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Chem 127 Class Notes - Week 2

by: Aenea Mead

Chem 127 Class Notes - Week 2 CHEM 160

Aenea Mead
Cal Poly

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About this Document

These notes cover most of what will be on our first quiz. The rest of the material can be found in my notes from week 1.
General Chemistry for Agriculture and Life Science I
Class Notes
Photoelectric Effect, light, wavelength, frequency, amplitude, Subatomic Particles, atom, blackbodyradiation, Molar Mass
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This 5 page Class Notes was uploaded by Aenea Mead on Saturday October 1, 2016. The Class Notes belongs to CHEM 160 at California Polytechnic State University San Luis Obispo taught by unknown in Fall 2016. Since its upload, it has received 7 views. For similar materials see General Chemistry for Agriculture and Life Science I in Chemistry at California Polytechnic State University San Luis Obispo.


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Date Created: 10/01/16
Vocabulary ​ Atom: Smallest unit of a chemical element ​ lement: one type of atom Co​ mpound: two or more elements combined ​ Ion: An atom that has gained or lost electrons and therefore has a charge ​ Cation: Positively charged ion ​ Anion: Negatively charged ion ​ Isotope: An atom of an element with a different number of neutrons ​ Mole:​ amount of substance containing the number of atoms as exactly 12 grams of carbon-12 ● Called Avogadro's number ● Equals 6.022 10 23 * ​ Molar Mass: mass (g) per mole of a substance ​ Frequency (ν): number of cycles that pass through in a given period of time 1 −1 ● Units: s or s = Hertz (Hz) ​ Wavelength (λ): Distance between two set points on a wave (ie. Crest to crest) ● Units: meter ​ Amplitude (a): Vertical height of the crest of a wave ● Relates to intensity / brightness Content / Concepts The Atom: ● Smallest unit of an element ● Electrically neutral ● Nucleus is surrounded by electrons Subatomic Particles Particle Symbol Location Charge Mass* Proton p + Inside nucleus 1+ “heavy” 0 Neutral n Inside nucleus 0 “heavy” − Electron e Outside nucleus 1- ~0 *The exact masses of the three particles are not relevant, what matters is that the proton and the neutral weigh roughly the same, and the mass of the electron, while existent, is so miniscule by comparison that it can be disregarded. Atomic number (z): ● Equals the number of protons ● identifies the element ● if atom is neutral: # protons = # electrons Mass Number (A):​ # protons + # neutrons Rather than writing out an element’s full name, whether it is electrically neutral, as well as how many neutrons it has, scientists have come up with an easier way. ​The element symbol is large and in the middle, the charge is written in the upper right corner, the mass in the upper left corner, and the atomic number in the bottom left corner. ​The atomic number, though, is not necessary because it gives the same information as the element symbol. Both ways to write it are accepted. The diagram above is an example of how a cation of sodium-23 might be written. Atomic Mass on the Periodic Table:​ You might have noticed that the atomic mass on the periodic table is not a whole number, which may seem odd considering the mass of each individual atom will always be a whole number (you cannot have partial protons or neutrons). The mass on the periodic table is a reflection of the mass and the percent composition of all the different isotopes of each element. How to determine the atomic mass of an element: mass of isotope 1 * abundance of isotope 1 + mass of isotope 2 abundance o* isotope 2 *abundance is in decimal form (i.e. if abundance is 40%, it is represented in this formula by 0.4) Units of Atomic Mass:​ The mass on the periodic table has two different units. Depending on what you are trying to do with it, you can use either. - amu / atom - Grams / mol The molar mass of a substance can be found by adding the atomic mass of each atom in a compound together. Ex. Atomic mass of O is 32g because one mole of oxygen weighs 16 grams, and since O 2as two oxygen atoms per molecule, then one mole of O will equal two moles of oxygen atoms and therefore the 16 grams is multiplied by two. Ex. Atomic mass of NaCl is 58.5g because sodium weighs 23g and chlorine weighs 35.5g. Since there is only one atom of each in the mixture the atomic mass if found by adding 23g and 35.5g. ​ Frequency (ν): number of cycles that pass through in a given period of time ● Units: 1 or s −1 = Hertz (Hz) s ​ Wavelength (λ): Distance between two set points on a wave (ex. Crest to crest, to trough to trough) ● Units: meter ​ Amplitude (a): Vertical height of the crest of a wave ● Relates to intensity / brightness Wavelength and frequency are inversely related. The following equation can be used to solve for one when given the other: Wave Interference Constructive Wave Interference: When two waves collide and their troughs and crests line up, the result is one wave with a combined amplitude from the original two waves. Destructive Interference: When two waves collide but do NOT line up. The result is that they cancel each other out and there are no longer any waves. Blackbody radiation: ​when a substance is heated up, the excess energy will be emitting in the form of light Composition of a Wave Classical Wave Theory (old): A wave’s energy depends on its intensity (amplitude) New Theory by Einstein (current): ​a wave’s energy depends on its ​frequency ● Einstein also theorized that a wave is not a single unit, but rather a stream of particles (photons) ● This led to the theory of ​wave-particle duality​, which states that light is both wavelike and particle-like Quantum Mechanics now states that ​emitted light energy from atoms can only come in discrete (quantized, not continuous) values​. Imagine many packets of light (called photons) moving within every wave. Because of this theory we can calculate the energy of a photon in a wave using the following: E = hν ​ = energy in joules of one photon −34 h = 6.626*10 (Planck’s constant - no units) The two equations above can be manipulated to form: hc E = ν Relationships to Note: - As frequency increases, wavelength decreases (inversely related) - As energy increases, frequency increases and wavelength decreases - Amplitude has no relation and instead has to do with the brightness / intensity of the light Photoelectric Effe​ ​The diagram on the right shows that when photons in light waves with enough energy hit metal, electrons are ejected off the metal.​ The photons each need to hit with a minimum energy to emit an electron (varies depending on the element), but any excess energy the photon has beyond that gets translated to kinetic energy of the electron. The kinetic energy of the electrons can be harnessed for various uses. This is how we get solar energy from solar panels. Higher Intensity = more photons Higher Energy = short wavelength / high frequency The kinetic energy of an electron can be determined using the following equations: KE = E p −Φ KE = 1m v 2 2 e Φ = Work Function/Binding Energy (energy required to emit an electron) E = the kinetic energy of the photon p −31 m e mass of electron (9.1* 10 kg) v = velocity (meters / second) KE = kinetic energy (Joules) **Note: All of what is written in these notes is important, otherwise it wouldn’t be here. What is highlighted in yellow, however, are the things I feel are the most important to know. It is also important to note that it is not important to memorize the highlighted equations because they will likely be provided for you, but rather it is important to understand the theories behind them and why they exist.


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