Unit 5: Covalent Compounds
8.1: Covalent Bonds
Bond formed from atoms sharing electrons
Covalent compounds are called molecules
Ionic compounds are called formula units
Covalent bonds form so that atoms become stable (octet in their valence shell because of the shared electrons)
Covalent compounds form between nonmetals
Diatomic elements are elements in nature that are bonded to themselves
H2 N2 O2 F2 Cl2 Br2 I2
A single covalent bond is when one pair of electrons is shared
A double covalent bond is when two pairs of electrons are being shared
A triple covalent bond is when three pairs of electrons are being shared
The strength of covalent bonds depends on the length of a bond, the shorter the stronger
Atomic radius makes the bond length increase
Bond length decreases as the number of bonds increases
Strength increases as the number of bonds between the carbon atoms increase
Bond Dissociation Energy is the amount of energy required to break a specific covalent bond
Bond dissociation energy is always a positive value
Breaking bonds is endothermic
The shorter the bond length the greater the bond dissociation energy
8.2: Naming Molecules
Covalent Compound naming
First element: write the name with the correct prefix based on subscript (never use mono)
Second element: write the name with the correct prefix based on subscript, change the ending to -ide (all prefixes are usable)
When there is o-o or a-o, first value is left out
Covalent Compound formulas
First element: write the symbol and add the subscript based on prefix (no prefix = no subscript)
Second element: write the symbol of the second element and add the subscript based on prefix
DO NOT REDUCE THE SUBSCRIPTS
Binary acids do not contain oxygen (HX)
Starts with the prefix hydro
Hydro_ic acid = -ide
Oxyacids contain oxygen (HXO)
Starts with the root of the anion
Never starts with hydro
_ous acid = -ite (sprite is delicious)
_ic acid = -ate (I ate something icky)
Hydrogen ions are added to the anion in order to make a neutral compound
Chapter 21: Hydrocarbons
Hydrocarbons contain only 2 elements (hydrogen and carbon)
Simplest hydrocarbons are called alkanes
Mnemonic for the first four prefixes
Monkeys - Meth (1)
Eat - Eth (2)
Peeled - Prop (3)
Bananas - But (4)
Alkanes formula: CnH2n+2
Alkenes formula: CnH2n
Alkynes formula: CnH2n-2
Alcohol formula: CnH2n+1OH
Unsaturated: if bond contains double/triple bonds
Saturated: if bond contains only single bonds
Straight-chain Hydrocarbons: contains any number of carbon atoms, one after the other, in a chain pattern
Cyclic Hydrocarbons: form a ring (draw a polygon with n sides), add prefix cyclo, do not follow alkane formula
Cyclic Alkanes
Cyclopropane
Cyclobutane
Cyclopentane
Cyclohexane
Cycloheptane
Aromatic Compounds: contain the benzene ring
Typically used to make dyes/moth repellent
Alcohol covalent bonding occurs with carbon, not ionic bonding with a metal therefore not basic
To name alcohols, drop the -ane, add -anol
ACIDS - If the compound starts with H; Use the naming acids rules.
ORGANIC - If the compound starts with C and contains quite a few H’s and perhaps some O’s; Use the naming organic compounds rules.
IONIC - if the compound starts with a metal or ammonium ion, it is most likely ionic; Use the naming ionic compounds rules.
COVALENT - If the compound starts with a nonmetal other than H or C; use the naming binary molecular compounds rules.
8.3: Molecular Structures
How to draw Lewis Structures
Sum the valence electrons from all the atoms.
If it is an ion - add an electron for each negative charge and subtract an electron for each positive charge.
Divide your sum by 2 to figure out the number of pairs
Determine your central atom - it is the least electronegative element (Usually occurs first and least in the formula)
Write the symbol for the central atom and connect the terminal ends (bonded atoms) with single bonds.
NOTE: Hydrogen can NEVER be the central atom
Complete the octets of atoms bonded to the central atom
NOTE: Hydrogen can only have a duet. It’s special like that.
If there are leftover pairs of electrons - place them on the central atom
NOTE: Central atom can hold more than an octet ONLY if the element is from periods 3-7
If you don’t have enough electrons to give the central atom an octet - try using multiple bonds
Resonance: a condition that occurs when 2 or more valid structures can be written
8.4: Molecular Geometry
VSEPR Theory: Valence Shell Electron Pair Repulsion
After pairing electrons repel each other in molecules
Unshared pairs (lone pairs) exert a greater repulsion force, they take up more space
How do you determine the geometry of a molecule?
Draw the Lewis structure
Add up bonding pairs & lone pairs on the central atom
Count double/triple bonds as ONE BOND
Molecular geometry is determined by the ratio of bonding pairs to lone pairs
Electron geometry is determined by the amount of bonds or lone pairs coming off the central atom
Linear
2 bonding pairs, 0 lone pairs
180 degrees
Molecular geometry: linear
Electron geometry: linear
Trigonal Planar
3 bonding pairs, 0 lone pairs
120 degrees
Molecular geometry: trigonal planar
Electron geometry: trigonal planar
Tetrahedral
4 bonding pairs, 0 lone pairs
109.5 degrees
Molecular geometry: Tetrahedral
Electron geometry: Tetrahedral
Trigonal Pyramidal
3 bonding pairs, 1 lone pair
106.7 degrees
Molecular geometry: Trigonal pyramidal
Electron geometry: Tetrahedral
Bent
2 bonding pairs, 2 lone pairs
104.5 degrees
Molecular geometry: Bent
Electron geometry: Tetrahedral
Trigonal Bi-Pyramidal
5 bonding pairs, 0 lone pairs
120 degrees equatorial, 90 degrees axial
Molecular geometry: Trigonal bi pyramidal
Electron geometry: Trigonal bi pyramidal
Octahedral
6 bonding pairs, 0 lone pairs
90 degrees
Molecular geometry: octahedral
Electron geometry: octahedral
Seesaw
4 bonding groups, 1 lone pair
90 and 120 degrees, 173 degrees axial
Molecular geometry: seesaw
Electron geometry: trigonal bipyramidal
T-shaped
3 bonding groups, 2 lone pairs
90 degrees, 175 degrees axial
Molecular geometry: t shaped
Electron geometry: trigonal bipyramidal
Square Pyramidal
5 bonding groups, 1 lone pair
90 degrees
Molecular geometry: square pyramidal
Electron geometry: octahedral
Square Planar
4 bonding groups, 2 lone pairs
90 degrees
Molecular geometry: square planar
Electron geometry: octahedral
8.5: Bond Character & Molecular Polarity
What determines molecular polarity?
Bond Character (evaluated using electronegativity)
Shape/symmetry of the molecule
Nonpolar Covalent: equal sharing (Zero)
Polar Covalent: unequal sharing because of electronegativity differences(Greater than zero, less than 1.7)
Ionic: complete transfer of electrons (Greater than 1.7)
Electronegativity: measure of the ability of an atom to attract electrons to form chemical bonds
More electronegative atoms have a greater pull on the electrons in their shared bond
Ionic Bond - the electrons completely taken by the nonmetal from the metal therefore there is a large difference in electronegativity
Covalent Bond - electrons are shared between two nonmetals, there is a smaller difference in electronegativity
Bond character is determined by calculating the electronegativity difference between both bonded atoms
All of the terminal atoms must be identical, if it is linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral, then its nonpolar
Square planar is the only structure that is symmetrical and has lone pairs
Shapes are not symmetrical when the central atoms have lone pairs or different terminal ends
Bent, trigonal pyramidal, seesaw, t shaped, and square pyramidal will be polar
Covalent bonds are strong but intermolecular forces are weak, therefore..
They have lower melting/boiling points
Many exist as gases or they vaporize easily
Relatively soft in solid state