Atomic Structure. Chemistry Timeline #1 B.C. 400 B.C. Demokritos and Leucippos use the term "atomos. Chemistry Timeline #2

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1 Atomic Structure Ch 2 Atomic Structure and Periodicity Big Idea #1: The chemical elements are fundamental building materials of matter, and all matter can be understood in terms of arrangements of atoms. These atoms retain their identity in chemical reactions. Image courtesy of I CAN: Use evidence to explain why atomic models have changed throughout history. Calculate the average atomic mass of an atom from mass spectrometry and isotopic data. Name compounds and write formulas for binary and ternary compounds and acids. Chemistry Timeline #1 B.C. 400 B.C. Demokritos and Leucippos use the term "atomos 2000 years of Alchemy 1500's Georg Bauer: systematic metallurgy Paracelsus: medicinal application of minerals Chemistry Timeline #2 1800's Joseph Proust: The law of definite proportion composition John Dalton: The Atomic Theory, The law of multiple proportions Joseph Gay Lussac: Combining volumes of gases, existence of diatomic molecules Amadeo Avogadro: Molar volumes of gases Jons Jakob Berzelius: Relative atomic masses, modern symbols for the elements Dmitri Mendeleyev: The periodic table J.J. Thomson: discovery of the electron Henri Becquerel: Discovery of radioactivity 1600's Robert Boyle:The Skeptical Chemist. Quantitative experimentation, identification of elements 1700's Georg Stahl: Phlogiston Theory Joseph Priestly: Discovery of oxygen Antoine Lavoisier: The role of oxygen in combustion, law of conservation of mass, first modern chemistry textbook 1900's Robert Millikan: Charge and mass of the electron Ernest Rutherford: Existence of the nucleus, and its relative size Meitner & Fermi: Sustained nuclear fission Ernest Lawrence: The cyclotron and trans uranium elements 1

2 John Dalton Dalton s Atomic Theory (1808) All matter is composed of extremely small particles called atoms Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in simple wholenumber ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged Modern Atomic Theory Several changes have been made to Dalton s theory. Dalton said: Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties Modern theory states: Atoms of an element have a characteristic average mass which is unique to that element. Modern Atomic Theory #2 Dalton said: Atoms cannot be subdivided, created, or destroyed Modern theory states: Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions Laws Law of Definite Proportion - a given compound always contains the same proportion of elements by mass example: H 2 O Law of Multiple Proportions - when two elements form more than one compound, the ratios of the masses of the second element that combine with one gram of the first can be reduced to small whole numbers. example: H 2 O and H 2 O 2 2

3 Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. Thomson s Atomic Model J.J. Thomson Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. Thomson believed that the electrons were like plums embedded in a positively charged pudding, thus it was called the plum pudding model. The oil drop apparatus Mass of the Electron 1909 Robert Millikan determines the mass of the electron. Mass of the electron is x kg Conclusions from the Study of the Electron Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons Electrons have so little mass that atoms must contain other particles that account for most of the mass hill.com/sites/ /student_view0/chapter2/animations_center.htm 3

4 Rutherford s Gold Foil Experiment Try it Yourself! In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target? hill.com/sites/ /student_view0/chapter2/animations_center.htm Alpha particles are helium nuclei Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded The Answers Target #1 Target #2 Rutherford s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected Like howitzer shells bouncing off of tissue paper! Conclusions: The nucleus is small The nucleus is dense The nucleus is positively charged 4

5 Atomic Particles The Atomic Scale Most of the mass of the atom is in the nucleus (protons and neutrons) Electrons are found outside of the nucleus (the electron cloud) Most of the volume of the atom is empty space Helium-4 Image: User Yzmo Wikimedia Commons. About Quarks Protons and neutrons are NOT fundamental particles. Isotopes Isotopes are atoms of the same element having different masses due to varying numbers of neutrons. Protons are made of two up quarks and one down quark. Neutrons are made of one up quark and two down quarks. Quarks are held together by gluons Images: Arpad Horvath, Wikimedia Commons. 5

6 Atomic Masses Atomic mass is the weighted average of all the natural isotopes of that element. Carbon = Atomic Number Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope. Mass # = p + + n Arsenic Phosphorus

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