Week 2: Flashcards
Periodic Table:
quantization of energy level in atoms into
orbitals and shells (n) and subshells (s,p,d,f)
ordering of the elements by the number
of electrons in their valence shell.
empirically realized by Dmitri Ivanovich
Mendeleev (1834-1907) long before quantum
mechanics were discovered/developed
The Aufbau Principle:
Aufbau: building up
- Atomic orbitals are filled starting from the lowest
to higher energies with quantum number n
defining a shell and l defining a subshell
Madelung Rule:
Orbital energies increase as (n+l) increases. For two
orbitals with the same (n+l), the one with the smaller n lies lower in
energy
drop in energy is not linear with increasing Z (diagram with wavy lines)
e curves cross each other, meaning the order of filling
the orbitals is not only dependent on the quantum numbers.
* Suttle differences are due to shielding and penetration, which we
will discuss in just a moment.
Three rules give us the proper order for interactions between electrons:
- Electrons are placed in orbitals to give the lowest total electronic
energy. Quantum numbers n and l are filled first. - Hund’s rule of maximum multiplicity requires that electrons be
placed in orbitals to give the maximum total spin possible
(maximum numbers of parallel spins). Spin multiplicity is number
of unpaired electrons plus 1 (see text table 2.6). - The Pauli exclusion principle requires that each electron in an
atom have a unique set of quantum numbers. Two electrons in the
same orbital must be spin correlated, i.e. have opposite spins.
Orbital Stabilization:
As the nuclear charge increases
and the electron repulsions, the
3d and 4s levels both become…
…more stable, but 3d moves down
in energy faster than 4s.
-The levels cross each other at
some point. 3d starts higher but
then moves lower than 4s and the
complex way in which the orbital
energies change with increasing
nuclear charge accounts for the
orbital filling.
Effective nuclear charge:
net positive charge experienced by an electron
in a polyelectronic atom.
Z* = Z - S
Z* = effective nuclear charge, Z = nuclear charge, S = shielding parameter
There are two effects that result in an effective nuclear charge:
1) The direct shielding of the outer electrons from the nuclear charge by the
“inner” electrons.
2) The penetration of “inner” electron density by “outer” electron density.
calculation of the energy:
-Pwermitted by Slaters rules:
E = -13.6 (Z/n)2
Ionization Energy:
Energy required to remove an electron from a gaseous atom
or ion
- Affected by shielding effects.
- Generally, observe an increase in ionization energy as the nuclear charge
increases
Electron Affinity:
Energy required to remove an electron from a negative
ion (endothermic process)
Atomic Radii:
As nuclear charge increases, the electrons are pulled in towards the
center of the atom, and the size of any particular orbital decreases. The opposite is
true for an increase in nuclear charge.
Ionic radius:
Radius of a monoatomic ion in an ionic crystal structure.
Covalent radius:
Measure of the size of an atom that forms part of one covalent
bond.
Van der Waals radius:
radius of an imaginary hard sphere representing the
distance of closest approach for another atom.
The covalent radius is half of the inter-nuclear distance between two identical (or
almost) atoms bonding by a single covalent bond.
- The Van der Waal’s radius is half of the inter-nuclear distance between the nuclei
of two non-bonding adjacent atoms belong to different molecules.
Cations are always smaller than the…
neutral atom and anions are always larger.
- Cations are always smaller than the neutral atom and anions are always larger.
Electronegativity: (Linus Pauling)
(“The Nature of the Chemical Bond, 1948):
“the power of an atom in a molecule to attract electrons to itself
- Pauling noticed that the bond energies of polar bonds (A-B) were greater than
the average of the bond energies of the two homonuclear species (avg. A-A &
B-B). - Derives values for electronegativity P by comparing bond energies:
Electronegativity: (Mulliken)
- Derives values for an Absolute Electronegativity M from ionization energy and
electron affinity of atoms or ions
The absolute electronegativity (x) is
defined as the change in energy of the
system as a function of the number of
electrons N present. E.g., for a neutral
species, N → N+1 = EA and N → N-1
= IP, (x) is defined as the negative
tangent at N to the curve shown above
Absolute Hardness:
Resistance of the chemical potential to change the number of
electrons. The harder a chemical species, the more difficult it will be to change its
oxidation state.
n = 0.5(IE - EA)
Polarizability:
An atom’s ability to be distorted by an electric field.
- Atoms are highly polarizable if their electron distribution can be distorted readily.
- High polarizability is most likely to occur when unfilled atomic orbitals lie close in
energy to the highest occupied atomic orbital(s), i.e. when there is a small energy
difference between HOMO and LUMO. - Closely separated frontier orbitals are typically found for large, heavy atoms and
ions, whereas small, light atoms typically have widely spaced energy levels
Fajan’s Rules: can be used to
predict whether…
a chemical bond
is expected to be predominantly
ionic or valent, and depend on
the relative charges and sizes of
the cation and anion.