Atomic structure and periodicity Flashcards
The behaviors of the verysmall
Electrons are incredibly small
Electrons behavior determines much of the behavior of atoms
Directly observing electrons in the atom is impossible; the electron is so small that observing it changes its behavior
Why Quantium mechanics?
Classical mechanics (Newton’s mechanics) and Maxwell’s equations (electromagnetic theory) can explain MACROSCOPIC phenomena such as motion of balls rockets
Until the beginning of the 20th century it was believed that all physical phenomena were deterministic (definite, predictable future)
The beginning of quantum mechanics
Quantum mechanics: used to explain microscopic phenomena such as photon-atom scattering and flow of the electrons in a semiconductor
Work done at that time by many famous physicists discovered that for sub-atomic particles, the present condition does not determine the future conditions
What is the best tool for studying the structure of atoms?
Electromagnetic radiation
One of the ways energy travels through space is through electromagnetic radiation
What we call light, is just one form of electromagnetic radiation
It is importantly to know the properties of light in order to understand how atomic structure is revealed by electromagnetic radiation
Wavelength
The distance between two successive rests
Units: meters or nanometers
Frequency
The number of waves passing a certain point over a unit of time
Units: s^-1 = Hz
Amplitude
Height of the wave measured from the axis of propagation, a measure of intensity
Equation
c = hv
h = c/v
v = c/h
Color
The color of light is determined by its wavelength or frequency
White light is a mixture of all the colors of visible light
A spectrum
Red, orange, yellow, green, blue, indigo, violet
Interference in two overlapping light waves
When two or more waves meet the can interact (interfere) either constructively or destructively
Diffraction
Before the 19th century very little was known about the nature of light and one the great debates about light was over the question of whether light was made of a bunch of “light particles” or whether light was a wave
When traveling waves encounter a obstacle of opening in a barrier that is about the same size as the wavelength, they bend around it—this is called diffraction
Young’s double-slit experiment
Around 1800, Thomas Young settled the question by performing an experiment in which he shone light through very narrow slits and observed the results
Refraction of light
As the light passes from one medium to another it’s speed changes, causing the light to bend
Quantum theory
The nature of energy
The wave of nature of light does not explain how an object can glow when its temperature increases
The nature of energy and matter
In 1900, Planck found that if light can only be emitted in small bursts, rather than continuous wave, the resulting calculation matched what people measured experimentally
Postulated that energy can be gained or lost only in whipped number multiples of the quantity: hv
h (planck’s constant): 6.626 x 10^-34
The change in energy for a system
E = nhv
n = integer, v = frequency
The photoelectric effect
Heinrich Hertz, 1888
Light sticking the surface of certain metals causes ejection of electrons
Below a certain frequency, no electrons were observed, no matter what the intensity
The energy of the ejected electrons increases linearly with the frequency of light
The number of emitted electrons increased with light intensity
All metals show the same pattern, but each metal has a different threshold frequency
Einstein’s explanation
Postulated that if one assumes that light is made up particles
Light has two properties
Wave-like
Particle-like
The “particles” are called photons, which are packets of energy
A high energy photon (particle) bumps into an atom, ejecting an electron
Is light a wave or a flow of particles?
It’s neither one. Light is a “quantum vector field”
This multiple personality of light is referred to as “wave-particle duality”
Light behaves as a wave or as particles, depending on what we do with it, and what we try to observe
Its wave-particle duality that lies at the heart of the Heisenberg uncertainty
Equation for photoelectric effect
1/2 mu^2
m = mass
u = speed
At frequencies greater than v0:
Vs = k(v-v0)
Ek = Ephoton - Ebinding
Atomic spectra
For atoms and molecules one does not observe a continuous spectrum, one gets from a white light source
Only a line spectrum of discrete wavelengths is observed
Rydberg’s spectrum analysis
Analyzed the spectrum of hydrogen and found that it could be described with an equation that involved an inverse square of integers
1/h = -R( 1/n^2 —1/n^2)
R = 1.096776 x 10^7 m^-1
Rutherford nuclear model
The atom contains a tiny dense center called the nucleus
The nucleus is essentially the entire mass of the atom
Positively charged
The electrons move around in the empty space of the atom surrounding the nucleus
Problems with Rutherford’S nuclear model
Elections are moving charged particles
Moving particles give off energy
Electrons should lose energy, crash into the nucleus and the atom should collapse
But it doesn’t
The Bohr atom
Niels Bohr adopted planck’s assumption and explained these phenomena in this way:
- Electrons in an atom can only occupy certain orbits
- Electrons in permitted orbits gave specific , “ allowed” energies; these energies will not be radiated from the atom
- Energy is only absorbed or emitted in such a way as to move an electron from one “allowed” energy state to another; the energy is defined by E = hv
En = -RH/n^2
Energy level diagram for the hydrogen atom
The energy absorbed or emitted from the process of electron promotion or demolition can be calculated by the equation:
E= -RH (1/n^2 —1/n^2)
Ephoton = hv =. hc/
Rh = 2.18 x 10^-18 J
