Covalent Bonds – The nice bonds that share!

How do we know the type of bond that will be formed? Simplistically, Ionic bonds occur when electrons are given and taken, and covalent bonds occur when electrons are shared. However… there is a bit more to the story. There’s really a spectrum of bonding, from taking to partial sharing, to unequal sharing, to complete sharing of valence electrons.

What determines what really happens to electrons in a chemical bond? Electronegativity of an atom determines what happens to electrons in a bond! (Remember electronegativity = the ability of an atom to attract electrons) To determine the bond type, you have to find the electronegativity difference between the atoms.

Bond Polarity Most bonds are a blend of ionic and covalent characteristics. Difference in electronegativity determines bond type.

How do I find Electronegativity difference? Use your green electronegativity chart to determine the difference of electronegativity between atoms (remember to subtract big-small, and that the electronegativities for all atoms are listed on the purple sheet)

Electronegativity differences and their bond type Once you know the electronegativity difference, you can determine the type of bond the atoms will form. See the chart below: Bond Type Electronegativity Difference Ionic Greater than 1.7 Polar Covalent 1.7 ≥X > 0.3 (less than or equal to 1.7, greater than 0.3) Covalent (pure /non-polar) <0.3 (less than or equal to 0.3

Bond Polarity Examples: NaCl 3.0-0.9=2.1 Ionic HCl 3.0-2.1=0.9 Polar 3.0-3.0=0.0 Non-Polar

Lets Classify some bond types based on electronegativity. Ni and Sb C and Br H and O K and Cl

General properties of Covalent Compounds

Molecules and Molecular Compounds Compounds that are NOT held together by an electrical attraction, but instead by a sharing of electrons. Atoms held together by a sharing of electrons are joined by a covalent bond.

Molecules and Molecular Compounds A molecule is a neutral group of atoms joined together covalent bonds. A compound composed of molecules is called a molecular compound. The chemical formula for a molecule is called the molecular formula.

Properties of Molecular Compounds Composed of two or more nonmetals. Usually gases or liquids at room temperature.

Properties of Molecular Compounds Molecular compounds tend to have a relatively lower melting and boiling point than ionic compounds. Reason: There are no (or few and weak) bonds holding the molecules together in molecular compounds. Molecular Compound Ionic Compound

Properties of Molecular Compounds Do not conduct electricity. They form nonelectrolytes in solution. Reason: Molecular compounds do not break apart into ions in solutions.

Comparing Bonds IONIC COVALENT Bond Formation e- are transferred from metal to nonmetal e- are shared between two nonmetals Type of Structure crystal lattice true molecules Physical State solid liquid or gas Melting Point high low Electrical Conductivity yes (solution or liquid) no Other Properties Low volatility Odorous and volatile

Properties of Polar Covalent bonds Unequal sharing of a pair of electrons Form a molecule, but since unequal sharing, one atom has a stronger pull on the electrons than on the other. Intermediate strength: 88% of all bonds (Ionic bonds are strongest, 10% of all bonds) Soft, solids, and liquids.

Properties of Polar Covalent Bonds Even though the electrons are shared, the fact that the electrons are more attracted to the Cl atom results in a partial negative charge.

+ - Bond Polarity Polar Covalent Bond e- are shared unequally asymmetrical e- density results in partial charges (dipole) + -

Water is a polar molecule– you need to know this handy fact!

Properties of Non-Polar Bonds Equal sharing of electrons Weakest bond type, about 2% of all bonds Usually identical atoms or nonmetals very close on periodic table.

Oil is a non-polar compound. You need to know this!

Like Dissolves Like The rule “like dissolves like” refers to the fact that nonpolar substances do NOT mix with polar substances. Polar mixes with polar. Ionic compounds also mix with polar substnaces Nonpolar mixes with nonpolar. Example: Oil is nonpolar and does not mix with water which is polar.

Bond Polarity Nonpolar Polar Ionic View Bonding Animations.

Covalent Bonding and the Octet Rule Compounds that are NOT held together by an electrical attraction, but instead by a sharing of electrons. Atoms attain an octet (also called noble gas electron configurations) by sharing electrons. The bonds that form from this sharing can be single, double or triple. Triple bonds are shorter and stronger than double bonds, which are shorter and stronger than single bonds.

To draw Lewis structures for covalent bonds, use the NAS method: N (Needed): Find the number of electrons needed to form full octets for all elements involved. For most nonmetals, the number needed is 8. Hydrogen is the exception, it needs only 2. A (Available): Find the number of electrons available by adding up all of the valence electrons for all elements involved. S (Shared): Subtract the two numbers. S= N-A A bond is formed with two electrons, so divide by two to tell you how many bonds to draw between the elements. Draw the molecule. Put first atom in the center. H’s are always outside. Draw in the bonds, then fill in the rest of the electrons. Check to ensure all atoms have a full octet.

H• •H

Example #1: CH4

Example #2: CO2

Example #3: N2

Naming Binary Molecular Compounds (2 elements) When naming molecular compounds Greek prefixes (listed on your yellow periodic table) are used. The least electronegative element is named first and the most electronegative element is named second. Put prefixes in front of element names to tell how many atoms are there and change the ending to –ide. The prefix mono is only used with oxygen.

Example #1: N2O4 :_________________ Example #2: SF6 :___________________ Example #3: diphosphorus pentoxide:______ Example #4: phosphorus pentachloride :_______ Greek Prefixes Mono – 1(Oxygen only) Hexa – 6 Di – 2 Hepta – 7 Tri – 3 Octa – 8 Tetra – 4 Nona – 9 Penta – 5 Deca – 10