 Covalent Bonds

2 Ways Elements Want To Be Like a Noble Gas  Gain or lose electrons to form an ionic compounds  Share electrons with other elements to form covalent bonds

How/Why Does This Happen?  Covalent Bonds:  Both elements have similar electro- negativities so neither will give electrons to the other.  As a result, they’re forced to share electrons

How do 2 nonmetals share electrons?  Let’s look at what happens when two fluorine atoms bond:  Because nonmetals bond by sharing valence electrons, they’re called “covalent compounds”.

Definitions:  Covalent Compound: A compound formed when nonmetals bond by sharing 2 or more valence electrons.  Covalent Bond: A chemical bond formed when nonmetal atoms share two valence electrons

Oxygen Bonds With Oxygen  Double bonds involve 4 shared electrons  Double bonds are stronger than single bonds!  Called “Bond Dissociation Energy”

More examples of covalent bonding  N 2 shares 6 electrons causing triple bonds  Have higher bond dissociation energies

Properties of Covalent Compounds  All properties of covalent bonds are determined by the fact that covalent compounds form molecules.  1. Covalent Bonds have low melting and boiling points.  Why? Because these molecules have very weak forces called “Van der Waals” forces

Properties of Covalent Compounds  2. Covalent compounds are soft and squishy.  3. Covalent compounds usually don’t dissolve in water as well as ionic compounds.  4. Covalent compounds do NOT conduct particles.  Since there are no charged particles & no delocalized electrons, they can’t conduct electricity at all  5. Covalent compounds sometimes burn when containing Carbon or Hydrogen.

Naming Covalent Compounds  All names have two words:  The first word is the same as the name of the first element in the formula.  The second word is the same as the name of the second element with “-ide” at the end.  Use prefixes in front of each word to indicate how many of each atom is present # of AtomsPrefix 1Mono- 2Di- 3Tri- And so on until 10…And so on ‘til Deca-

Exceptions to these rules…  Water – H 2 O  Ammonia – NH 3  Methane – CH 4  Elements: If a compound contains only one element, the name of the compound is the same as the name of the element.  F 2 - Fluroine

Examples  PCl 3 – Phosphorus Tribromide  S 8 – Sulfur  N 2 S 3 – Dinitrogen Trisulfide  CO – Carbon Monoxide

Naming acids (Compounds that start with H)  For acids without OXYGEN, the name is written “hydro[anion]ic acid”.  Examples:  H 2 S = “hydrosulfuric acid”  HBr = “hydrobromic acid”  HCN = “hydrocyanic acid”

Naming Acids with Hydrogen  For acids that conatin oxygen (oxyacids): The acid name is “[anion name][suffix] acid”.  The suffix depends on the name of the anion:  If it ends in “-ate”, the suffix is “-ic”.  HNO 3 = nitric acid  H 2 SO 4 = sulfuric acid  H 3 PO 4 = phosphoric acid  If it ends in “-ite”, then the suffix is “-ous”.  HNO 2 = Nitrous Acid  H 2 SO 3 = Sulfurous Acid

3 Types of formulas  Molecular formulas: Tell you how many of each type of atom are present in a molecule  Example: C 6 H 6 has 6 carbon atoms and 6 hydrogen atoms.

Empirical formulas: Reduced molecular formulas that tell you the ratios of the elements to each other. Example: C 6 H 6 reduces to CH.

 Structural formulas: Formulas that not only tell you how many of each type of atom are present, but also tell you where they are.

How to draw structural formulas: Lewis Structures  Example: CH 4 1. Count the total number of valence electrons in the molecule.  C: 4 electrons x 1 atom = 4 valence electrons  H: 1 electron x 4 atoms = 4 valence electrons  Total = 8 valence electrons

2. Find the number of “octet” electrons for the molecule.  Hydrogen ALWAYS want 2 octet electrons!  Beryllium ALWAYS wants 4 octet electrons!  Boron wants 6 electrons for neutral molecules, 8 if it’s in an anion.  For our example:  C: 8 octet electrons x 1 atom = 8 octet electrons  H: 2 octet electrons x 4 atoms = 8 octet electrons  Total = 16 octet electrons

3. Subtract the number of valence electrons from the number of octet electrons to find the number of bonding electrons.  Example: 16 – 8 = 8 bonding electrons 4. Divide the # of bonding electrons by 2 to find the # of bonds in the molecule.  Example: 8/2 = 4 bonds

5. Draw an arrangement for the atoms that has the # of bonds you found in Step 4 and follow these rules.  Hydrogen and halogens ALWAYS bond once.  Oxygen’s family and Beryllium bond twice in a neutral molecule.  Nitrogen’s family and Boron bond 3 times in neutral molecules.  Carbon’s family ALWAYS bonds 4 times.

6. Add pairs of electrons to the structure until all atoms have the number of electrons around them that we said they needed in Step 2. Handy Hint: You NEVER need to add any pairs of electrons to carbon’s family, the halogens, or hydrogen

Example Practice!  Ammonia  Carbon Dioxide  NH 4 +1

Resonance Structures  When more than one possible valid Lewis structure can be drawn to express the structure of a molecule.  Resonance structures can only be found in molecules containing double or triple bonds.