4: Atomic and Molecular Structure Flashcards
coulombic potential does what
coulombic potential around the positive nucleus defines the energy and possible position of the electrons, which in turn define all possible properties of atoms and molecules.
second law of thermo determines
what happens or doesnt happen
overview of Bohr atomic model
- nucleus has Z = #protons, N = # neutrons
mass: A = Z+N
proton and neutron mass»_space; electron mass
standard probability distribution for the place of electrons (distance from nucleus vs probability )
what are the quantum numbers
n, l, ml, ms
n = principal #
l = angular momentum qn, l= 0, 1, 2… (n-1)
- l is limited by n
ex: n= 2: l = 0, 1
m = magnetic quantum #
- component of orbital angular momentum along a specific direction
m = -l,- (l-1), -(l-2)….0,…. (l-2), (l-1), 1
m has 2l+1 possibilities
ms: spin #, +/- 1/2
Schrödinger eqn
wave function = Y
prob density function = Y^2
Schrödinger eqn (total energy = kinetic + potential energy)
EY = HY
hydrogen atom soln of schrodinger
slide 11-13
orbital fillin principles
- aufbau
- hund’s rule
- pauli exclusion principle
aufbau: electrons occupy orbitals of increasing energy
Hund’s rule: electrons occupy all degenerate orbitals (same n and l quantum numbers) before putting two electrons in the same orbital
pauli exclusion principle: no two electrons have the same set of 4 quantum numbers
do elements have a stable electron configuration ?
most dont, bc valence shell is usually not filled completely, so there will be net attractive forces to a similar neighbour
noble gases have full valence shells, so theyre stable
what can we say abt elements of similar valence structure ?
similar properties
review orbital filling, and filling my periodic table
def electronegativity
tendency of a nucleus to pull an electron cloud towards it. depends on how shielded the nucleus is by the core electrons. it has important effects on chemical bonds
what side of periodic table has lower electronegativity ?
left
explain metallic bonding
atomic orbitals combine to form a delocalized electron cloud shared by a large number of electrons
def ionic, covalent bonds
Ionic: elements w greatly varying electronegativity bond, one takes neg charge, one takes pos
covalent: similar electronegativity, the electron pair becomes shared in molecular orbital
van der Waals bonding
hydrogen bonding
van der walls: weak bonding induced by fluctuating or permanent molecular dipoles
hydrogen bonding: bonding between hydrogen protons and an available electron pair
study all curves in notes
on a force vs interatomic seperation curve, where is the ro, what does the well represent ?
ro = the bond length at the point where the sum of forces = 0 (r at x intercept)
well = work done
what relationship do the force vs r and potential vs r curve share?
potential curve is the integral of the force vs r curve
what is the main type of bonding in ceramics
ionic
what material has covalent bonding
- polymers have it within each chain (but van der walls between each molecule)
- diamonds
covalent bond hybridization
combining of s and p orbitals to form set of equivalent orbitals for bonding
metallic bonding (longer explanation)
what does this bonding structure impact?
sea of delocalized valence electrons which move relatively freely around the ion cores constituted of the protons and core electrons. the bonds are non-directional.
this bonding structure has an impact on electrical thermal, mechanical properties
which bonds are directional? non-directional?
directional: covalent, secondary
non-directional: ionic, metallic
the force and potential created by chemical bonds influence what properties
- melting temp
- Elastic modulus
- thermal expansion coefficient
- hardness
- etc
describe how metallic bonding affects melting temperature. what main property influences melting temp
main parameter : depth of the potential well (Eo, bond energy)
if the well is deeper, higher Mp
if well is shallower, lower Mp
if bond energy is low, what does the E vs r curve look like ? what is melting point ?
low bond energy = shallow well = lower mp
melting temperature of a solid correlates directly with____.
bonding energy (Eo)
what property affects elastic modulus (referring to E vs r)
the curvature of the well.
a deeper well = higher curvature
describe how curvature and elastic modulus are related
higher curvature = high E
so visually, deeper well = higher E
what is the main parameter for thermal expansion. how does the parameter determine thermal exp
the symmetry of the well (alpha)
if the well is more symmetric, smaller thermal exp (small alpha)
if well is less symmetric, (big alpha), bigger thermal exp
note - deeper wells are more symmetric
def hardness.
what is it influenced by?
resistance of surface to plastic (permanent) deformation
influenced by height of the total force curve (thus, by Eo)
- (integral under the force curve)
materials with high Tm are hard or soft ? why?
high Tm = hard
because Tm is proportional to Eo and hardness is proportional to Eo
how does the nature of the bond affect electrical conductivity
ionic and covalent: poor conductors, electrons aren’t free to leave atoms used in bonding
metallic: low ionization energy, electrons are free within crystal lattice, good conduction
how does nature of bond affect thermal conductivity
- high in metallic (freely moving electrons)
- low in ionic (electrons aren’t free to leave atom used in bonding)
is a good electrical conductor always a good thermal conductor ?
yes
is a good thermal conductor always a good electrical conductor ?
no, not always, bc thermal conduction can happen 2 ways: electron transport and phonon transport.
so if a good thermal conductor is good bc of its phonon transport, it might not be good at electron transport, thus not a good electrical conductor
is bond energy large/small for ceramics, metals, polymers
ceramics: large
metals: variable
polymers: directional properties, so secondary bonding dominantes
ceramics:
- Eo, Tm, E, alpha
Eo = large
Tm = large
E = large
alpha = small
polymers:
- Eo, Tm, E, alpha
directional properties, secondary bonding dominates
Tm = small
E = small
alpha = large
electron band structure
when atoms are in a solid, they exert a field on the other atoms around them. thus, each atomic state splits in many closely spaced states, which forms an electron band
where does splitting for electron bands occur ?
how does effect change when close to nucleus?
first outer shells
effect is smaller towards nucleus
the number of energy states in splitting levels is proportional to ____.
number of atoms in a solid
def valence band
band having the highest energy electrons (valence electrons) at 0K
def conduction band
next band at energy greater than valence band
next free energy band above valence band, next available band
def fermi energy
at 0K, all electrons have energy smaller or equal to Ef
(limit between separation of energy from valence to conduction band)
how does electron band structure affect a material being a insulator, conductor, semi-conductor
some bands overlap, some dont, leaving a band gap between the valence and conduction band
at 0K, all electrons are at lowest levels possible (below Ef). electrons need energy to move to higher levels, some need more than others.
electron band structure for conductors (energy)
energy needed to move in a conduction region and become free electrons is VERY SMALL
electron band structure for insulators (energy)
a large energy band gap exists between the full valence band and the conduction region
electron band structure for semi-conductors (energy)
a small band gap exists between the full valence band and the conduction region
study resistivity
