1 The Structure of the Atom

2 Early Theories of Matter

BC Democritus l Greek Philosopher l Named atom »smallest unit of matter »means indivisible

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John Dalton: Atomic Theory l Revived and revised Democritus’ ideas and began developing the modern atomic theory

6 Dalton’s Atomic Theory l all elements are composed of tiny indivisible particles called atoms l atoms of same element alike. Each element is different from atoms of other elements l atoms of different elements combine in simple whole number ratios to form compounds l chemical reactions occur when atoms are rearranged

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8 Picture shows: Conservation of Mass Element combing in simple whole number ratios

9 Subatomic Particles & the Nuclear Atom

10 Discovering the Electron

William Crookes l investigated electrical discharge in gases l cathode ray tube l Cathode rays are streams of negatively charged particles. l The particles are found in all forms of matter

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J.J. Thomson l determined nature of cathode ray l determined charge to mass ratio of electron l Found that atoms were divisible - Dalton & Democritus were wrong

J.J. Thomson l positive beam experiments l plum pudding model of atom or chocolate-chip cookie dough model of the atom

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Robert Milliken l determined charge of electron l oil drop experiment l with Thomson’s charge to mass ratio: able to determine the mass of e- l Mass of electron = 9.1x grams

Ernest Rutherford l discovered nucleus l gold foil experiment disproved plum pudding model l small dense central part of atom = nucleus l (+) charge

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Rutherford l Refined concept of nucleus l Concluded that nucleus contained positively charged particles called protons

James Chadwick l identified neutron l same mass as proton l no charge

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25 How Atoms Differ

26 NUCLEUS 1. Protons with (+) charge 2. Neutrons with no charge. 3. Protons & neutrons have about the same mass. 4. (+) charge is responsible for most of mass of atom (dense central part).

27 ELECTRONS l move around nucleus l responsible for most of volume of atom l (-) charge l negligible mass

28 ATOMIC NUMBER & MASS NUMBER

29 ATOMIC NUMBER l # of protons in nucleus l it identifies the element l elements in Periodic Table are listed in increasing order of atomic # l if atom is neutral: the # of protons equals # of electrons

30 MASS NUMBER l sum of protons & neutrons in nucleus l written as part of name: must be given to you Neon-20 mass #20 p + n = 20 atomic #10 p = 10 n = 10

31 mass # p + n Symbol atomic # p ? # neutrons

32 Oxygen-17 mass # 17 atomic # 8 p + n = 17 p =_8_ 9 n

33 l To calculate electrons for an ion you must look at the charge written in the upper right corner l To determine the number of electrons: »If the charge is positive then subtract that number from the number of protons. »If the charge is negative then add that number to the number of protons

34 ISOTOPES l Atoms of the same element are not all identical - may differ in # of neutrons l Isotopes - atoms of the same elements (same # of protons), but different mass # (different # neutrons) and therefore different masses

C C C ? # neutrons 6 7 8

36 ATOMIC MASS

37 ATOM l smallest unit of an element that can exist alone and still have the properties of that element

38 Atomic Mass l Average Atomic Mass - weighted average of the masses of the naturally occuring isotopes »relative mass based on carbon-12 as the standard »Carbon-12 is defined as having a mass of exactly 12 amu l atomic mass unit (amu) - 1/12 of the mass of a carbon-12 atom

39 Weighted Average Example 50% test70 30% Lab80 20% Daily90 = 50%(70) + 30%(80) + 20%(90) =.5(70) +.3(80) +.2(90) = = 77

40 Isotopes of Hydrogen H-1 protium % H-2 deuterium % H-3 tritium

41 Calculating Average Atomic Mass Multiply the percent (as a decimal #) by the mass and then add each together. = % ( amu) % ( amu) = ( amu) ( amu) = amu amu = amu

42 Example chlorine % chlorine % Will the average atomic mass be closer to 35 or 37? (35 because higher %) 75.8%(35) %(37) = 35.5 amu