1. What are the structural differences between ionic and molecular compounds? 2. How do those differences affect their chemical formulas?

The chemical formula for a covalent compound is called a molecular formula; The chemical formula for an ionic compound is called a formula unit.

The chemical formula for a covalent compound is called a molecular formula; The chemical formula for an ionic compound is called a formula unit. Example: H 2 O is the molecular formula of water Example: NaCl is the formula unit of common table salt

What’s the dif?

A nice VISUAL look at the difference will help out:

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

NaCl does not exist as a 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 bonded to six oppositely charged neighbors:

Let’s shrink the ions so we can take a peek inside

Let’s shrink the ions so we can take a peek inside

We’ll take a look at this Cl - ion.

How many Na + ions is the Cl - close enough to bond with?

We’ll start again from the head-on view

We’ll start again from the head-on view

And, how many Cl - ions is each Na + ion bonded 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 are part of one single bonded unit, and the crystal below would be Na 32 Cl 32.

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 for the sodium chloride crystal. NaCl is the smallest whole number ratio 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 for the sodium chloride crystal. NaCl is the smallest whole number ratio 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 H 2 O molecule remains as a single unit of three covalently bonded atoms.

In large amounts of H 2 O, any attraction between H 2 O molecules is due to weak forces, and so the molecules behave as individual units, not one large bonded unit as in an ionic crystal.

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

NaCl This difference in structure between molecules and ionic compounds is reflected in their symbols: versus H2OH2O 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 by the formula are all covalently bonded. In a collection of those molecules, the molecules are attracted to each other by weak forces, but no covalent bonds occur between molecules--the molecules exist as individual units of covalently bonded atoms. BUT, in an ionic compound, where the bond is the electrostatic attraction between oppositely charged ions, each ion is bonded to each of its oppositely charged neighbors! So, for any ionic compound, all the ions in the whole crystal 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.