Question

MCAT Tutorial: Basics of Quantum Chemistry

Answer 1

\({\bf{Atomic~Structure:}}\)
atoms: composed of protons + neutrons + electrons
protons: in the nucleus, +1 charge, 1 amu
neutrons: in the nucleus,0 charge, 1 amu
electrons: far away from nucleus, -1 charge, ~ 0 amu [mass is considered negligible for practical purposes]

\({\bf{Atomic~Properties:}}\)
- mass number: # protons + # of neutrons
- isotopes: atoms of the same element (same # of protons) w/ diff. # neutrons, named by saying the name of the element, then a dash, then the mass #, ex. carbon-14
- average atomic mass: weighted average of the mass of isotopes and their relative abundance

\({\bf{Atomic~Models:}}\)
- note: will most likely not have to worry about the plum pudding model b/c it's crap and nobody uses it
- Bohr model: planetary model where nucleus is made of protons + neutrons, w/ electrons "orbiting" the outside at fixed energy levels
- Schrodinger/Electron cloud model: the more accurate model, electrons are in clouds of high probability density for finding an electron

\({\bf{Emission~and~Absorption:}}\)
- E = hv (quantized energy, v = frequency, h = planck's constant)
- E = R_h (1/nf^2 - ni^2) where R_h = Rydberg constant, describes energy emitted/absorbed during electron transition
- general energy equation E = hc/lambda where h = planck's, c = speed of light, lambda = wavelength

- emission spectrum: bands that represent radiation given off when e- transitions from high to low energy
- absorption spectrum: low to high energy

- transitions to n = 1: Lyman
- transitions to n = 2: Balmer
- transitions to n = 3: Paschen

Answer 2

\({\bf{Quantum~Numbers:}}\)
n: principal, represents energy level, takes integer values from 1 to infinity

l: azimuthal/angular, represents orbital shape, takes integer values from 0 to (n-1)

> 0,1,2,3 --> s, p, d, f, respectively, probably won't have to worry about g+ orbitals

ml: magnetic, represents orbital orientation, takes integer values from -l to +l [that's a lower case l not a 1]

ms: spin, represents its behavior in a magnetic field (it's kind of complicated), takes either +1/2 or -1/2 as its values

Answer 3

\({\bf{Orbital~Diagrams:}}\)
- Aufbau's Principle: electrons fill lowest energy level before moving to higher energy levels [a lot of exceptions to this rule, primarily to fill unoccupied orbitals]
- Hund's Rule: electrons will preferentially remain unpaired (think of it like a bus where people will space themselves out and fill up seats individually before pairing up)
- Pauli Exclusion Principle: each electron has its own set of 4 quantum numbers

- order of orbitals: either memorize this pattern or use the ptable [have to know which elements correspond to which blocks/energy levels]

Answer 4

as noted there are several exceptions to the aufbau principle to fill up d orbitals

Answer 5

Some misc. definitions:
- diamagnetic: all electrons are paired [will be able to see this from the orbital diagram], repulsion from mag field
- paramagnetic: some electrons unpaired, attracted to mag field
- effective nuclear charge: relative attraction of the electrons to the nucleus, depends on proton # and shielding effects
- valence electrons: electrons with the highest energy level/furthest apart from the atom, involved in bonding
- noble gas configuration: take the nearest noble gas "below" the atom in question, write that noble gas in brackets, then write the remaining electron configuration as normal

Answer 6

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT