Ionic Compounds

Unit 4

Ions

• An atom is electrically neutral because it has the same # of protons (+) and electrons (-)

• An atom becomes charged when it either gains or loses electrons.

• A charged atom is called an ion

Ionic compounds

• A cation (charge is +) is formed when an atom loses electrons. Usually metals are cations.

• An anion (charge is -) is formed when an atom gains electrons. Usually nonmetals are anions

• Cations and anions have opposite charges and are attracted to one another.

• These attractive forces can hold the ions together in an ionic bond, forming a compound.

• Ionic compounds are usually made up of a metal and nonmetal.

• Ionic compounds are often called: salts

Valence electrons review

• Valence electrons are the electrons that are in the highest energy level of an atom.

• These electrons are involved in forming bonds with other atoms.

• Elements (except helium) have the same # of valence electrons as their group #.

Octet rule• The charge of an ion is related to the number of

valence electrons on the atom.• In order to become stable, atoms tend to either

gain (anions) or lose (cations) valence electrons so that its highest energy level will become full with 8 electrons, similar to a noble gas. (except He, which has 2).

• This is called the octet rule.

Example of an ionic compound:NaCl

Example of an ionic compound:NaCl

Example of an ionic compound

Ion Electron Configurations• When we write the electron configuration of

a positive ion, we remove one electron for each positive charge:

Na → Na+

1s2 2s2 2p6 3s1 → 1s2 2s2 2p6

• Notice: the electron configuration of Na+ is that of Ne, a noble gas (having a stable octet electron configuration)

• When we write the electron configuration of a negative ion, we add one electron for each negative charge:

O → O2-

1s2 2s2 2p4 → 1s2 2s2 2p6

• Also with O2- there are 8 electons in the outermost energy level.

Practice

• Give the electron configuration for:• F-

• Mg2+

• P3-

• Li+

Properties

• Properties of ionic compounds:high melting pointlow malleability – break and shatter easilycan conduct electricity under certain conditions

(disssolved in water or when molten)

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.

Naming Binary 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 Binary Ionic Compounds

• Negative monatomic ions end in –ide.Cl- chloride ionO2- oxide ionN3- → nitride ion

Trends in charge of ions

• Group 1: always 1+• Group 2: always 2+• Al: always 3+• Group 7 (halogens): always 1-• Group 6: always 2-• Group 5 (N, P): always 3-

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• KBr

Since 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

• Give the name of the following compounds:

Na2O

BaI2

Give the formulas for:

Magnesium bromide

Aluminum fluoride

Calcium nitride

Metal ions with more than one common charge

• Certain metals can form more than one type of cation

• Roman numeral (in between parenthesis) tells the charge on the cation

Examples: • Iron (II) chloride, FeCl2

• Iron (III) chloride, FeCl3

Examples

• Fe: 2+ and 3+• Cu: 1+ and 2+• Sn: 2+ and 4+• Pb: 2+ and 4+• Co: 2+ and 3+• Mn: 2+, 6+ and 7+

Naming

• Determine the charge of the cation:• Total charge of the molecule is neutral, so use the

known charge of the anion• Example: PbCl4; 1 Pb and 4 Cl atoms per

molecule• Each Cl- charge is – 1; molecular charge is 0.• So Pb charge is 4+• Name: lead (IV) chloride

Naming polyatomic ions

• 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 Þ same charge, per- prefixperchlorate = ClO4

-

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

-

• -ate ion minus 2 O Þ 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

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 charge, then when we write the name we must indicate which charge 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!!!

More practice

• Give the name for:• MgSO4

• Sn(NO3)4

Naming Hydrates• Hydrates are compounds containing water

molecules.• 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 Prefix1 mono-2 di-3 tri-4 tetra-5 penta-6 hexa-7 hepta-8 octa-9 nona-

10 deca-

Practice

• Magnesium carbonate pentahydrate

• NiSO4.6H2O (Ni needs Roman numeral)

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

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)