Ionic Compounds Unit 6. Writing Formulas Elements occur in constant whole number ratios in a compound (Law of Definite Proportions). In a chemical formula.

Ionic Compounds

Unit 6

Writing Formulas• Elements occur in constant whole number ratios in

a compound (Law of Definite Proportions).• In a chemical formula symbols and subscripts are

used to describe this ratio.• For molecules the chemical (molecular) formula

shows the actual number of atoms of each element in the compound

• In an ionic compound, the formula shows the lowest whole number ratio of ions in the ionic crystal.

• This ratio is called a formula unit.

Identifying Ionic Compounds• Any substance made of metal cation and nonmetal

anion OR contains any polyatomic ions• Ionic bond = transfer of electrons

1 atom loses, 1 atom gains (cation/anion formed) the + and – particles are attracted to one anther and form a

very stable bond (electrostatic attract.)

• Properties of ionic bonds:high melting point low malleability – break and shatter easilycan conduct electricity under certain conditionsCoordination number – the number of ions of opposite

charge that surround the ion in a crystal

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Naming Ionic Compounds

• Simply name the two ions, positive ion first (the positive ion is always first).

• Positive monatomic (one type of element) ions have the same name as the element.Na+ sodium ionAl3+ aluminum ion

Naming Ionic Compounds

• Negative monatomic ions end in –ide.Cl- chloride ionO2- oxide ion

• Negative polyatomic ions end in –ate or –ite.• There is not a hard and fast rule which will allow

you to determine if a polyatomic ion is –ate or –ite (which is why you have to memorize them), but there is one helpful shortcut.

Patterns for Polyatomic Ions• The most common form of a polyatomic ion

formed from a nonmetal combined with oxygen ends in –ate.

• -ate ionchlorate = ClO3

-

• -ate ion plus 1 O same charge, per- prefixperchlorate = ClO4

-

• -ate ion minus 1 O same charge, -ite suffixchlorite = ClO2

-

• -ate ion minus 2 O same charge, hypo- prefix, -ite suffixhypochlorite = ClO-

Patterns for Polyatomic Ions

Elements in the same column on the Periodic Table form similar polyatomic ionssame number of O’s and same charge

ClO3- = chlorate BrO3

- = bromate If the polyatomic ion starts with H, add hydrogen- before the ions name and add +1 to the charge

CO32- = carbonate HCO3

- = hydrogen carbonate

Writing Ionic Formulas

• IMPORTANT – When writing formulas, use appropriate subscripts so the total number of positive and negative charges cancel out and the compound is neutral.

• Now, lets see how good we are at writing some formulas and naming some ionic compounds.

Practice

• KBrSince this has a metal (K) and a nonmetal (Br),

we say it is an ionic compound.So we name the positive ion – potassium and

the negative ion with the ending changed to –ide, bromide.

• Potassium bromide• Binary compound

Practice

• Calcium ChlorideAgain a metal and a nonmetal so it is ionic.Calcium would form an ion with a 2+ charge and chloride would be 1-.Ca2+Cl- in order for the compound to be neutral, how

many Cl- would there need to be for every Ca2+??2 Cl- for every 1 Ca2+

• So the formula would be CaCl2

• Binary compound

Practice

• Na2CO3

In this compound there are two ways to identify is as ionic.

First, it has a metal and a nonmetal.Second, it has a polyatomic ion.So we name the ions, positive ion first.

• Sodium carbonate

• Ternary compound

Practice• Magnesium Phosphate

magnesium – Mg2+ ; phosphate – PO43-

In order for the compound to be neutral we have to find the least common multiple between our two charges, 2 and 3. The LCM is 6.

2 goes into 6 – 3 times so Mg3; 3 goes into 6 – 2 times so (PO4)2.

3 x +2 = +6 AND 2 x -3 = -6 Our compound is neutral.Mg3(PO4)2

Ternary compound

Important Precautions

• If we need more than one polyatomic ion (like in the previous example), it must be surrounded by parentheses before you add the subscript.

• Also, notice that if the subscript is 1 we do not write it.

• If the positive ion has more than one possible oxidation number, then when we write the name we must indicate which oxidation state it is.

• This is done by writing a roman numeral in parentheses which is equal to the positive charge.

Practice• Copper (I) Oxide

(I) indicates that the copper has a charge of 1+.Copper - Cu1+ ; Oxide – O2-

• Cu2O• Iron (III) Oxide

(III) indicates iron has a 3+ charge.Iron - Fe3+ ; Oxide – O2-

• Fe2O3

Practice and More Practice!!!

Naming Hydrates

• Hydrates are crystalline compounds which attract and hold water molecules.

• The water is called the water of hydration and can be removed (evaporated) by heating.

• After water is removed the crystal is said to be anhydrous.

Naming Hydrates

• To name hydrates simply name the compound (usually ionic) and then indicate the number of water molecules by using the same prefixes as in molecular compounds.

• CuSO4 • 5 H2OCopper (II) sulfate pentahydrate

• Sodium carbonate heptahydrateNa2CO3 • 7 H2O

Number Prefix

1 mono-

2 di-

3 tri-

4 tetra-

5 penta-

6 hexa-

7 hepta-

8 octa-

9 nona-

10 deca-

Bonding In Metals• valence electrons of metal atoms can be

modeled as a sea of electrons – they are mobile and can drift from one part of the metal to the other

• Metallic Bond – the attraction of these “free-floating” electrons for the metal ions

• these bonds hold metals together and explain many of their physical properties

• but can we actually bond 2 metals together?

Alloys• an alloy is a MIXTURE that is composed of

two or more elements, at least one of which is a metal

• you can also think of an alloy as a solid solution made by dissolving metals in other metals

• alloys are important because their properties are often superior to those of their component elements

Substitutional Alloy

• atoms of approximately the same size replace each other in the crystal

• sterling silver, coins, solder, brass, 18-carrat gold, bronze

Interstitial Alloy

• atoms of smaller size fit into the space (interstices) between the larger atoms

• cast iron, steel, stainless steel, surgical steel

Amalgam

• metal solution that contains mercury

• this type of alloy may be a solid or liquid

• used to be very useful in dental amalgams (mixture of Hg, Ag, and Zn)