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1 Atomic Structure Image courtesy of

2 Chemistry Timeline #1 B.C. 400 B.C. Demokritos and Leucippos use the term "atomos” 1500's  Georg Bauer: systematic metallurgy  Paracelsus: medicinal application of minerals 1600's Robert Boyle:The Skeptical Chemist. Quantitative experimentation, identification of elements 1700s'  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  2000 years of Alchemy

3 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 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

4 Dalton’s Atomic Theory (1808)  Atoms cannot be subdivided, created, or destroyed  Atoms of different elements combine in simple whole-number ratios to form chemical compounds  In chemical reactions, atoms are combined, separated, or rearranged  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 John Dalton

5 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.

6 Modern Atomic Theory #2 Dalton said: Modern theory states: Atoms cannot be subdivided, created, or destroyed Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions

7 Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

8 Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. J.J. Thomson

9 Mass of the Electron 1909 – Robert Millikan determines the mass of the electron. The oil drop apparatus Mass of the electron is x kg Mass of the electron is x kg

10 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

11 Rutherford’s Gold Foil Experiment  Alpha particles are helium nuclei  Particles were fired at a thin sheet of gold foil  Particle hits on the detecting screen (film) are recorded

12 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?

13 The Answers Target #1Target #2

14 Rutherford’s Findings  The nucleus is small  The nucleus is dense  The nucleus is positively charged  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:

15 Atomic Particles

16 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.

17 About Quarks… Protons and neutrons are NOT fundamental particles. 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.

18 Atomic Number Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.

19 Mass Number Mass number (A) is the number of protons and neutrons in the nucleus of an isotope. Mass # = p + + n Arsenic Phosphorus

20 Atomic Masses average Atomic mass is the average of all the naturally isotopes of that element. Carbon =

21 Isotopic Notation Isotopic Notation: X Write the isotopic notation for the following elements: Boron, Titanium, and Carbon Z A

22 Isotopes Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.

23 Using Isotopic Ratios to calculate the average mass of an element Silver (Ag; Z = 47) has 46 known isotopes, but only two occur naturally, 107 Ag and 109 Ag. Given the following data, calculate the atomic mass of Ag: IsotopeMassPercent Abundance 107 Ag Ag

24 Try one… Gallium has two naturally occurring isotopes: 69 Ga (isotopic mass amu, 60.11% abundance) and 71 Ga (isotopic mass amu, 38.89% abundance) Calculate the average atomic mass

25 Another one… Magnesium has three naturally occurring isotopes seen in the table below: IsotopeMassPercent Abundance 24 Mg Mg Mg

26 A little more complicated… Copper has two naturally occurring isotopes, 63 Cu (isotopic mass amu) and 65 Cu (isotopic mass ). If copper has an atomic mass of amu, what is the percent abundance of each isotope?


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