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Atoms
The Basis of All Materials
RPI - ERTH 2330
The Atom
• Neutron - neutral particle
• Proton - positively charged particle
• Electron - negatively charged particle
E.B. Watson
Our story begins with light…
Visible light – part of the EM spectrum
Longer l
Higher f
v = lf, and v = 299,792,458 m / s (~3 E 8) in a vacuum
l is length of a cycle in m, f is Hz (cycles per second)
Wave Terms
FREQUENCY (f) number of cycles per unit time
[units = Hertz (Hz)]
1 Hz = 1 cycle/s
T = 1/f; f = 1/T; T f = 1
Refraction – producing constituent wavelengths
Dispersion
High f – more “bending” in prism
Refraction
Waves are bent
as the move
through materials
with different
wave
propagating
properties
E.B. Watson
The spectrum of the hydrogen atom
E.B. Watson
Violet 1
Violet 2
Green
Red
7.3E14 Hz
6.9E14 Hz
6.2E14 Hz
4.6E14 Hz
What are the
wavelengths (v=c)?
l=v/f
Photo-Electric Effect
Incident light causes movement of charge.
Electrons move from the surface of the
negatively-charged metal plate
The intensity of the light determines how many
electrons are produced, but not their kinetic energy.
Wavelength changes kinetic energy
PHOTONS
Small packets, or quanta, possessing specific
amounts of energy.
An incident photon is either totally absorbed by
"target" matter or not absorbed at all*
The energy of the photons in a monochromatic
beam of light
E = hf
h is (Planck’s constant 6.626 E -34 Js)
*Quantization of photons is like counting children you can’t have a partial child
Energy transitions in
Hydrogen
Ephoton = EH - EL = h f
Balmer relationships
Hydrogen spectra
1
1
 R  2  2
2
l
n 
1
Line
n
Violet
6
Violet
5
Blue-green 4
Red
3
Any hot gas at low pressure will produce a line
spectrum, although the spectra of other gases
are more complex than that of hydrogen.
Bohr model - a "solar-system" model.
assumptions:
•an electron is in specific "allowed" orbits
•the allowed orbits are described by
mvr=nh/2p
Planck’s const.
Angular momentum
F = m v2 / r
and
F = k q1 q2 / r2
Bohr saw this as charged particles on
circular path set the forces equal
Bohr specific radii for electrons
E.B. Watson
j
k
l
It requires energy to remove electrons
Energy increases with orbital distance.
Lowest energy configuration – ground state
Energy level at
quantum number
Quantum number
EL
En  2
n
Energy level at
ground state
EL
En  2
n
You can use this to determine changes in
the amount of energy
E L EG
E n  2 
n
1
Describes the change in energy of an electron moved
from 1st orbital (the ground state in H) to the n orbital
E.B. Watson
Electrons are attracted to the protons in the nucleus.
Great, how big is the nucleus?
E.B. Watson
E.B. Watson
E.B. Watson
Elements differ from one another by the number
of protons they contain (Z)
Hydrogen (H) – 1 proton
Helium (He) – 2 protons
Sodium (Na) – 11 protons
Francium (Fr) – 87 protons
A neutral atom will have as many electrons as
protons.
Many atoms for each element will have the same
number of neutrons as protons
Elements of the same Z that differ in the number
of neutrons are isotopes.
Atoms
Mass
Charge
Electron
9.109 E -31 kg
(-1) 1.602 E -19 coul.
Neutrons
1.673 E -27 kg
(0) None
Proton
1.673 E -27 kg
(+1) 1.602 E -19 coul.
Z = number of protons
N = number of neutrons
A = atomic mass number (N + Z)
Element = unique Z
Isotope = unique Z, different N
What happens to A?
14C
Isotope
Same Z, different N
Carbon (C ) Z = 6
Carbon-12
Carbon-13
Carbon-14
12C
13C
14C
6 protons, 6 neutrons
6 protons, 7 neutrons
6 protons, 8 neutrons
Atomic mass units (u)
Every isotope is scaled to 12C (12.000 u)
98.89%
1.11%
trace
The total mass (“atomic weight”) of any
element is the sum of the weighted mass of its
isotopes.
For example, Oxygen has three isotopes
mass (rel 12C)
Abundance
16O
15.99491
99.759%
17O
16.99914
0.037%
18O
17.00016
0.204%
(15.994191 x 0.99759) + (16.99914 x 0.00037)
+ (17.00016 x 0.00204) = 15.9994 u
•The interactions of electromagnetic radiation and
electrons reveal the energy structure of the atoms
•The interaction of charged nuclei reveal the size of
atomic nuclei
•The number of protons(+) determines the element’s
identity
•It also determines the number of electrons (-)
•The number of electrons controls the behavior of the
atom
•The number of neutrons may vary - isotopes
•Atomic mass is normalized to 12C
•Atomic mass for an element is a function of isotope
abundance and mass.