Molecular Compounds

Oct. 21, 2008

Oct. 21, 2008

Review of Ionic Compounds

• Ionic compounds form when ions come together to balance out their overall charge.

• This way the atoms have a complete outer shell and have an overall charge of zero.

BUT, there is another way to attain a complete outer shell AND an overall charge of zero …

Oct. 21, 2008

Molecular Compounds

• Involve COVALENT BONDS (instead of Ionic Bonds)

• Electrons of neutral non-metal atoms are shared to form a molecule.

Oct. 21, 2008

Another Familiar Example • In order to create pure Water (H2O), Hydrogen

and Oxygen form covalent bonds by sharing electrons.

H

H

O

H

H

O

Both the Hydrogen and the Oxygen remain electrically neutral, AND have been able to completely fill their outer valence shell.

Shared electrons

Oct. 21, 2008

Other examples in your text:

Oct. 21, 2008

Formulas for Molecular Compounds

• The Combining Capacity is the number of covalent bonds that an atom would need to form a stable molecule.

H

H

O

H

H

O

Oct. 21, 2008

Example • Carbon has four electrons in its outer valence.

• It would need to share 4 more electrons with other atoms in order to fill its outer valence.

• Therefore it has a combining capacity of 4.

C

H

H

H

H

The molecule produced is Methane

Oct. 21, 2008

Combining Capacity of some non-metal Atoms

4 3 2 1 H

C N O F

Si P S Cl

As Se Br

I

Oct. 21, 2008

Chemical Formula

• Follow the same “shortcut” steps used for Ionic Compounds

• Use the COMBINING CAPACITY instead of the ionic charge.

Oct. 21, 2008

Example • How would you write the formula for a

compound formed between carbon and sulfur?

C S

1. Write the symbols for the two elements. Which one is written first this time?

2. Write the combining capacity of each element above the symbol.

4 2

3. Crisscross the combining capacity to produce subscripts.

The formula is C2S4

4. Reduce the subscripts if possible.

The correct formula is CS2

Oct. 21, 2008

Naming for Molecular Compounds

• Molecular compounds use prefixes to indicate the number of atoms in a molecule.

Prefix Number Example (formula)

mon(o) 1 carbon monoxide

di 2 carbon disulfide

tri 3 sulfur trioxide

tetra 4 carbon tetrafluoride

pent(a) 5 phosphorus pentabromide

CO

CS2

SO3

CF4

PBr5

Oct. 21, 2008

Oct. 21, 2008

Practice

• Write the names for: – CO2

– PF3

– As2S3

• Write the formulas for: – Sulfur dichloride

– Boron triiodide

– Diphosphorus tetraoxide

Textbook References

• Nelson:

– Section : 5.10

– Pages: 206 - 209

Oct. 21, 2008