Chem chapter 8 exam 4 Flashcards
octet rule exceptions
odd-electron species
incomplete octet
expanded valance shell
odd electron species (octet rule exception)
-an octet can’t be achieved if there are an odd-number of electrons in a compound
-they have free radicals, or an unpaired electron, which makes the atom/compound more reactive
incomplete octet (octet rule exceptions)
boron is generally stable with 6 electrons
expanded valance shell (octet rule exceptions)
-elements (the central atom) in row 3 or lower can expand their valance shell to hold more than eight electrons
-can help reduce formal charges`
in organic lewis structures, the carbons…
connect to each other and the non-carbons connect to the carbons
electron geometry
the geometry of all of the atoms and the electrons in a compound
molecular geometry
the geometry of the atoms in a compound (still affected by the lone pairs)
how to determine electron groups
how many bonding pairs and lone pairs there are
for example, in NH3, the lewis structure has 3 lines and 1 set of lone pairs on the central atom, so they have 4 electron groups
2 electron groups
linear
3 electron groups
trigonal planar
4 electron groups
tetrahedral
5 electron groups
trigonal bipyramidal
6 electron groups
octahedral
molecular geometry: linear with zero lone pairs
] linear
] 180 degrees
molecular geometry: trigonal planar with zero lone pairs
] trigonal planar
] 120 degrees
molecular geometry: trigonal planar with one lone pairs
] bent
] <120 degrees
molecular geometry: tetrahedral with zero lone pairs
] tetrahedral
] 109.5 degrees
molecular geometry: tetrahedral with one lone pairs
] trigonal pyramidal
] <109.5 degrees
molecular geometry: tetrahedral with two lone pairs
] bent
] <109.5 degrees
molecular geometry: trigonal bipyramidal with zero lone pairs
] trigonal bipyramidal
] 120 degrees, 90 degrees
molecular geometry: trigonal bipyramidal with one lone pairs
] seesaw
] <120 degrees, 90 degrees
molecular geometry: trigonal bipyramidal with two lone pairs
] T-shaped
] 180 degrees, 90 degrees
molecular geometry: trigonal bipyramidal with three lone pairs
] linear
] 180 degrees
molecular geometry: octahedral with zero lone pairs
] octahedral
] 90 degrees
molecular geometry: octahedral with one lone pairs
] square pyramidal
] 90 degrees
molecular geometry: octahedral with two lone pairs
] square planar
] 90 degrees
when looking at a molecule with multiple central atoms, how can we find the electron geometry?
look at each central atom and count for lone and boding pairs
no or very little difference in electronegativity (EN)
covalent
moderate difference in EN
polar covalent
large difference in EN
ionic
ΔEN less than or equal to 0.4
covalent
ΔEN between 0.4 and 1.8
polar covalent
ΔEN greater than 1.8
ionic
in a polar covalent bond, the bonds…
involve unequal sharing of electrons, which will result in partial charges
- the more EN will have more of a (-) charge and the less EN will have more of a (+) charge
polar covalent bonds on pictures of the molecule
more blue means more (+) so less e-
more red means more (-) with means more e-
a polar bond dosent always mean the molecule is polar because it requires a … and a polar bond creates a …
-it requires a net dipole moment
-a polar bond creates a dipole, but the molecular geometry can cancel it out
for a polar molecule, there must be…
- must have a polar bond
- the polar bonds must not cancel out
(usually happens when the electron and molecular geometry match)
for example: o=c=o, the oxygens are more EN and pull on opposite sides of the carbon, but since there is a pull on both sides, they cancel out and the bond is non polar.
bond order
the number of bonding electron pairs shared by two atoms in a molecule
- single bonds: 1
- double bonds: 2
-triple bonds: 3
higher order bonds vs lower order bonds
higher order bonds: shorter and stronger
lower order bonds: weaker and longer bonds
why do we get short order bonds
more electrons shared between two elements results in more overlap between the electron clouds thus shorter
why do we have stronger order bonds
more electrons shared in bonds between two elements requires more energy to break thus stronger
a single bond is
the longest and weakest
a double bond is
in the middle with length and strength
a triple bond is
the shortest and strongest
bond length and atomic radii
when comparing between different pairs of elements depend on the atomic radii
- smaller elements make shorter bonds
bond order formula=
(number of shared pairs in all X-Y bonds)/ (number of X-Y links in the molecule or ion)
or think of it as number of lines divided by the groups of lines
average bond energy (KJ/mol)
the amount of energy to break a bond and these are endothermic since you need energy going into the compound to break the bond(s)
heats or reaction to get average bond energies
Σ(BE reactants)- Σ(BE products)
how can you tell if a reaction is endo or exothermic
-if the reactants have weak bonds and the products have strong bonds, exothermic
-if the reactants have strong bonds and the products have weak bonds, endothermic
