatory School, All Rights = Reserved.
Last=20 Modified February=20 07,=20 2008