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Atom Unit
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Standard 7.PS Develop and use Models to illustrate the structure of Atoms, including the Subatomic Particles with their Relative Positions and Charges.
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Element Substance that CANNOT be broken down into simpler Substances. It is a Matter made of Atoms of only one kind Example—Iron is only made of iron atoms 118 Elements on Periodic Table
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Atom Smallest part of an Element
Consist of a Nucleus surrounded by one or more Electrons
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I. Scientists who contributed to the knowledge of the Atom
The Theory of the Atom has changed over time
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Timeline BCE means Before Common Era AD means Anno Domini
(Year of Our Lord)
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A. Democritus Born 460 BCE Died 370 BCE
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Democritus A Greek Philosopher, who theorized that if you cut an object in half repeatedly, you would end up with a particle that could not be cut. (ATOM) Atom (Greek word: ATOMAS) meaning “not able to be divided”
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B. Aristotle Born 384 BCE Died 322 BCE
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Aristotle Greek Philosopher, who believed that you would never end up with a particle that could not be cut More Influential He was WRONG
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C. John Dalton Born AD 1766 Died AD 1844
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John Dalton British School Teacher / Chemist
Combined his idea of Element with Greek Theory of the Atom He proposed the following ideas about Matter:
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Matter is made up of Atoms.
Atoms cannot be divided into smaller pieces. All the Atoms of an Element are exactly alike. Different Elements are made of different kinds of Atoms.
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D. William Crooke Born AD 1832 Died AD 1919
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William Crooke Start of Scientific Evidence
(No Lab Experiments before this time) Glass tube that had almost all the air removed from it. The glass tube had two pieces of metal called Electrodes.
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Electrode is a piece of Metal that can Conduct Electricity.
Anode an Electrode that has a Positive End. Cathode an Electrode that has a Negative End.
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Crooke hypothesized that the green glow in the tube was caused by Rays, or Streams of particles.
Rays were called Cathode Rays Used in TV and Computer Screens
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E. J.J. Thomson Born AD 1856 Died AD 1940
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J. J. Thomson Tested Crooke’s Experiment
Was the Greenish Glow Light, or was it a stream of charged particles? Placed a Magnet beside the tube from Crooke’s experiment.
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The Beam is bent in the direction of the Magnet.
Light cannot be bent by a Magnet, as the beam couldn’t be Light. Conclusion—must be a charged particle that came from the Cathode.
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He showed that there was a mistake with Dalton’s Theory
He showed that there was a mistake with Dalton’s Theory. The Atom could be divided into smaller pieces. Electrons Negatively charged particle that exists in an Electron Clouds formation around an Atom’s Nucleus
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F. Ernest Rutherford Born AD 1871 Died AD 1937
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Ernest Rutherford Former student of Thomson’s
Designed an experiment to study the parts of the Atom He aimed a beam of small, positively particles at a thin sheet of Gold Foil He placed a coated screen around the foil The coating glowed when hit by the positively charged particles
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Hypothesized that almost all the Mass of the Atom and all of its Positive charge are crammed in the center of the Atom called the Nucleus. Proton Positive charge particle present in the Nucleus of all Atoms.
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An Atom’s Electron have almost No Mass.
Neutron Neutral particle that has the same Mass as a Proton and is found in an Atom’s Nucleus
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G Niels Bohr Born AD 1885 Died AD 1962
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Niels Bohr Danish scientist who worked with Rutherford
Studied the way Atoms react with Light Electrons travel in a region surrounding the nucleus called the Electron Cloud.
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II. Parts of the Atom Size of an Atom: Dot on your paper
2 Million Atoms Penny – 20,000,000,000,000,000,000,000 Atoms or 2 X 10²² Atoms of Copper and Zinc
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A. Nucleus (Center Atom)
Proton Positively Charged Particle --All Protons are Identical Neutron No Electrical Charged Particle --All Neutrons are Identical
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Nucleus (Center Atom) Neutrons are slightly Larger than Protons
Majority of Mass in Atom (Protons/Neutrons)
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B. Outside the Nucleus Electron Negatively Charged Particle
Has hardly any Mass Very Small (1,800 Electrons equal Mass of 1 Proton)
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Outside the Nucleus Electrons are found around the Nucleus in what is called Electron Clouds
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Mass Proton = Mass Neutron
Mass Proton > Mass Electron Mass Neutron > Mass Electron
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III. How do Atoms in an Element Differ?
Elements differ by their number of Protons, Neutrons, and Electrons in each Atom. The different numbers of each will give the Atom Special Properties
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How do Atoms in an Element Differ?
Elements can differ in charge The Element will either have a Neutral Charge (Proton = Electron), Positive Charge (Protons > Electrons), or Negative Charge (Electrons > Protons)
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Ion Atom that is No longer Neutral because it has gained or lost an Electron
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Problem 1 Protons 50 _________ Find the Amount of Electrons in the following Charges +1 = +12 = -15 = -100 =
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Protons 50 Mercury Find the Amount of Electrons in the following Charges +1 = +12 = -15 = -100 =
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Protons 50 Mercury Find the Amount of Electrons in the following Charges +1 = 79 e- +12 = -15 = -100 =
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Protons 50 Mercury Find the Amount of Electrons in the following Charges +1 = 79 e- +12 = 68 e- -15 = -100 =
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Protons 50 Mercury Find the Amount of Electrons in the following Charges +1 = 79 e- +12 = 68 e- -15 = 95 e- -100 =
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Protons 50 Mercury Find the Amount of Electrons in the following Charges +1 = 79 e- +12 = 68 e- -15 = 95 e- -100 = 180 e-
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Problem 2 Protons 45 _________ Find the Amount of Electrons in the following Charges +5 = +15 = -30 = -60 =
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Protons Rhodium Find the Amount of Electrons in the following Charges +5 = +15 = -30 = -60 =
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Protons Rhodium Find the Amount of Electrons in the following Charges +5 = 40 e- +15 = -30 = -60 =
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Protons Rhodium Find the Amount of Electrons in the following Charges +5 = 40 e- +15 = 30 e- -30 = -60 =
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Protons Rhodium Find the Amount of Electrons in the following Charges +5 = 40 e- +15 = 30 e- -30 = 75 e- -60 =
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Protons Rhodium Find the Amount of Electrons in the following Charges +5 = 40 e- +15 = 30 e- -30 = 75 e- -60 = 105 e-
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IV. Atomic Structure of an Atom
All Atoms are made up of 3 Subatomic Particles Proton Neutron Electron *** Most of an Atom is Empty Space
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You must know 3 things about each Subatomic Particle
What is the Charge? Where can it be Found? What is its JOB?
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A. Proton Positive Charge Found in Nucleus of an Atom
Identification of Element
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B. Neutron Neutral Charge Found in Nucleus of Atom
Radioactive (Yes or No) Yes if Neutrons > Protons No if Protons > Neutrons
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C. Electron Negative Charge
Found Outside the Nucleus in a region called the Electron Cloud Binding or Glue to form Compounds
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D. Atomic Number Is the number of Protons in the Nucleus of an Atom of that Element. The Atoms of different Elements contain different numbers of Protons.
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Examples Hydrogen has 1 Proton, Carbon has 6 Protons, Oxygen has 8 Protons, Iodine has 53 Protons, Mercury has 80 Protons, and Plutonium has 94 Protons Atoms of an Element are identified by the number of Protons because this number never changes without changing the identity of the Element.
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Mass Number The sum of Neutrons and Protons in the Nucleus of an Atom.
To calculate the number of Neutrons in an Isotope you must subtract the Atomic Number from the Mass Number of the Isotope.
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2 He 4.00 Protons = Electrons = Neutrons =
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2 He 4.00 Protons = 2 Electrons = Neutrons =
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2 He 4.00 Protons = 2 Electrons = 2 Neutrons =
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2 He 4.00 Protons = 2 Electrons = 2 Neutrons = 2
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6 C 12.00 Protons = Electrons = Neutrons =
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6 C 12.00 Protons = 6 Electrons = Neutrons =
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6 C 12.00 Protons = 6 Electrons = 6 Neutrons =
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6 C 12.00 Protons = 6 Electrons = 6 Neutrons = 6
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82 Pb 207.2 Protons = Electrons = Neutrons =
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82 Pb 207.2 Protons = 82 Electrons = Neutrons =
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82 Pb 207.2 Protons = 82 Electrons = 82 Neutrons =
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82 Pb 207.2 Protons = 82 Electrons = 82 Neutrons = 125
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80 Hg 200.6 Protons = Electrons = Neutrons =
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80 Hg 200.6 Protons = 80 Electrons = Neutrons =
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80 Hg 200.6 Protons = 80 Electrons = 80 Neutrons =
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80 Hg 200.6 Protons = 80 Electrons = 80 Neutrons = 120
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1 H Protons = Electrons = Neutrons =
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1 H Protons = 1 Electrons = Neutrons =
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1 H Protons = 1 Electrons = 1 Neutrons =
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1 H Protons = 1 Electrons = 1 Neutrons = 0
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92 U 238.0 Protons = Electrons = Neutrons =
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92 U 238.0 Protons = 92 Electrons = Neutrons =
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92 U 238.0 Protons = 92 Electrons = 92 Neutrons =
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92 U 238.0 Protons = 92 Electrons = 92 Neutrons = 146
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E Neutrons The Atomic Number is the number of Protons, but what about the number of Neutrons in an Atom’s Nucleus. A particular type of Atom can have a varying number of Neutrons in its Nucleus
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Isotopes Are Atoms of the same Element that have different numbers of Neutrons. Carbon-12 (Normal) Carbon-13 (Isotope) Carbon-14 (Isotope)
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Helium Isotopes
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Radioactive Decay Many Atomic Nuclei are stable when they have about the same number of Protons and Neutrons. Some become unstable when too many or too few Neutrons.
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Examples Uranium and Plutonium
Repulsion builds up because the Nucleus must release a particle to become Stable When particles are Released, Energy is Given Off
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Radioactive Decay release of Nuclear particles and Energy from Unstable Nuclei
Transmutation the changing of one Element into another through Radioactive Decay Example – Smoke Alarm
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Transmutation (Fire Alarm)
Americium --- Neptunium (Am) (Np) 95 Protons 93 Protons 146 Neutrons 144 Neutrons -- Makes Electric Current
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Half-Life Half-Life time needed for one-half of the mass of a sample of a radioactive isotope to decay.
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Half –Life Problem: Tritium has a Half-Life of 12.5 years. If you start with 20 grams of Tritium, how much is left after 50 years?
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Half –Life Problem: Cycles = __50 Years__ 12.5 Years Cycles = 4 Cycles
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Half –Life Problem: Grams = 10 g 2 Grams = 5g
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Half –Life Problem: Grams = 2.5 g 2 Grams = 1.25 g
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Carbon Dating Carbon-14 Half-Life of 5,730 Years.
Radioactive Decay is useful in determining the Age of Artifacts and Fossils. Carbon-14 Half-Life of 5,730 Years.
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Carbon Dating In a living Organism, the amount of Carbon-14 remains in Constant Balance with the levels of the Isotope in the Atmosphere or Ocean. The Balance occurs because Living Organisms take in and release Carbon.
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Carbon Dating While life processes go on, any Carbon-14 Nucleus that Decays is replaced by another from the Environment. When the Plant and Animal dies, the Decaying Nuclei no longer can be replaced.
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V. Calculating the Mass of an Element
Atomic Mass of an Element is the weighted average of the Masses of all the naturally occurring Isotopes of that Element Atomic Mass Unit = AMU
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Atomic Mass Problem Chlorine-35 makes up 76% of all the Chlorine in nature, and Chlorine-37 makes up the other 24%. What is the Atomic Mass of Chlorine?
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Atomic Mass Problem Cl – 35 x .76 = 26.60 Cl – 37 x .24 = 8.80
= amu
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VI. Forces of an Atom Gravitational Force – depends on Mass and Distance Apart -- since Atom very small this Force is Small
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Forces of an Atom 2. Electromagnetic Force – objects that have the same Charge Repel each other, while objects with Opposite charge Attract each other -- Strong Force
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Forces of an Atom Strong Force -- Protons push away from each other
-- Strongest Force
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Forces of an Atom 4. Weak Force – Occurs in Radioactive Atoms when Neutrons change to Protons -- Weak Force
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