Untitled Flashcards Set

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