Naming Molecular Compounds

CH4 methaneBCl3 boron trichloride

CO2 Carbon dioxide

All are formed from two or more nonmetals.

Ionic compounds generally involve a metal and nonmetal (NaCl)

1. What are the structural differencesbetween ionic and molecular compounds?

2. How do those differences affecttheir chemical formulas?

The chemical formula for a covalent compoundis called a molecular formula;

The chemical formula for an ionic compoundis called a formula unit.

Example: H2O is the molecular formula of water

Example: NaCl is the formula unit of common table salt

What’s the dif?

AVISUAL look

at the differencewill help out:

Like any ionic compound,sodium chloride exists as a crystal:

Let’s shrinkthe ions sowe can takea peek inside

The formula unit for sodium chloride is NaCl.However, if we look inside the crystal,the Na+ and Cl- ions are each bondedto six oppositely charged neighbors:

Because each Na+ is bonded to more than one Cl- ion,and each Cl- is bonded to more than one Na+ ion, then

all the ions in the whole crystal arepart of one single bonded unit,

and the crystal below would be Na32Cl32.

This is impractical, given the enormous numbers of ions in a crystal large enough to see with our eyes.

So, only the ratio of ions is given (its empirical formula),

which is called a formula unit.

NaCl is the formula unit forthe sodium chloride crystal.

NaCl is the smallest whole numberratio of sodium and chloride ions

in the crystal.

What about molecules?

water is covalently bonded hydrogen and oxygen.

‘space filling’ model ‘ball and stick’ model

A single water molecule

The H2O molecule remains as a single unit of threecovalently bonded atoms.

The H2O molecule remains as a single unit of covalently bonded molecules

NaCl

This difference in structure between molecules andionic compounds is reflected in their symbols:

versus

H2O

The molecule is represented by a molecular formula,showing all the atoms present in the unit.The ionic compound is represented by the ratio of its ions, which is called a formula unit.

Molecular (Covalent) Nomenclaturefor two nonmetals

Prefix System (binary compounds)

1. Less electronegative atom comes first.

2. Add prefixes to indicate # of atoms. Omit mono- prefix on the FIRST element. Mono- is OPTIONAL on the SECOND element.

3. Change the ending of the second element to -ide.

PREFIXmono-di-tri-tetra-penta-hexa-hepta-octa-nona-deca-

NUMBER123456789

10

Molecular Nomenclature Prefixes

CCl4

N2O

SF6

carbon tetrachloride

dinitrogen monoxide

sulfur hexafluoride

Molecular Nomenclature: Examples

arsenic trichloride

dinitrogen pentoxide

tetraphosphorus decoxide

AsCl3

N2O5

P4O10

More Molecular Examples

Learning Check

1. P2O5 a) phosphorus oxide

b) phosphorus pentoxide

c) diphosphorus pentoxide

2. Cl2O7 a) dichlorine heptoxide

b) dichlorine oxide

c) chlorine heptoxide

3. Cl2 a) chlorine

b) dichlorine

c) dichloride

Mixed Practice

1. Dinitrogen monoxide

2. Potassium sulfide

3. Copper (II) nitrate

4. Dichlorine heptoxide

5. Chromium (III) sulfate

6. Iron (III) sulfite

7. Calcium oxide

8. Barium carbonate

9. Iodine monochloride