Mid Term 4

Quantum mechanics
explaining the periodic table and the behavior of the elements in chemical bonding – as well as providing the practical basis for lasers, computers, and countless other applications
The quantum-mechanical model

explains the manner in which electrons exist and behave in atoms

It helps us understand and predict the properties of atoms that are directly related to the behavior of the electrons:
•    why some elements are metals and others are nonmetals •    why some elements gain one electron when forming an anion,
whereas others gain two •    why some elements are very reactive while others are
practically inert •    and other periodic patterns in the properties of the elements

Light
Light is a form of electromagnetic radiation composed of perpendicular oscillating waves, one for the electric field and one for the magnetic field
 electric field
a region where an electrically charged particle experiences a force
a magnetic field
a region where a magnetized particle experiences a force All electromagnetic waves move through space at the same, constant speed
Amplitude
The amplitude is the height of the wave, the distance from node to crest or node to trough
The amplitude is a measure of how intense the light is – the larger the amplitude, the brighter the light
Wavelength
The wavelength (”lamb’ da;) is a measure of the distance covered by the wave, the distance from one crest to the next or the distance from one trough to the next (or the distance between alternate nodes)
Frequency
The frequency is the number of waves that pass a point in a given period of time
Color
The color of light is determined by its wavelength or frequency
White light is a mixture of all the colors of visible light (red, orange, yellow, green, blue, ; violet)
When an object absorbs some of the wavelengths of white light and reflects others, it appears colored. The observed color is predominantly the colors reflected
The Electromagnetic Spectrum

Visible light comprises only a small fraction of all the wavelengths of light ; which collectively is called the electromagnetic spectrum

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Shorter wavelength (high-frequency) light has higher energy

Interference

The interaction between waves

;;;; When waves interact so that they add to make a larger wave it is called
constructive interference; waves are in-phase

;;;; When waves interact so they cancel each other it is called destructive
interference; waves are out-of-phase

Diffraction

When traveling waves encounter an obstacle or opening in a barrier that is about the same size as the wavelength, they bend around it ; this is called diffraction

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;;;; Traveling particles do not diffract

;;;; The diffraction of light through two slits separated by a distance comparable to
the wavelength results in an interference pattern of the diffracted waves

;;;; An interference pattern is a characteristic of all light waves

The Photoelectric Effect
It was observed that many metals emit electrons when a light shines on their surface; this was called the photoelectric effect
Classic wave theory

attributed this effect to the light energy being transferred to the electron

According to this theory, if the wavelength of light is made shorter, or the light waves; intensity made brighter, more electrons should be ejected
As the energy of a wave is directly proportional to its amplitude and its frequency, this idea predicts if a dim light were used there would be a lag time before electrons were emitted to give the electrons time to absorb enough energy

The Photoelectric Effect
In experiments it was observed that there was a minimum frequency needed (called the threshold frequency) before electrons would be emitted, regardless of the intensity!
Einstein;s Explanation
Einstein proposed that the light energy was delivered to the atoms in packets, called quanta or photons
The energy of a photon of light is directly proportional to its frequency (and thus inversely proportional to its wavelength)
Ejected Electrons

  • One photon at the threshold frequency gives the electron just enough energy for it to escape the atom
  • When irradiated with a shorter wavelength photon, the electron absorbs more energy than is necessary to escape
  • This excess energy becomes kinetic energy of the ejected electron

Spectra

When atoms or molecules absorb energy, that energy is often released as light energy (e.g., fireworks, neon lights, etc.)

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emission spectrum
When that emitted light is passed through a prism, a pattern of particular wavelengths of light is seen that is unique to that type of atom or molecule ; the pattern
Rydberg;s Spectrum Analysis
Rydberg analyzed the spectrum of hydrogen and found that it could be described with an equation that involved an inverse square of integers
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