chem midterm studying

valence bond theory

1. when atoms approach each other for bonding, the atomic orbitals of one atom overlap with the atomic orbitals of the other atom
2. each of the atomic orbitals that overlap should have one electron each with opposite spins
3. after the orbitals overlap. a air of electrons occupy the overlapped region

strength of bond

the strength of the bond depends on the extent of orbital overlap. the greater the orbital overlap, the closer the nuclei are to the bonded electrons, and the stronger the bond

hybridization

certain atomic orbitals inside an atom mix with each other to form a new set of orbitals. the process of orbital mixing is called hybridization and the new set of orbitals formed are called hybrid orbitals

features of hybridization

1. the number of hybrid orbitals formed is exactly equal to the number of atomic orbitals combined

2. the hybrid orbitals formed are equivalent in shape and energy

3. the hybrid orbitals are more effective in forming bonds than unhybridized orbitals

4. the hybrid orbitals formed orient themselves in 3d space to maximize the distance between them and minimize the repulsions between the electrons

sp hybrid orbital

sp2 hybrid orbital

sp3 hybrid orbital

sp3d hybrid orbital

sp3d2 hybrid orbital

sigma bond

a bond formed when two atomic orbitals combine to form a molecular orbital that is symmetrical around the axis connecting the two atomic nuclei

pi bond

pi bond is than a sigma bond

weaker because the sideways overlap is less effective than the end-to-end overlap

double bond consists of

one sigma bond and one pi bond

triple bond consists of

one sigma bond and two pi bonds

the polarity is measured in , units

dipole moment, debye (D)

for molecules with more than 2 atoms, overall molecular polarity depends on 2 properties

1. individual bond polarity

2. shape of the molecule

a molecule is polar if

it contains one or more polar bonds AND the individual bond dipoles do not cancel

boiling point is proportional to intermolecular forces

directly, the stronger the forces the more energy it takes to separate atoms

intermolecular forces

arise from the attraction between molecules with partial charges, or between ions and molecules

intermolecular forces are _ than bonding forces because

weaker because q1 and q2 are low (charges) and r value is large (distance from one molecule to another is larger than one atom to another)

dipole-dipole forces

attractions between oppositely charged regions of polar molecules

greater dipole moment = dipole-dipole forces = _ boiling point

higher, higher

what kind of molecule exhibits dipole-dipole forces

any molecule with a net dipole moment

hydrogen bonding is possible for molecules that have…

a hydrogen atom covalently bonded to a small, highly electronegative atom with lone electron pairs (specifically N, O, or F)

an intermolecular hydrogen bond is the attraction between…

the H atom of one molecule and a lone pair of the N, O, or F atom of another molecule

hydrogen bonds increase

boiling point

H bonds are stronger than

dipole-dipole forces

when atoms are close together, the instantaneous dipole in one atom induces a

dipole in the other

instantaneous dipole

temporary dipole that occurs for a brief moment in time when the electrons of an atom or molecule are distributed asymmetrically

dispersion forces or london forces

arise when an instantaneous dipole in one particle induces a dipole in another, resulting in an attraction between them

exist in all particles, increasing the energy of attraction in all matter

stronger in more polarizable particles

in general larger particles are more and experience __ dispersion forces than smaller ones

polarizable, stronger

polarizability correlates closely with __ for similar particles

molar mass

What couldn't be explained by valence bond theory and lewis dot structures?

the paramagnetic nature of molecular oxygen:

associated with presence of unpaired electrons

VB theory and Lewis dot do not show unpaired electrons between 2 Os

molecular orbital (MO) theory

delocalized electrons

molecular orbitals in molecules similar to atomic orbitals in atoms

when individual atoms combine the atomic orbitals on the bonding atoms combine to form molecular orbitals

the molecular orbitals formed are spread out over the entire molecule

Linear Combination of Atomic Orbitals (LCAO)

The mathematical process of combining atomic orbitals to generate molecular orbitals

combination of atomic orbitals to form molecular orbitals can be described as

combination of atomic wave functions to form a molecular wave function

constructive interference

Ψa + Ψb = Ψab (bonding molecular orbital)

destructive interference

Ψa - Ψb = Ψab*

of atomic orbitals combined =

molecular orbitals created

electron present inside bonding MO has ___ compared to electron present in atomic orbital where it is attracted by only one nucleus

lower energy

electrons in the antibonding molecular orbital are

excluded from the region between the nuclei

electrons in the antibonding molecular orbital __ the molecule

destabilize because they repel each other more than they attract the nuclei

energy between atomic orbital and bonding MO =

energy between atomic orbital and antibonding MO

how do we fill MOs

with the same rules as atomic orbitals:

1. filled in order of increasing energy (Aufbau principle) 2

2. MO can hole a maximum of 2 eletrons with opposite spins (Pauli exclusion principle)

3. degenerate orbitals are half-filled, with spins parallel, before pairing spins (Hund's rule)

electrons in the antibonding MO have more energy because

because of node between nuclei, zero electron density there

lack of electron density between the nuclei results in stronger repulsion between the nuclei, as the electrons are not there to shield them from each other, which destabilizes the bond and raises energy

as number of electrons in a bonding MO increases what else increases?

stability of the molecule. vice versea for electrons in antibonding orbitals

bond order =

1/2 (# bonding electrons - # antibonding electrons)

the higher the bond order,

the stronger the bond

mole

6.022 x 10^23 entities (Avogadro's number)

one mole is the amount of a substance that contains the same number of particles or entities as there are atoms in exactly 12g of the Carbon 12 isotope

molar mass

the mass of one mole of a substance in grams

numerically equal to atomic mass but expressed in grams

molar mass of compounds

sum of all the molar masses of all the atoms in a molecule

mass percent

percentage mass of a particular element present in a compound

percent composition of compound

molecular formula shows

1. type of each element

2. the actual number of atoms of each element

present in a molecule of the compound

empirical formula

shows the simplest whole-number ratio of various elements in a compound

relationship between molecular formula and empirical formula

molecular formula is a whole-number multiple of the empirical formula

relationship between molar mass and empirical formula mass

molar mass is a whole-number multiple of the empirical formula mass

steps to determine molecular formula from experimental data

1. determine the empirical formula from the masses or mass percent values of various elements present in a compound

2. determine molecular formula from molar mass of the compound and empirical formula