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THE PERIODIC TABLE AND THE ATOM Overview The atom, while now a familiar concept, is a relatively new theory. It is truly a theory, since nobody has every actually seen one individual atom. As we will see, the evidence for this theory is overwhelming. Dalton’s Atomic theory In 1802: An element is made up of tiny particles called atoms Atoms of an element are identical. Atoms of different elements are different in some fundamental way. Compounds are formed by combinations of different elements, whose ratio of atoms remains the same. Reactions involve reorganizing atoms; the atoms themselves are unchanged. The First Picture J. J. Thomson in 1897 Experimenting with Cathode Ray Tubes Found that the beam deflected when a magnet was brought near If the beam was made of one charge of matter, the rest of matter must have opposite charge Plum Pudding Model of the Atom Cathode Ray Tube Plum Pudding Model Positive matter Negative Particles Getting to the Core Ernest Rutherford in 1912 “Gold Foil Experiment” Assumed: Like Thompson said, the atom is full of positive matter with negative chunks distributed throughout Assumed the large positive radiation would blast through the atoms’ matter All radiation would be detected on other side of foil Gold Foil Experiment Rutherford’s Model Actual Results Majority of radiation went through Few deflections to sides No radiation stuck in foil; all went through Atomic Model • Atom is mostly empty space • Positively charged nucleus • Electrons very little mass, randomly in empty space Rutherford’s Model Why is this thing so important? Gives information on individual elements or atoms Visual representation of properties of the elements Predicts the existence and properties of elements yet to be discovered Individual Atoms Atomic Mass 79 Symbol Se Name Selenium Atomic Number 34 Atom smallest particle of an element that retains the properties of the element. Atom Contains… Protons (Nucleus) Neutrons (Nucleus) Electrons (Shell) Atom Protons (Positive Charge) Neutrons (Neutral Charge) Electrons (Negative Charge) What are those things? Name: Typically named by discoverer, but must be approved by the IUPAC (International Union of Pure and Applied Chemistry) Typically named after people or places Thorium: Thor, god of thunder Einsteinium: Albert Einstein, scientist Berkelium: University of California at Berkely What are those things? Symbol: Once named, the symbol has 1 or 2 letters Second letter always lowercase N (Nitrogen) Ti (Titanium) Co (Cobalt: pretty blue dye used in art) CO (Carbon Monoxide: colorless gas used in death) Sometimes symbol is based off of something unusual Pb (lead): based on Latin word “plumbus”, which by the way gave us the word “plumber” as well, since pipes used to be made out of lead! W (Tungsten): element found in the mineral Wolframite What are those things? Atomic Number: Represents number of protons in one atom Under usual conditions protons = electrons PT organized by atomic number Atomic Mass: Represents the mass of one atom It is calculated by adding the protons + neutrons So why isn’t the value shown on the PT a whole number? Example How many protons, neutrons, and electrons are in one atom of Gold? 197 Au Protons = 79 Electrons =79 Neutrons =197-79 = 118 Gold 79 Periodic Table Basics Valence Electron Electrons found in the outer shell. Rules… 1st shell can hold 2 electrons. 2nd, 3rd, 4th… can hold 8 electrons. Review Atoms are mostly ____. A. positive B. negative C. solid spheres D. empty space Review Whose work led to the modern atomic theory? A. Dalton B. Rutherford C. Einstein D. Aristotle Review What are the two fundamental subatomic particles found in the nucleus? A. proton and electron B. proton and neutron C. neutron and electron D. neutron and positron Isotopes Atoms with the same number of protons but different numbers of neutrons are called isotopes. Uses: dating matter, nuclear power, smoke detectors Mass of Atoms The mass number is the sum of the protons and neutrons in the nucleus. Mass of Atoms The atomic mass of an element is the weighted average mass of the isotopes of that element. Review Elements with the same number of protons and differing numbers of neutrons are known as what? A. isotopes B. radioactive C. abundant D. ions Review An unknown element has 19 protons, 19 electrons, and 3 isotopes with 20, 21 and 22 neutrons. What is the element’s atomic number? A. 38 B. 40 C. 19 D. unable to determine