A.P. Chemistry
Atomic Structure
The wave nature of light
Electromagnetic waves
Electromagnetic wave - A wave that consists of an electric field and a magnetic field that
are perpendicular to each other and to the direction the wave is moving.
The energy of an electromagnetic wave changes back and forth between the electric field
and the magnetic field as the wave moves, once an electromagnetic wave starts, it no
longer depends on the source (self propagating)
Origin – any time electrical charges are accelerating, an electromagnetic wave is
formed
Velocity – All electromagnetic waves move at the speed of light (c =3x108 m/s in air)
Energy – The energy of electromagnetic waves depends on the frequency
Electromagnetic Spectrum
Properties of waves
wavelength - the distance between crests on adjacent waves
frequency - the number of crests per second
velocity - the speed and direction of the wave
amplitude - the height of the wave above its equilibrium position
period – the reciprocal of the frequency
c= λf
Units : c = 3.0 x 108 m/s;  = m; f = Hz
Quantum nature of energy
E = ‘h f = ‘hc / λ
‘h = 6.63 x 10-34 J∙s = 4.14 x 10-15 eV∙s
Trix - ‘hc= 1240 eV∙nm
Photoelectric effect – light strikes the surface of certain metals and electrons are ejected
Bohr Model of the atom
negative: energy is increased as it is moved away from the nucleus
Energy levels
E = (-RH) (1 / n2)
n = principle quantum number
RH = Rydberg constant
2.18 x 10-18 J = 13.6 eV for hydrogen
∆E = Ef – Ei = ‘hf
Emission Spectra – characteristic set of bright lines emitted as electrons fall back to ground state
from higher energy levels
Absorption spectra – characteristic set of dark lines in the continuous spectrum as electrons jump
to higher energy levels (quantum leap)
Handout
Wave particle duality principle
- Light consisted of a stream of massless bundles of wave energy called photon
hence light has both energy and momentum
by analogy, matter has a wavelength . . . . . deBroglie wavelength
λ = ‘h/mv
Heisenberg Uncertainty Principle:
It1.is Enthalpy
inherently
impossible
know simultaneously
both
is an extensive
property to
– magnitude
of ΔH is directly amount
of
the exact momentum of an electron and its exact location
in space.
What this means with respect to electron orbits.
It is not appropriate to think of electrons moving in well-defined circular orbits about the
nucleus
Quantum mechanics and atomic orbitals
Orbitals and quantum numbers
Orbital ⇒ a specific distribution of electron density in space; each orbital has a
characteristic energy and shape
Unlike the Bohr model which used a single number, n, to describe an orbital, the quantum
mechanical model uses three quantum numbers, n, l, and ml .
-
Principle quantum number (n) –
1, 2, 3 --- as n increases, the size of the orbital increases
bigger n = more energy in the electron
-
Azimuthal quantum number (l) –
Integer between 0 and n-1 for each value of n
Defines the shape of the orbital (0 = s; 1 = l; 2 = d; 3 = f)
-
Magnetic quantum number (ml) –
Integer between -l and l (including zero)
Defines the orientation of the orbital
Electron Shell – collection of orbitals with the same value of n.
Subshells – collection of orbitals with the same n and l values
Example:
NOTE:
1. A shell with a principle quantum number n will consist of exactly n subshells
2. For a given value of l, there are 2l +1 values of ml
l = 0 . . . . s = 1 orbital . . . . . l = 1 . . . . p = 3 orbitals
l = 2 . . . . d = 5 orbitals . . . . . l = 3 . . . . f = 7 orbitals
3. The total number of orbitals in a shell is n2
n
1
2
3
4
n2
1
4
9
16
number of orbitals
2 (1s)
8 (2s and 2p)
18 (3s, 3p, and 3d)
32 (4s, 4p, 4d, and 4f)