Chemistry

10/21/11

Brainteaser

• Name these ionic compounds:– Fe2S3

– Ca(OH)2

– MnCrO4

• Write the correct formula of these compounds– Zinc sulfate– Magnesium nitrate– Potassium permanganate

Gaining an Octet

• There are two ways that elements want to be like the nearest noble gas:

1) Gain or lose electrons to form an ionic compound.

 

2) Share electrons with other elements to form covalent compounds.

How does this happen?• Whenever two nonmetals bond to each other,

electrons don’t get transferred because both elements have similar electronegativities (i.e. they both want to gain electrons to be like the nearest noble gas).– In ionic bonding (as with NaCl, Cl is electronegative

and Na isn’t, so Na doesn’t mind giving electrons to Cl). This way, both fill their octets.

– In covalent bonding (as with F2), both elements have similar electronegativities so neither will give electrons to the other. As a result, they’re forced to share electrons.

Definitions

• Covalent compound: A compound formed when nonmetals bond by sharing two or more valence electrons.–Share valence electrons co”valent”

compound

• Covalent bond: A chemical bond formed when nonmetal atoms share two valence electrons.

The The OctetOctet Rule and Rule and Covalent CompoundsCovalent Compounds

Covalent compounds tend to form so that each atom, by sharing electrons, has an octet of electrons in its highest occupied energy level.

Covalent compounds involve atoms of nonmetals only.

The term “molecule” is used exclusively for covalent bonding

The The OctetOctet Rule: Rule: The Diatomic Fluorine Molecule The Diatomic Fluorine Molecule

F

F

1s

1s 2s

2s 2p

2p

Each has sevenseven valence

electrons

F F

The The OctetOctet Rule: Rule: The Diatomic Oxygen Molecule The Diatomic Oxygen Molecule

O

O

1s

1s 2s

2s 2p

2p

Each has sixsix valence

electrons

O O

The The OctetOctet Rule: Rule: The Diatomic Nitrogen MoleculeThe Diatomic Nitrogen Molecule

N

N

1s

1s 2s

2s 2p

2p

Each has fivefive valence

electrons

N N

Lewis structures show how valence Lewis structures show how valence electrons are arranged among atoms in a electrons are arranged among atoms in a molecule.molecule.

Lewis structures Reflect the central idea Lewis structures Reflect the central idea that stability of a compound relates to that stability of a compound relates to noble gas electron configuration.noble gas electron configuration.

Shared electrons pairs are covalent bonds Shared electrons pairs are covalent bonds and can be represented by two dots (:) or and can be represented by two dots (:) or by a single line ( - )by a single line ( - )

Lewis StructuresLewis Structures

Properties of Compounds

• All properties of covalent compounds are determined by the fact that covalent compounds form molecules, while ionic compounds form crystals.

• A good model for thinking of this:–Ionic compounds are like stacks of Legos all

locked together into a big block.–Covalent compounds are like rubber balls

thrown together into the same bucket.

Properties contd.• Low Melting and Boiling Points

–In covalent compounds, the molecules only have very weak forces (called Van der Waals forces) holding them to each other. As a result, covalent compounds can be found as solids, liquids, or gases at standard room conditions.• BONDS ARE NOT BROKEN WHEN THERE IS A

STATE CHANGE!!!

• How is this different than ionic compounds?–Magnetically stuck together in big crystals

Naming Binary Covalent Compounds

• Write the name of the first nonmetal• Write the name of the second nonmetal with the

ending changed to the suffix “–ide.”• Insert prefixes into the name to reflect subscripts

in the formula.mono =1 hexa= 6di = 2 hepta= 7tri = 3 octa = 8tetra = 4 nona= 9penta = 5 deca = 10

• Never start a name with the prefix “mono-”

Practice

• Name the following binary molecular compounds– CCl4– CO

– SO2

– NF3

Practice

• Write the formula of these binary molecular compounds– Dihydrogen monoxide– Tricarbon tetrachloride– Diphosphorus pentoxide– Sulfur dioxide

Review

• Rules for naming ionic compounds– Metal (+) and Non-metal (-)– Monatomic (only one atom present)

• O2-

– Polyatomic (more than one atom present)• Example: NO3

-

• Rules for naming covalent compounds– Non-metal(-) and Non-metal(-)

• Hydrogen is a non-metal (if it comes after a non-metal)– CH4

– Use Prefixes (Never start with Mono in the first non-metal)