Chapter 7: Periodicity and Modern Atomic Theory Part 2 AP Chemistry

Chapter 7: Periodicity and Modern Atomic Theory THE BIRTH OF QUANTUM MECHANICS

Quantum Mechanics The branch of chemistry and physics that studies the nature and behavior of matter and energy on the atomic and subatomic level

Physics at Beginning of 20th Century “Classical Physics” “Given for one instant an intelligence which could comprehend all the forces by which nature is animated and the repective positions of the beings which compose it…. Nothing would be uncertain and the future as the past, would be present to its eyes.” - Pierre-Simon, marquis de Laplace (c. 1820) “The more important fundamental laws and facts of physical science have all been discovered… our future discoveries must be looked for in the sixth place of decimals.” - Albert Michelson (c.1903)

Physics at Beginning of 20th Century “Classical Physics” “There is nothing new to be discovered in Physics now. All that remains is more precise measurement.” - Lord Kelvin (c. 1900)

Accomplishments of Classical Physics / Chemistry Empirical – from 1700-1800 collected and organized much data about natural phenomenon Used patterns from data to enable them to make predictions about how nature behaved Didn’t really understand WHY For example, they had organized elements into periodic table based on observable properties, but idea of an atom still wasn’t universally accepted; electron had just been discovered and its existence wasn’t fully confirmed

Hallmark of Classical Physics Measurement without Disturbance Core assumption that measuring a system could reveal information without changing the system Determinism Based on belief that if one knows precisely the initial values of position and velocity of all particles in a system at one moment in time, then all future behavior of the system can be predicted fundamentally believed that natural events were predictable… scientists could study matter enough to write an equation that would allow them to predict future behavior if they knew the present state

Accomplishments of Classical Physics Mechanics Laws of Motion, based on Newton’s Laws Culminating in Kepler’s Law, which showed that Newtonian laws of motion could even be applied to motion of planets Thermodynamics Energy conservation & energy interaction with matter, based on works of Carnot, Clausius, and Lord Kelvin Electromagnetism (Electricity, Magnetism, Optics) largely due to the work of Oersted, Faraday and Maxwell Demonstrated light was electromagnetic wave & knew its speed and that it could travel without a medium (no “ether”)

At the end of the 1800’s Matter and Energy were distinctly different Matter was thought to consist of distinct particles Energy in the form of light was described as a wave Massless and delocalized (their position in space could not be specified) No intermingling of matter and light

Problems with Classical Physics & Accepted View of Matter & Energy: Three Unexplained Experiments Blackbody Radiation Photoelectric Effect Spectral Lines

Classical Physics Predictions from classical physics still work – as long as you are on the MACROscopic scale Equations from classical physics work for large objects that are not approaching the speed of light

Light – Is it a Particle or a Wave?!? Newton suggested light was a particle around 1700 Two contemporaries, Christiaan Huygens & Robert Hooke, tried to prove that light was a wave based on different explanations for the same phenomena, but Newton’s theory was accepted In early 1800’s, Thomas Young showed that light has properties of waves (double slit experiment)

Light – Is it a Particle or a Wave?!? In the 1830’s, Michael Faraday explained that light is electromagnetic radiation Electric and magnetic fields are produced by source of light Instead of a substance to transmit the waves (like air), the field lines propagate without a medium, and that light is the vibration of these field lines

Light – Is it a Particle or a Wave?!? In 1864, James Clerk Maxwell published four equations to describe motion of light as a wave Speed of light is constant – 3.0 x 108 m/s

To understand quantum mechanics, we first need to know how to describe light as a wave….

The amplitude (A) is the height of the wave. The distance from node to crest or node to trough The amplitude is a measure of light intensity—the larger the amplitude, the brighter the light. The wavelength (l) is a measure of the distance covered by the wave. The distance from one crest to the next The distance from one trough to the next, or the distance between alternate nodes

The frequency (n) or (f) is the number of waves that pass a point in a given period of time. The number of waves = the number of cycles. Units are hertz (Hz) or cycles/s = s−1 (1 Hz = 1 s−1). The total energy is proportional to the amplitude of the waves AND the frequency. The larger the amplitude, the more force it has. The more frequently the waves strike, the more total force there is.

For waves traveling at the same speed, the shorter the wavelength, the more frequently they pass. Speed (c) – speed of light (2.9979×108 m/s)

The Nature of Waves

Classification of Electromagnetic Radiation

Amplitude and Wavelength

Problems with Classical Physics Three Unexplained Experiments Blackbody Radiation Explained by Max Planck (1900) Photoelectric Effect Explained by Albert Einstein (1905) Spectral Lines Explained by Neils Bohr (1913)

The Nature of Energy The wave nature of light does not explain how an object can glow when its temperature increases.

Planck’s Constant & Concept that Energy is Quantized Explained blackbody radiation “blackbody” – an object that absorbs 100% of the energy that hits it (no transmission or reflection) When energy is absorbed, it causes molecules to vibrate, causing the object to heat up and eventually emit electromagnetic radiation (light!)

How do heated objects radiated light? Puzzle confronting physicists at turn of 20th century: How do heated objects radiated light? heat was known to cause the molecules and atoms of a solid to vibrate, and it was known that the molecules and atoms were themselves complicated patterns of electrical charges. Maxwell’s predictions that oscillating charges emitted electromagnetic radiation had been confirmed the problem was that according to classical physics, there should be no limit to the amount of energy radiated from a heated object, so they should eventually start to give off UV light

Planck’s Conclusions: Each oscillator can only emit a packet of energy that corresponds to the frequency with which it oscillates DE = nhn As temperature is increased, more molecules vibrate with higher frequency, thus shifting the total color emitted from red to white (but each oscillator only emits one color)

The Nature of Energy Max Planck explained it by assuming that energy comes in packets called quanta.

KEY RESULT FROM PLANCK: ENERGY IS QUANTIZED Energy can be gained or lost only in whole number multiples of .  A system can transfer energy only in whole quanta (or “packets”). Energy seems to have particulate properties too. Energy has mass Dual nature of light: Electromagnetic radiation (and all matter) exhibits wave properties and particulate properties. E = mc2

Electromagnetic radiation is a stream of “particles” called photons. Planck’s constant = h = 6.626 × 10-34 Js 37 37

Photoelectric Effect & Concept that Energy is Quantized Many metals will emit electrons when light shines on their surface

Using the classical Maxwell wave theory of light, the more intense the incident light the greater the energy with which the electrons should be ejected from the metal. That is, the average energy carried by an ejected (photoelectric) electron should increase with the intensity of the incident light.  Scientists found the energies of the emitted electrons to be independent of the intensity of the incident radiation. 

Experimental observations indicate the following: A minimum frequency was needed before electrons would be emitted regardless of the intensity called the threshold frequency.

Experimental observations indicate the following: For light with frequency lower than threshold frequency, no electrons are emitted regardless of intensity of light.

Experimental observations indicate the following: For light with frequency greater than the threshold frequency, the number of electrons emitted increases with the intensity of the light The kinetic energy of the electrons stays the same for more intense light of same frequency

Experimental observations indicate the following: For light with frequency greater than the threshold frequency, the kinetic energy of the emitted electrons increases linearly with the frequency of the light.

Einstein proposed that the light energy was delivered to the atoms in packets, called quanta or photons ; light did not interact with matter like a wave of energy The energy of a photon of light is directly proportional to its frequency. Inversely proportional to its wavelength The proportionality constant is called Planck’s Constant, (h), and has the value 6.626 × 10−34 J ∙ s.

Kinetic Energy = Ephoton – Ebinding Binding Energy, f (aka – Work Function, W) The energy needed to eject an electron from the atom One photon at the threshold frequency gives the electron just enough energy for it to escape the atom. When irradiated with a longer wavelength photon, electron doesn’t absorb enough energy to overcome the binding energy and no electrons are emitted 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. Kinetic Energy = Ephoton – Ebinding KE = hn − W

KEY RESULT FROM EINSTEIN & PLANCK: ENERGY IS QUANTIZED Energy is quantized. It can occur only in discrete units called quanta Electromagnetic radiation, which was previously though to exhibit only wave properties, also has characteristics of matter as well. Wave-Particle Duality of Light