Electrons and Light Production

Understanding the Atom1Learning ObjectivesTSWBAT = The student will be able to:Explain the different models of the atomExplain how electronic transitions produce atomic spectra (light)Explain the wave-particle duality theoryExplain the evidence that suggest that light exists as a waveExplain the composition of wavesCalculate frequency & wavelengthExplain the Michelson ExperimentDescribe how Rutherfords experiment provides evidence that light consisted of particles

2ACOS CorrelationACOS #3c: Utilizing benchmark discoveries to describe the historical development of atomic structure, including photoelectric effect, absorption, and emission spectra of elements

3He basically postulated that everything in life was composed of atoms. Democritus didnt know much about chemical behavior. He defined the atom to be small indestructible particles of matter. No experimental evidence, simply theoryTheory #1 p101 (Democritus)

4Expanded on Democritus Believes that everything is made of atomsAtoms of the same element have the same mass/different elements have different massesCompounds contain atoms of more than one elementIn a particular compound, atoms of different elements always combine in the same way ex H2OWas created to explain why compounds always join in the same fashion or had a fixed composition.Theory #2: John Dalton

5Sizing Up The AtomThe diameter of an atom is approximately 110-8cm

To help explain this concept, lets use a copper penny.

Now, lets say I grind the penny to find particles.

Lets say, I pull a piece of the dust and grind it up to the tiniest piece. This tiniest particle is known as an atom.

6How do we visualize the atom?By using a scanning electron microscope!

7Basically identified that the atom contained subatomic particles (charges).Proved his hypothesis using the machine on p103 Figure B. Basically he sent an electric current through a gas tube. Once you sent an electric current through the gas tube, energy was released in the form of light ex: fluorescent light bulbs. He then placed negative/positive magnets outside the tubing and noticed that the beam deflectedHe discovered the electronsOften called the chocolate chip ice cream modelTheory #3: Joseph John Thomson

8Robert MillikanMillikan supported Thomsons theories by defining the charge and mass of the Electron.The charge of the electron is negativeThe mass of the electron is approximately 9.11 x 10-31 kg.Because the mass is so small, it isnt used in determining the mass of the atom.

9Eugen GoldsteinGoldstein supported Thomsons theories by identifying the presence of a protonsThe charge of the proton is positiveThe mass of the proton is approximately (1840)x (9.11 x 10-31 kg)

10James ChadwickChadwick supported Thomsons theories by identifying the presence of a neutron.The charge of the neutron is neutralThe mass of the neutron is approximately (1840)x (9.11 x 10-31 kg)Notice: The mass of the neutron is identical to that of the proton.

11Used alpha particles in uraniumAn alpha particle is a fast moving particle that has a positive chargeContradicts Thompsons model because we identified the positively charged nucleusUsed the Gold Foil experiment that enabled him to confirm his beliefs about the nucleus.Theory #4 p107- p108 (Ernest Rutherfords Theory)

12The basic premise of the Gold Foil experiment was that he shot a beam of uranium light towards a gold piece of aluminum foil. He observed that some particles were deflected off. After further research he observed that the charges being deflected were positively charged and they were only deflected when they came in proximity with the positively charged nucleus.Believed that the electrons remained in a fixed orbital in a three-dimensional rotation.Theory #4 p107- p108 (Ernest Rutherfords Theory)

13Believed that the e- move with constant speed in an more orderly system around the nucleus.At atoms energy can change if we gain or lose an electronThe possible energy that an electron in an atom can have is called and energy level.Energy levels increase as you go away from the nucleus.This theory is the most important to learn in our class because it helps us to understand the details of the periodic table.Theory #5 p128-129 (Bohrs Theory)

14NotesWe learned in the past section that electrons can gain or lose energy, ultimately causing them to change energy levels.

We will extend upon this knowledge to explore how light is produced.

15Atomic SpectraAtomic spectra atomic display of energy emission (see below)In laymans terms, it means its a visual image that occurs as a result of energy being released into the system.As energy is released, it produces the colors below

Why is this important?No two elements have the same emission spectra (patterns) therefore it can be used as an identifier for atomic spectraCan definitely be used in forensic science

16Notes:In order to see color, light must be present.Evidence: Turn off your lights at home, what colors are you able to see?Lets now explore how light is produced. Key Terms:Quantum numbers (n) these numbers are used to correspond to the energy levelsPhotons packets of energy that carry light

17 How is light produced?Emission of light occurs as a single abrupt step called an electronic transition going from a high energy level to a lower energy level (Bohrs Model relates to this)

Light is produced because electrons absorb energy. When energy is absorbed, e- are bumped into a higher energy level.

Once in the higher energy level, e- release heat and light as they return to the lowest energy level.

18Wave Particle DualityTheories suggests that light behaves as both a wave and as a particle19Wave BehaviorWhat evidence supports the idea that light behaves as a wave?It is believed that light can behave as a wave because of its interference patterns

20Evidence that light has wave-like behaviorThomas Young did an experiment using light and comparing the pattern formed by manipulating light.Constructive Interference light is amplified or is more intense because of the manipulationDestructive Interference light is reduced based on certain non-helpful manipulations ie blocking the light patterns.

21Wave CompositionAmplitude height of the waveWavelength the distance between two identical points on a waveCrest the peakTrough the valley

22Frequency Frequency is defined as the number of cycles (revolutions) divided by time.

Written as v (pronounced mu)SI Unit = the hertz (s-1 or 1/s)

23Frequency

Frequency and time are inversely proportionateFrequency and energy are directly proportionateFrequency and wavelength are inversely proportionateLarge frequency/high energy can be very dangerous to human health i.e. tanning salons. 24PeriodThe time it takes for one complete cycle to occur

25Why does this matter?Color is a reflection of a wave frequencyLight consists of electromagnetic wavesVisible wavelengths are 10-2 through 10-11 (m) approximatelyKnowing this information can help us design devices

26 Calculating wavelength and frequencyc= speed of light & =wavelengthSpeed of light is approx. equal to 3 x 108 m/sSolving for frequency (v= c/):A microwave (from an antenna) is part of the electro-magnetic spectrum therefore its speed is the speed of light. What is its wavelength if its frequency is 3.44 x 10 9 hertz?Solving for wavelength ( = c/v)Calculate the wavelength of an FM radio with frequency waves of 102.5 x 102 m.

27How did we calculate the speed of light?Michelson ExperimentIt helped us to place a value on the speed of light. He used mirrors to time how fast the light would travel based on known data for the mirrors. He basically used the formula for speed.

28Evidence that Light behaves as particlesLets revisit Rutherfords experimentHe shot a beam of Uranium (U) light into a piece of (Gold)Au and noticed particle deflection. If light didnt exist in particles, we wouldnt have observed this deflection

29The Photoelectric EffectThe photoelectric effect is the name given to the observation that when light is shone onto a piece of metal, a small current flows through the metal. The light is giving its energy to electrons in the atoms of the metal and allowing them to move around, producing the current. 30So where do we stand now?31Heisenberg Uncertainty Principle

States that it is impossible to know exactly both the velocity and the position of a particle at the same time.When the e- collides with a photon of light, it changes the velocity/direction of the e- in ways that scientists are unsure of . Based on what we learned about motion in Physical Science, we must know the speed and direction of an object in order to clearly describe its motion.

32Louis de Broglie & Schrodingers

Both individuals did tremendous work on explaining how electrons move in this wavy like pattern thus explaining why light behaves as both as wave and a particle. (p144-145)

33Why does any of this matter?Understanding properties of light have helped scientist to enhance technology. For example: light telescopes manipulate light to help us view the starselectron microscopes manipulate light, mirrors and electrons to view imageslasers manipulate light during surgery to cut things34Contradicts Bohrs model because we know that the electrons move around in a less predictable way.The density of the cloud is greater in the region where the cloud is the thickestThe thicker the region, the higher probability of finding an electronAn orbital is a region of space where an electron is most likely to be foundGround state refers to the lowest energy level.There are four known orbitals: s,p,d,f

Electron Cloud Model of the Atom

35Counting Atoms2H203CO2

Al2O3C2H6

FeSO4Pb(SO4)3

Na2SO4 3K3Fe(SCN)636 Most elements can be found on earth (with the exception of those elements that too unstable and thus must be synthesized in the laboratory).

Since all elements have isotopes then we must consider how much of one isotope of an element exists versus another isotope of the same element.

These are called the "natural" abundances on earth.

Isotope Masses of XIsotopeMass (amu)Xa54Xb56Xc59 Next, we can inquire what the mass of element X is?

Since each isotope has a different mass (because each isotope has a different number of neutrons) the simplest answer is to give the "average" mass of element X - the atomic weight.

After more analysis the mass of each isotope is determined to be the following:

Then the average mass (atomic weight) is given by:

Rubidium has two common isotopes, 85Rb and 87Rb. If the abundance of85Rb is 72.2% and the abundance of 87Rb is 27.8%, what is the average atomic mass of rubidium?2) Uranium has three common isotopes. If the abundance of 234U is 0.01%, the abundance of 235U is 0.71%, and the abundance of 238U is 99.28%.

What is the average atomic mass of uranium?3) Titanium has five common isotopes: 46Ti (8.0%), 47Ti (7.8%), 48Ti (73.4%), 49Ti (5.5%), 50Ti (5.3%).

What is the average atomic mass of titanium?Here are three isotopes of an element: 612C613C 614CThe element is: __________________The number 6 refers to the _________________________The numbers 12, 13, and 14 refer to the ________________________How many protons and neutrons are in the first isotope? ________________How many protons and neutrons are in the second isotope? _________________How many protons and neutrons are in the third isotope? _______________

Isotope name atomic #mass ## of protons# of neutrons# of electrons 92 uranium-23592 uranium-2385 boron-105 boron-11Complete the following chart:

Naturally occurring europium (Eu) consists of two isotopes was a mass of 151 and 153. Europium-151 has an abundance of 48.03% and Europium-153 has an abundance of 51.97%.

What is the atomic mass of europium? Strontium consists of four isotopes with masses of 84 (abundance 0.50%), 86 (abundance of 9.9%), 87 (abundance of 7.0%), and 88 (abundance of 82.6%).

Calculate the atomic mass of strontium.