Module 4 Lesson 2

Ionic Naming and Formulas

Formulas• Chemical formula

– Shows kinds and numbers of atoms in smallest representative unit• NaCl, H2O

• Formula unit– Ionic compounds form CRYSTALS that vary in size, because the ions

attract from all sides, the ions pack together in an alternating pattern, so the formula shows the lowest whole number ratio of ions

Oxidation Numbers• Oxidation numbers are the charges that an atom takes on to obey the

Octet Rule• The oxidation number of an element depends on the Group number of

the element.Group Number Oxidation Number1 +12 +23-12 Usually +2 except Ag and Zn13 +314 +415 -316 -217 -118 0

Mark your Periodic TableAll the transition metals except Ag and Zn can have more than one charge and need a Roman numeral (Pb and Sn need one too!)

+1+2

+1

+20

-1-2

-3+3+4

Formulas• Practice

Al + O Be + S Al + BrK + I Mg + I Na + O

• Sometimes one loses more e- than the other needs, or vice versa, so the ratio cannot be 1:1

• look at the charges, criss-cross them so that they become the subscripts, then reduce if possible; the idea is that the overall charge in a chemical formula is ZERO

• Al+3 O-2 Al2O3

• Be+2 S-2 Be2S2 BeS• Try the rest of the practice at the top of the page and then check yourself

on the following slide

2:2 reduces to 1:1

Formulas and Naming• Practice• Al + O Be + S Al + Br

K + I Mg + I Na + O• AlBr3

• KI

• MgI2

• Na2O• Naming these are easy, you just say the name of the cation and the name

of the anion (ending is changed to –ide)• Writing these names and formulas is easy because each ion always has the

same charge, but this isn’t always the case

Aluminum bromide

Potassium iodide

Magnesium iodide

Sodium oxide

Naming Ionic Compounds

• Remember, ionic compounds always have a metal• Cations have the same name as the metal (add “ion”)• Anions have the ending changed to –ide, or they are polyatomic and

have a special name• Metals that can form more than one type of ion use a Roman numeral

equal to the charge to designate the type• Fe+2 is iron(II)• Fe+3 is iron(III)

Variable Oxidation States• As stated on this previous slide, some elements can have more than one

oxidation state. • This includes many of the Transition Elements as well as Sn and Pb.• These require Roman Numerals to show the oxidation state.

For example Pb can be +2 or +4 and would thus be represented:Pb (II) and Pb (IV)

• These Roman Numerals are also required to name these compounds.– For example– PbCl2

– PbCl4

Lead (II) chloride

Lead (IV) chloride

Polyatomic IonsFrom the … Reference Tables

Writing Formulas• Polyatomic ion: a group of covalently bonded atoms with an overall charge• Criss cross as you did for the other ionic formulas placing the superscript

form one as the subscript of the other.• K+ PO4

-3 K3PO4

• Mg+2 NO3- Mg(NO3)2

• Note that you must add parentheses if the number goes for the entire polyatomic ion

• Try the following:– Ca + hydroxide– K + sulfate– Al + carbonate– Ammonium + O

Writing Formulas• Ca + Hydroxide

Ca+2 OH-Ca(OH)2

• K + SulfateK+ SO4

-2 K2SO4

Al + carbonateAl+3 CO3

-2 Al2(CO3)3

Ammonium + ONH4

+ O-2 (NH4)2O

Naming Polyatomics• Naming ionic compounds containing polyatomics is simple!• Name as usual except just use the name of the polyatomic ion when it

occurs.• Example:• Ca(OH)2 Name the cation (Calcium) and then name the polyatomic ion

(hydroxide)– Calcium hydroxide– Go to the next slide to see more examples

Naming Ionic Compounds

• Ca + HydroxideCa+2 OH-Ca(OH)2

• K + SulfateK+ SO4

-2 K2SO4

Al + carbonateAl+3 CO3

-2 Al2(CO3)3

Ammonium + ONH4

+ O-2 (NH4)2O

Calcium hydroxide

Potassium sulfate

Aluminum carbonate

Ammonium oxide