Compounds and Bonding

Ionic Bonding

Chemical bonds are the “glue” that hold groups of atoms together. Some bond “facts”:

All bonds form because of an electrostatic attraction between the positive and negative charges.

Energy is required to break chemical bonds. Energy is released when bonds form.

Chemical bonds are classified in TWO categories

IONIC and COVALENT

BONDING IS ALL ABOUT ELECTRONS

Recall: Electron configurations

To understand bonding you must have a very clear idea about valence electrons

Lewis dot diagrams are used to illustrate the number of valence electrons for an atom:

1.Determine the number of valence electrons.

2.Write the element symbol

3.Place one dot to represent electrons around the four sides of the element symbol. Do not place two electrons together until each side has one (exception is helium).

Lewis Dot structures – complete on the first worksheet

The basis for an ionic bond is the electrostatic attraction between two oppositely charged ions.

+ -ATTRACTIVE

FORCE HOLDS IONS TOGETHER

Cation (metal)

Positive ion

Has “lost” electrons

Anion (non-metal)

Negative ion

Has “gained” electrons

To be most stable, atoms form ions by gaining or losing electrons so as to complete the outer valence shell. This usually means 8 electrons so the rule is called the OCTET RULE.

Metal atoms have a tendency to lose valence electrons and form positive ions.

Non-metals have a tendency to gain valence electrons and form negative ions.

MgMg+2 ClCl-1

When metallic elements react with non-metallic elements . . . .

The electrostatic attraction between the positive magnesium and negative chloride ions is the basis for the ionic bond.

Mg2+ Cl-

Notice that the overall electrical charge of the Mg-Cl combination is still +1. THE CHARGES ARE NOT BALANCED! (neutral)

The electrical charges will be balanced if two chloride ions combine with the magnesium ion.

This is a convenient combination as the Mg atom has lost 2 electrons whereas each chlorine atom gained 1.

Mg2+ Cl-

This combination is the formula unit for the compound and has the formula MgCl2

Cl-

The electrical charge is always balanced in the chemical formula of an ionic compound.

For example – what ionic compound forms between calcium and bromine?

1.Locate the elements involved on the Periodic Table

2.Determine the ions that are formed by each element

calcium is in group II : loses 2 e- to become Ca2+

bromine is in group VII: gains 1 e- to become Br-1

3.Criss-cross the oxidation number (charge) to find the numbers of each atom in the compound. You may need to simplify.

Ca2+ Br-1

CaBr2

2.Determine the ions that are formed by each element

calcium is in group II : loses 2 e- to become Ca2+

oxygen is in group VI: gains 2 e- to become O-2

3.Criss-cross the ionic charges and simplify.

Ca2+ O-2

CaO

1.Locate the elements involved on the Periodic Table

Write the chemical formula for calcium oxide

Ionic Bonding Worksheet

Review ion formation, the octet rule, determining the number of protons, neutrons and electrons in atoms or ions AND showing ionic bonding with dot diagrams

See WIKI for answers

Naming Ionic Compounds

We know how to write the chemical formula of a simple ionic compound from its name. We must also know how to state the name of compound from it’s formula.

What would be the name of the ionic compound MgCl2?write the name of the metallic element first

magnesium

write the name of the non-metallic element but change its ending to “ide”

chlorideide

Calculating Formula Mass

We know that the average atomic mass of a single atom is given on the Periodic Table for each element.

What would be the formula mass ofthe ionic compound MgCl2?

The formula mass is the total mass of all the atoms in a single formula unit . The formula unit is given by the chemical formula.

Using data from your periodic tablesfor MgCl2

one Mg atom( 24.3) + two Cl atoms

(2 x 35.5) = 95.3

Calculating Formula Mass

Practice:

18.0

677.6

180.0

Ionic Compounds Containing Metals not in Group I, II or II

Recall Group I metals form +1 ionsGroup II metals form +2 ionsGroup III metals form +3 ions

+1+2 +3

the transition metals and some others are interesting because they can have more than one ionic chargeiron for example can form both Fe2+ and Fe3+ions Fe

Ionic Compounds Containing Metals not in Group I, II or II

Since iron can form both +2 and +3 ions, there are two possible chemical formulas for iron oxide. What would they be?

FeOand

Fe2O3

iron II oxideand

iron III oxide

Calling both compounds “iron oxide” would be confusing. Different names are required. Can you see from the examples how these compounds were named?

Ionic Compounds Containing Metals not in Group I, II or II

FeOand

Fe2O3

iron II oxideand

iron III oxide

IF a metal atom can form more than one possible ion, a ROMAN NUMERAL indicates the ionic charge.

If the metal can only form one possible ion (Group I, II, III, Zn and Ag) then a roman numeral is NOT included.

+3

+2

+1

III

Try naming these compounds

BeCl2

CuBr2

SnO2

Pb3P2

beryllium chloride

copper II bromide

tin IV oxide

lead II phosphide

Write the chemical formulas for these compounds

Na2S

CuO

NiCl3

FeN

sodium sulfide

copper I oxide

nickel III chloride

iron III nitride

Structure of Ionic Compounds

Ionic compounds are crystalline solids. Positive and negative ions arrange themselves in regular geometric patterns.

This crystal lattice structure is held together strongly by the ionic bonds.

These strong forces mean ionic compounds are brittle and have high melting points.

Crystals of sodium chloride are made of numerous Na+ and Cl- ions. Each ion is attracted to all the oppositely charged neighboring ions.

Most ionic compounds are soluble and dissolve in water to form solutions

Solutions of ions are electrolytes. They conduct electricity because the dissolved ions are free to move.