CHEM 1124Q Lecture Notes - Lecture 3: Magnetic Quantum Number, James Prescott Joule, Radiant Energy
Electronic Structure and Periodic Properties of the Elements
Chapter 3
Energy and Energy Changes
-Energy is the capacity to do work or transfer heat. All forms of energy are either kinetic or potential
(chemical bond).
-Kinetic energy (Ek) is the energy of motion. Ek = 1 mu2
2
- m is the mass of the object
- u is the velocity of the object
- The form of energy that is of the most interest in chemistry is thermal energy -- the energy associated with
the random motion of particles
-Potential energy is the energy possessed by an object by virtue of its position.
- There are two forms of potential energy of great interest to chemists: Chemical and Electrostatic energy.
Chemical energy is stored within the structural units of chemical substances. Electrostatic energy is
potential energy that results from the interaction of charged particles.
- Q1 and Q2 represent two charges separated by the distance, d
- Kinetic and potential energy are inter-convertible – one can be converted to the other.
- Although energy can assume many forms, the total energy of the universe is constant.
- Energy can neither be created nor destroyed. When energy of one form disappears, the same amount of
energy reappears in another form or forms. This is known as the law of conservation of energy.
Units of Energy
- The SI unit of energy is the joule (J), named for the English physicist James Joule.
- It is the amount of energy possessed by a 2-kg mass moving at a speed of 1 m/s.
- Ek = ½ mu2 = ½(2 kg)(1 m/s)2 = 1 kg∙m2/s2 = 1 J
- The joule can also be defined as the amount of energy exerted when a force of 1 newton (N) is applied
over a distance of 1 meter.
- 1 J = 1 N ∙ m
- Because the magnitude of a joule is so small, we often express large amounts of energy using the unit
kilojoule (kJ).
- 1 kJ = 1000 J
- Energy = Forces x Distance
The Nature of Light
- Visible light is only a small component of the electromagnetic spectrum. (There is an electrical and
magnetic component to it.)
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- The speed of light (c), through a vacuum is a constant: c = 2.99792458 x 108 m/s (Normally rounded to
3.00 x 108 m/s
- Speed of light, frequency and wavelength are related:
-
- λ is expressed in meters
-v is expressed in reciprocal seconds (s−1)
- s-1 is also known as hertz (Hz)
- All forms of electromagnetic radiation travel in waves.
- Waves are characterized by:
1. Wavelength (λ; lambda) – the distance between identical points on successive waves
2. Frequency (ν; nu) – the number of waves that pass through a particular point in 1 second.
3. Amplitude – the vertical distance from the midline of a wave to the top of the peak or the bottom of the
trough.
- An electromagnetic wave has both an electric field component and a magnetic component.
- The electric and magnetic components have the same frequency and wavelength.
- When light passes through two closely spaced slits, an interference pattern is produced
- Constructive interference is a result of adding waves that are in phase.
- Destructive interference is a result of adding waves that are out of phase.
- This type of interference is typical of waves and demonstrates the wave nature of light
Quantum Theory
- Early attempts by nineteenth-century physicists to figure out the structure of the atom met with only limited
success. They were using the laws of classical physics.
- These laws describe the behavior of macroscopic objects. Over time, the realization and acceptance was
the behavior of subatomic particles is not governed by the same physical laws as larger objects.
- A quantum of energy is the smallest quantity of energy that can be emitted (or absorbed).
Quantization of Energy
- When a solid is heated, it emits electromagnetic radiation, known as blackbody radiation, over a wide
range of wavelengths.
- The amount of energy given off at a certain temperature depends on the wavelength.
- Classical physics assumed that radiant energy was continuous; that is, could be emitted or absorbed in
any amount.
- Max Planck suggested that radiant energy is only emitted or absorbed in discrete quantities, like small
packages or bundles.
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- A quantum of energy is the smallest quantity of energy that can be emitted (or absorbed).
- The energy of E of a single quantum of energy is E = hv
- h is called Planck’s constant: 6.63 x 10-34 Js… Js x 1 = J
s
- The idea that energy is quantized rather than continuous is like walking up a staircase or playing the
piano. You cannot step or play anywhere (continuous), you can only step on a stair or play on a key
(quantized).
- Photons and the Photoelectric Effect:
- Albert Einstein used Planck’s theory to explain the photoelectric effect.
- Electrons are ejected from the surface of a metal exposed to light of a certain minimum frequency, called
the threshold frequency.
- The number of electrons ejected is proportional to the intensity (or brightness) of the light. Below the
threshold frequency no electrons were ejected, no matter how intense the light.
- Einstein proposed that the beam of light is really a stream of particles.
- These “particles” of light are now called photons. Each photon must posses the energy given by the
equation: Ephoton = hv
- Shining light onto a metal surface can be thought of as shooting a beam of particles, photons – at the
metal atoms.
- If the ν of the photons equals the energy the binds the electrons in the metal, then the light will have
enough energy to knock the electrons loose. High frequency x high energy
- If we use light of a higher ν, then not only will the electrons be knocked loose, but they will also acquire
some kinetic energy.
- This is summarized by the equation: hv = KE + W. KE is the kinetic energy of the ejected electron and W
is the binding energy of the electron
Niels Bohr and the Beginnings of Quantum Theory
- Sunlight is composed of various color components that can be recombined to produce white light.
- The emission spectrum of a substance can be seen by energizing a sample of material with some form of
energy.
- The “red hot” or “white hot” glow of an iron bar removed from a fire is the visible portion of its emission
spectrum.
- The emission spectrum of both sunlight and a heated solid are continuous; all wavelengths of visible light
are present
- Line spectra are the emission of light only at specific wavelengths
- Every element has its own unique emission spectrum.
Atomic Line Spectra
- The Rydberg equation can be used to calculate the wavelengths of the four visible lines in the emission
spectrum of hydrogen. Equation =
- R∞ is the Rydberg constant (1.09737317 x 107 m−1)
- λ the wavelength of a line in the spectrum
- n1 and n2 are positive integers where n2 > n1. 1,2,3,4….
The Line Spectrum of Hydrogen
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Document Summary
Electronic structure and periodic properties of the elements. Energy is the capacity to do work or transfer heat. All forms of energy are either kinetic or potential (chemical bond). Kinetic energy (ek) is the energy of motion. M is the mass of the object u is the velocity of the object. The form of energy that is of the most interest in chemistry is thermal energy -- the energy associated with the random motion of particles. Potential energy is the energy possessed by an object by virtue of its position. There are two forms of potential energy of great interest to chemists: chemical and electrostatic energy. Chemical energy is stored within the structural units of chemical substances. Electrostatic energy is potential energy that results from the interaction of charged particles. Q1 and q2 represent two charges separated by the distance, d. Kinetic and potential energy are inter-convertible one can be converted to the other.
