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Chapter 3 History of the Atom History of Atom Part 1 Dalton’s Atomic Theory (1808) 1. All matter is made of indivisible and indestructible atoms. 2. All atoms of the same element are identical in their physical and chemical properties. 3. Atoms of different elements have different properties. 4. Atoms of different elements combine in simple whole-number ratios to form compounds. 5. Atoms cannot be subdivided, created, or destroyed when they are combined, separated, or rearranged in chemical reactions. Changes to Dalton’s Theory Are atoms actually indivisible? Are all atoms identical? The Atom the smallest unit of an element that retains that elements properties Made of protons, neutrons, and electrons Can you see inside an atom? How did they know it contains protons, neutrons and electrons? Obscertainer Activity History of Atom Part 2 JJ Thomson’s Cathode Ray Tube Cathode: Negatively Charged Anode: Positively Charged Cathode Ray Tube When connected to electric current the remaining the gas forms a BEAM OF LIGHT. The beam always started at the NEGATIVE electrode and flowed to the POSITIVE electrode. The electrode is named by what type of particle it attracts Cathode: Negative (-) Anode: Positive (+) Cathode Ray Conclusions 1. The beam was attracted to a POSITIVE magnet. 2. The beam must be made of NEGATIVE particles. JJ THOMSON DISCOVERED A NEGATIVE PARTICLE CALLED THE: ELECTRON! Plum Pudding Model Plum Pudding OR Chocolate Chip Cookie Radioactivity Becquerel discovered uranium RADIATION would expose photographic plates in the dark The properties of an element changed as it gave off radiation Curie Discovered radium and polonium The radioactive emissions of alpha, beta and gamma rays were identified. Types of Radiation Radiation Type Alpha Beta Gamma Greek Symbol Charge Ernest Rutherford’s Gold Foil Experiment Set up Gold Foil with a detection sheet around it. Set up radioactive source emitting alpha particles. ALPHA PARTICLES shot at gold foil. MOST particles went through the gold foil SOME particles BOUNCED back Rutherford’s Gold Foil Experiment alpha particles shot through the gold foil bouncing back – like hitting the target going through – missing the target Gold Foil Conclusions 1. 2. 3. The atom is made up of mostly EMPTY SPACE The center of the atom contains a POSITIVE CHARGE Rutherford called this positive bundle of matter the NUCLEUS Rutherford’s Model of the Atom + – NUCLEUS EMPTY SPACE History of Atom Part 3 Goldstein and Wien - 1886 Used a cathode ray tube Discovered collection of positively charged particles **DISCOVERED THE PROTON!! Chadwick’s Experiment - 1932 Found that alpha particles shot at beryllium made a beam form The beam had the same mass of a proton but was electrically neutral **DISCOVERED THE NEUTRON!! Niels Bohr – 1913 Developed a new diagram of the atom Electrons can only be at certain energies Electrons must gain a specific amount of energy to move to a higher level, called a quantum **DISCOVERED ENERGY LEVELS!! Bohr’s Model of the Atom – + ENERGY LEVELS NUCLEUS Bohr Models n=3 n=2 n=1 Energy Level (n) 1 P+ no 2 Nucleus = correct number of protons & neutrons 3 # of Electrons How to Fill in Bohr Models Element Symbols Element Symbols Rule 1 First letter is capitalized H Hydrogen C Carbon O Oxygen Rule 2 Second letter is lower case Cl Chlorine He Helium Ne Neon Rule 3 Some are Latin names Na Sodium (natrium) Fe Iron (ferrum) Rule 4 Some elements are named for places, scientists, greek gods… Eu Europium Am Americium Hyphen Notation The mass number is written with a hyphen after the name of the element. Element – Mass# Uranium – 235 Calcium – 40 Nuclear Symbol The superscript indicates the mass number and the subscript indicates the atomic number. A X Z Mass Number Atomic Number 12 C 6 Element Symbol Subatomic Particles Subatomic Particles PARTICLE PROTON NEUTRON ELECTRON SYMBOL CHARGE LOCATION RELATIVE MASS ALL ATOMS ARE ELECTRICALLY NEUTRAL Must have same number of protons and electrons Atomic Number: number of protons NUMBER Every OF PROTONS NEVER CHANGES! atom of an element has the same number of protons which makes the element unique Atomic Number Examples 3 Li 6.94 Lithium: atomic number of 3 How many protons? Hydrogen: atomic number of 1 How many protons? If you add another proton, does it become positive? Mass Number Protons + Neutrons This is not the actual mass of the atom Masses on the periodic table are NOT whole numbers Examples: Silicon 14 protons Mass Number = ______ Fluorine 9 protons Mass number = 19 14 neutrons ____ neutrons ELEMENT SYMBOL ATOMIC NUMBER PROTONS NEUTRONS ELECTRONS Nitrogen MASS NUMBER 15 Cu 35 38 88 12 12 30 26 Isotopes and Ions Isotopes Atoms with the same number of protons and different number of neutrons Hydrogen has 3 isotopes: 1p0n 1p1n Hydrogen – 1 Hydrogen Hydrogen – 2 Deuterium 1p2n Hydrogen – 3 Tritium Do You Understand Isotopes? How many protons, neutrons, and electrons are in 146 C? 6 protons, 8 (14 - 6) neutrons, 6 electrons How many protons, neutrons, and electrons are in 116 6 protons, 5 (11 - 6) neutrons, 6 electrons C? ION : an atom, or group of atoms, that has a net positive or negative charge. cation – ion with a positive charge when an atom loses an electron Na 11 protons 11 electrons Na+ 11 protons 10 electrons anion – ion with a negative charge when an atom gains an electron Cl 17 protons 17 electrons Cl- 17 protons 18 electrons Do You Understand Ions? + 27 3 How many protons and electrons are in 13 Al ? 13 protons, 10 (13 – 3) electrons 2- ? Se How many protons and electrons are in 78 34 34 protons, 36 (34 + 2) electrons Summary NEVER CHANGES!!! Particle Protons Neutrons Electrons Atom Same Same Same Isotope Same DIFFERENT Same Ion Same Same DIFFERENT Average Atomic Mass Relative Atomic Mass Carbon is the standard for all masses on the periodic table. Carbon: 6 p and 6 n = 12 amu Atomic Mass Unit Periodic table lists weighted average atomic masses of elements (like a GPA calculation) Final Grade Calculation Calculation AVERAGE Atomic Mass 75% 20% 5% 133Cs 132Cs Steps: 1. Percent to a decimal 134Cs 2. Multiply by mass 3. ADD IT UP!
