FROM THE LAST UNIT, WE TALKED ABOUT THE OLD MODELS OF THE ATOM IN THE PAST. THIS UNIT, WE WILL BE FOCUSING ON THE CURRENT MODEL, WHICH IS THE QUANTUM MECHANICAL MODEL. QUANTUM MECHANICAL MODEL AND ELECTROMAGNETIC SPECTRUM

ELECTROMAGNETIC RADIATION Light is an electromagnetic wave. In empty space, light waves travel at 3x10 8 m/s, and is referred to as the constant, c. The wavelength, represented by λ (the Greek letter lambda), is the distance between the crests The frequency, represented by f, is the number of cycles (wavelengths) a wave has in a given amount of time Before you can understand the quantum mechanical model, you must understand how electromagnetic radiation works:

CALCULATING FREQUENCY AND WAVELENGTH The equation to calculate frequency and wavelength is as follows: c = f λ Quick Write #1: Based on this equation, what is the relationship between wavelength and frequency, direct or inverse? What happens to frequency when wavelength is increased? Quick Write # 2: Rearrange the equation to solve for frequency, f, then for wavelength, λ.

THE BASICS OF THE QUANTUM MECHANICAL MODEL 1. It is a statistical model – A model of chance! For example: if you shoot one electron down a path and measure where it lands, a second electron shot down the same path under the same conditions will not necessarily follow the same course but instead will most likely land in a different place!

THE BASICS OF THE QUANTUM MECHANICAL MODEL 2. Electrons can be only certain distances from the nucleus. Each distance corresponds to a certain quantity of energy that an electron can have. An electron that is as close to the nucleus as it can be is in its lowest energy level. The farther an electron is from the nucleus, the higher the energy level that the electron occupies. The difference in energy between two energy levels is known as a quantum (or quanta for plural) of energy.

HOW ENERGY OF AN ELECTRON IS QUANTIZED When white light (photons) passes through a prism, you can see that the light is made of a rainbow of colors. This spectrum of colors is called a continuous spectrum. A continuous spectrum is not quantized (it contains all energy levels).

HOW ENERGY OF AN ELECTRON IS QUANTIZED When a photon of light containing energy hits an atom, it is absorbed by an electron. This electron becomes excited from the absorbed energy for a short time. Unable to sustain that energy level, it goes back to its ground energy state emits another photon at a certain wavelength specific to that element. Different elements release different colors based on the energy levels of the electrons of that atom. This shows the element’s line emission spectrum. Becoming Excited (absorbing a photon) Going back to ground state (releasing a photon)

LINE EMISSION SPECTRUM ***THIS EXPLAINS WHY EVERYTHING IS THE COLOR THAT IT IS***

THE BASICS OF THE QUANTUM MECHANICAL MODEL 3. An electron behaves as both a particle and a wave just like photons do. This is called wave-particle duality. Electrons as Waves electrons must always have fixed energy levels, and therefore release electromagnetic radiation when it cannot sustain that energy level Young’s double slit experiment showed that electrons also behave like waves because the interference pattern of an electron looks the same as ripples of water (a wave). Electrons as particles J. J. Thomson’s cathode ray experiments demonstrated that electrons have mass like a particle The photoelectric effect of electrons also shows that both electrons AND light behave like particles

THE PHOTOELECTRIC EFFECT States that an electron’s ability to leave an atom depends on how much energy is given to the electron at one time. If electrons were waves, the electron would leave once it was given enough energy. This means that a photon must have just the right energy to cause the electron to leave the atom. This can only be done if a photon is a "packet" of energy, or a particle, colliding with the electron. Analogy : If you were to pelt 100 paper balls at someone’s head, it won’t knock them out because each ball doesn’t have enough energy to get that result. If you were to pelt 1 large stone at someone’s head, you will be able to knock them out even though you exerted the same amount of energy as throwing 100 paper balls.

THE PHOTOELECTRIC EFFECT The Intensity (energy) of the light shining on the particle depends on it the frequency of the light, not the length of time the light was on the particle. This was determined by Max Planck, who determined the relationship between frequency and energy of a photon using the following equation: E = hf h is a constant: 6.6x J/s Quick Write #3: Based on this equation, what is the relationship between energy of an electron/photon and frequency (direct or inverse)? What happens to frequency when energy is increased? Quick Write #4: Rearrange the equation above to solve for frequency

THE PHOTOELECTRIC EFFECT You can also determine the energy of an electron or photon of light from its wavelength as well using the following equation: λE=hc Quick Write #5: Based on this equation, what is the relationship between energy of an electron/photon and wavelength (direct or inverse)? What happens to energy when wavelength is increased? Quick Write #8: Rearrange the equation above to solve for wavelength, then for energy.