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.

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?

A niceVISUAL look

at the differencewill help out:

We’ll look at sodium chloride, NaCl,as an example ionic compound.

NaCl does not exist asa single unit like a molecule.

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

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:

Let’s shrinkthe ions sowe can takea peek inside

Let’s shrinkthe ions sowe can takea peek inside

We’ll take alook at thisCl- ion.

How manyNa+ ions isthe Cl- closeenough tobond with?

We’ll start againfrom thehead-on view

We’ll start againfrom thehead-on view

And,how manyCl- ions iseach Na+ ionbonded to?

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.

which is called a formula unit.

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),

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 a molecule.

‘space filling’ model ‘ball and stick’ model

water is covalently bonded hydrogen and oxygen.

‘space filling’ model ‘ball and stick’ model

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

In large amounts of H2O, any attraction betweenH2O molecules is due to weak forces, and so themolecules behave as individual units, not one largebonded unit as in an ionic crystal.

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

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.

IN SUMMARY, in a molecule, the atoms given bythe formula are all covalently bonded. In a collectionof those molecules, the molecules are attracted toeach other by weak forces, but no covalent bondsoccur between molecules--the molecules exist asindividual units of covalently bonded atoms.

BUT, in an ionic compound, where the bond is theelectrostatic attraction between oppositely chargedions, each ion is bonded to each of its oppositelycharged neighbors!So, for any ionic compound, all the ions in the wholecrystal can be thought of as part of one single bonded unit. So the formula unit shows the lowest whole number ratio of the ions, rather than representing ALL the ions in the crystal.