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Ch. 4 Sizing up the Atom  Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties.

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Presentation on theme: "Ch. 4 Sizing up the Atom  Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties."— Presentation transcript:

1

2 Ch. 4

3 Sizing up the Atom  Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that element  If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long  Despite their small size, individual atoms are observable with instruments such as scanning tunneling (electron) microscopes

4 An STM image of nickel atoms placed on a copper surface. Source: IBM Research

5 Red ridge is a series of Cesium atoms

6 Image of a ring of cobalt atoms placed on a copper surface. Source: IBM Research

7 Atom - smallest particle making up elements One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water!

8 Think about the technological advances of the past 100 years! They have been nothing short of miraculous! RadiosCalculators TelevisionsComputers AutomobilesCell phones Jet airplanesIpods PlasticVelcro RefrigeratorsInternet (thanks, Al Gore) PenicillinCD’s & DVD’s Insulinand, of course - Electric guitars Sliced Bread!

9 Development of Atomic Theory This explosion of technology occurred once we had a better understanding of the atom and how it behaves!

10 Where did it all begin? The word “atom” comes from the Greek word “atomos” which means indivisible. The word “atom” comes from the Greek word “atomos” which means indivisible. The idea that all matter is made up of atoms was first proposed by the Greek philosopher Democritus in the 5th century B.C. The idea that all matter is made up of atoms was first proposed by the Greek philosopher Democritus in the 5th century B.C.

11 Then came the idea of “The 4 Basic Elements” Earth, Air, Fire, & Water After that came Alchemy. The change to “real” Chemistry didn’t occur until the first true element was discovered! (1774) The first element discovered was

12 The discovery of oxygen is attributed to 3 scientists (working independently) Karl Scheele (1771) (German) Karl Scheele (1771) (German) first to prepare and describe oxygen first to prepare and describe oxygen Joseph Priestley (1774) (British) Joseph Priestley (1774) (British) isolated oxygen gas from mercuric oxide. isolated oxygen gas from mercuric oxide. observed accelerated burning observed accelerated burning Antoine Lavoisier (1784) (French) Antoine Lavoisier (1784) (French) made accurate measurements and interpreted Priestley’s results made accurate measurements and interpreted Priestley’s results

13 Carl Wilhelm Scheele beat Priestley to the discovery but published afterwards. Carl Wilhelm Scheele beat Priestley to the discovery but published afterwards. Too bad! – So sad!

14 Priestley Medal Source: Roald Hoffman, Cornell University Priestley gets the main credit for discovering oxygen!

15 Priestley produced a gas (oxygen) by using sunlight to heat mercuric oxide kept in a closed container. The oxygen forced some of the mercury out of the jar as it was produced, increasing the volume about five times. Priestley produced a gas (oxygen) by using sunlight to heat mercuric oxide kept in a closed container. The oxygen forced some of the mercury out of the jar as it was produced, increasing the volume about five times. 2HgO (s) → 2Hg (l) + O 2 (g)

16 Priestley: Scientific Contributions DISCOVERY OF 8 GASES Oxygen Oxygen Nitrogen Nitrogen Carbon Dioxide Carbon Dioxide Carbon Monoxide Carbon Monoxide Sulfur Dioxide Sulfur Dioxide Nitrous Oxide Nitrous Oxide Nitric Oxide Nitric Oxide Hydrogen Chloride Hydrogen Chloride

17 Priestley: Additional Scientific Contributions Discovered the interconnection between photosynthesis and respiration Discovered the interconnection between photosynthesis and respiration Discovered carbonated water Discovered carbonated water Discovered that India rubber removed graphite pencil marks - the first rubber eraser Discovered that India rubber removed graphite pencil marks - the first rubber eraser Now we can make mistakes!!

18 Antoine-Laurent Lavoisier Lavoisier: the Founder of Modern Chemistry Lavoisier continued the investigations of Priestly Quantitative experiments led to: Law of Conservation of Matter. He systematized the language of chemistry, its nomenclature and rhetoric. He was beheaded during the Reign of Terror for his role as a tax “farmer” prior to the Revolution (Priestley escaped to America!)

19 Lavoisier heated a measured amount of mercury to form the red mercuric oxide. He measured the amount of oxygen removed from the jar and the amount of red oxide formed. When the reaction was reversed, he found the original amounts of mercury and oxygen. Lavoisier heated a measured amount of mercury to form the red mercuric oxide. He measured the amount of oxygen removed from the jar and the amount of red oxide formed. When the reaction was reversed, he found the original amounts of mercury and oxygen. 2Hg (l) + O 2 (g) → 2HgO (s)

20 Properties of Oxygen Colorless Colorless Odorless Odorless Tasteless Tasteless Gas at room temperature Gas at room temperature Slightly soluble in water Slightly soluble in water Inflammable (does NOT burn) Inflammable (does NOT burn) Only part of air that supports combustion Only part of air that supports combustion Physical Property or Chemical Property? P PPPPCCPPPPCC

21 These properties of oxygen were later used to determine the properties of other substances. By the late 18 th century, scientists finally came to the conclusion that Oxygen was truly an element (can’t be broken down into simpler forms without losing its properties) Scientists began to search for & test other new elements.

22 Sometimes, when they tried to react substances together, nothing happened! Substances that DO NOT react are Inert They found that most materials will react to form new substances. These elements are said to be chemically active (reactive) Oxygen is very reactive, so is hydrogen which we will look at next! Increasing chemical reactivity inert Oxygenhydrogen

23 Discovery of Henry Cavendish (1766) Reacted various metals with acids producing a salt and hydrogen gas Acid + metal → hydrogen gas + salt Zinc + sulfuric acid → Hydrogen + zinc sulfate Zn (s) + H 2 SO 4(aq) → H 2 (g) + ZnSO 4 (aq) While testing the properties of Hydrogen he While testing the properties of Hydrogen he found that water is a compound found that water is a compound (1731 – 1810) Word Equation Chemical equation

24 Hydrogen + Oxygen Water Hydrogen + Oxygen Water 2H 2 + O 2 2H 2 O

25 Antoine Lavoisier Named Priestly’s newly discovered gas - “oxygen” - meaning “acid former” Named Priestly’s newly discovered gas - “oxygen” - meaning “acid former” Named Cavendish’s new gas “hydrogen” - meaning “water former” Named Cavendish’s new gas “hydrogen” - meaning “water former”

26 Dalton’s Atomic Theory John Dalton ( ) While his theory was not completely correct, it revolutionized how chemists looked at matter and brought about chemistry as we know it today (instead of alchemy) So, it’s an important landmark in the history of science.

27 Dalton’s Modern Atomic Theory (experiment based!) 3)Atoms of different elements combine in simple whole-number ratios to form chemical compounds 4)In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element. 1)All elements are composed of tiny indivisible particles called atoms 2)Atoms of the same element are identical. Atoms of any one element are different from those of any other element.

28 Law of Definite Proportions n Each compound has a specific ratio of elements by mass. n Ex: Water is always 8 grams of oxygen for each gram of hydrogen.

29 Discovery of the Electron Began with the invention of the Crooke’s Tube (cathode ray tube) c. 1875

30 Cathode Ray Tube Electric current sent through gases sealed in tube at low pressure Anode- positive electrode Cathode- negative electrode Voltage source Metal Disks - electrodes - + gas

31 Modern Cathode Ray Tubes  Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. TelevisionComputer Monitor

32 In 1897, J.J. Thomson used a cathode ray tube to study gases.

33 n Passing an electric current makes a beam appear to move from the negative to the positive end – so the ‘beam’ was called a “Cathode Ray” Thomson’s Experiment Voltage source + -

34 Thomson’s Experiment n Thomson found that cathode rays were deflected from a negatively- charged plate. -

35 Voltage source Thomson’s Experiment n and that cathode rays were attracted to plates with a positive charge n Does light bend like this? +

36 Light doesn’t ‘bend’ so the cathode ray must be made of particles rather than Light! Since they are attracted to a positive plate & repelled by a negative one the particles aren’t neutral – What charge must they have? That’s right! NEGATIVE!! Thomson called these negative particles – ELECTRONS

37 Mass of the Electron 1916 – Robert Millikan determined the mass of the electron: 1/1840 the mass of a hydrogen atom; and, has one unit of negative charge The oil drop apparatus Mass of the electron is 9.11 x g

38

39 Conclusions from the Study of the Electron: a)Cathode rays have identical properties regardless of the element used to produce them. Therefore, all elements must contain identically charged electrons. b)Atoms are neutral, so there must be a positive substance in the atom to balance the negative charge of the electrons c)Electrons have so little mass that atoms must contain other particles that account for most of their mass

40 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

41 Plum-Pudding Model Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 56

42 In 1903, An important discovery leading to further understandings of atomic structure happened by accident. Henri Becquerel discovered radioactivity Radioactivity is the spontaneous emission of energy from an object 1903: Shared a Nobel Prize with Pierre andNobel PrizePierre Marie CurieMarie Curie for discovering radioactivity.

43 Ernest Rutherford ( ) The Nobel Prize in Chemistry 1908 Studied under J. J. Thomson

44 3 Types of Radiation discovered by Ernest Rutherford Alpha ( ά ) – a positively charged helium nucleus 4 2 He +2 Alpha ( ά ) – a positively charged helium nucleus 4 2 He +2 Beta ( β ) – fast-moving electrons - eBeta ( β ) – fast-moving electrons - e Gamma ( γ ) – like high-energyGamma ( γ ) – like high-energyx-rays

45 Ernest Rutherford’s Gold Foil Experiment  Shot alpha particles at a thin sheet of gold foil  Particles that hit on a detecting screen (film) were recorded

46 Lead block Polonium Gold Foil Flourescent Screen

47 He Expected: The alpha particles to pass through the foil without changing direction very much. The alpha particles to pass through the foil without changing direction very much. Because… Because… The positive charges were spread out evenly (according to Thomson’s atomic theory). Alone they were not enough to stop the alpha particles. The positive charges were spread out evenly (according to Thomson’s atomic theory). Alone they were not enough to stop the alpha particles.

48 What he expected

49 Again, because he thought the mass was evenly distributed in the atom

50 What he got

51 Rutherford’s Observations Most of the particles went straight through the foil (what he expected) Most of the particles went straight through the foil (what he expected) A few particles were slightly deflected A few particles were slightly deflected Still fewer actually bounced back towards the source! Still fewer actually bounced back towards the source! Astonishing!!! Astonishing!!! Rutherford said it was like firing a Howitzer shell at a piece of tissue paper & having it bounce back & hit you! Rutherford said it was like firing a Howitzer shell at a piece of tissue paper & having it bounce back & hit you! “Like howitzer shells bouncing off of tissue paper!”

52 +

53 Rutherford’s Conclusions Since most of the particles went through the foil - atoms are mostly empty space. Since most of the particles went through the foil - atoms are mostly empty space. Because a few particles were deflected they must have come close to a positively charged core. Because a few + particles were deflected they must have come close to a positively charged core. Since a very few particles were deflected straight back, the positively-charged core must be very dense. Since a very few particles were deflected straight back, the positively-charged core must be very dense. This small dense positive area is the nucleus. This small dense positive area is the nucleus. +

54 The Rutherford Atomic Model Based on his experimental evidence: Based on his experimental evidence: The atom is mostly empty space The atom is mostly empty space All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus” All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus” The electrons are distributed around the nucleus, and occupy most of the volume The electrons are distributed around the nucleus, and occupy most of the volume His model was called a “nuclear model” His model was called a “nuclear model”

55 Discovery of Protons  Eugen Goldstein in 1886 observed particles with a positive charge passing through a perforated cathode.

56 In 1920, Rutherford studied these particles & called them protons. They have a charge of positive 1 and a mass of 1.7 x grams. This is not a ‘handy’ number to work with so we use a mass of 1 amu. Amu stands for “atomic mass unit”

57 1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton (1 amu). Discovery of the Neutron Rutherford predicted the existence of the neutron in Twelve years later, his assistant found it! So now we have a more complete picture of an atom!

58 Subatomic Particles ParticleCharge Mass (g) Location Electron (e - ) (e - ) 9.11 x g (virtually 0) outside nucleus Proton (p + ) (H + ) (H + )+1 1 amu (1.7 x g) in nucleus Neutron (n o ) (n o )0 1 amu 1 amu (1.67 x g) (1.67 x g) in nucleus

59 Elements are the new building blocks Hydrogen Nitrogen-7 Oxygen-8 Carbon-6

60 Between 1912 and 1914, the physicist H.G.J. Moseley conducted a series of experiments where he bombarded targets made out of different kinds of metals with cathode rays. Each metal he studied emitted X-rays of a characteristic frequency, almost like a set of "fingerprints". Henry Moseley (1887 – 1915)

61 The pattern that emerged when the observed X-rays were organized in order of increasing frequency suggested to Moseley a regular increase in the positive charge on the nuclei of the atoms. He called this positive nuclear charge- the Atomic Number of the element

62 Atomic Number Elements are different because they contain different numbers of PROTONS The “atomic number” of an element is the number of protons in the nucleus Since all atoms are neutral - the # protons in an atom = # electrons Henry Moseley – used x-ray spectra & came up with the idea of the Atomic Number

63 Atomic Number, Z All atoms of the same element have the same number of protons in the nucleus, Z 13 Al Atomic number Atom symbol AVERAGE Atomic Mass

64 Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = # protons + # neutrons

65 Subatomic Particles Most of the atom’s mass. NUCLEUS ELECTRONS PROTONS NEUTRONS NEGATIVE CHARGE POSITIVE CHARGE NEUTRAL CHARGE ATOM Atomic Number equals the # of... equal in a neutral atom

66 Isotopes Frederick Soddy ( ) in 1912 (worked with Rutherford) Frederick Soddy ( ) proposed the idea of isotopes in 1912 (worked with Rutherford) Isotopes are atoms of the same element having different mass numbers, due to varying numbers of neutrons. Isotopes are atoms of the same element having different mass numbers, due to varying numbers of neutrons. Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials. Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.

67 Isotopes Atoms of the same element (same Z) but different mass number (A). Atoms of the same element (same Z) but different mass number (A). Boron-10 (B-10) has 5 p and 5 n Boron-11 (B-11) has 5 p and 6 n Boron-10 (B-10) has 5 p and 5 n Boron-11 (B-11) has 5 p and 6 n 10 B 11 B

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69 Isotopes Radioisotopes (radioactive isotopes) - Radioisotopes (radioactive isotopes) - unstable isotopes that spontaneously decay emitting radiation They play an important part in the technologies that provide us with food, water and good health. They play an important part in the technologies that provide us with food, water and good health. Radio-carbon dating of fossils Radio-carbon dating of fossils In medicine, diagnosis, treatment, and research In medicine, diagnosis, treatment, and research Sterilization of meat Disinfestation of grain and spices Increasing shelf life (eg, fruits)

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71 Nuclear Symbols Contain the symbol of the element, the mass number and the atomic number (represent isotopes of elements) Contain the symbol of the element, the mass number and the atomic number (represent isotopes of elements) X Mass number Atomic number Subscript → Superscript → Element symbol REMEMBER! number of electrons = number of protons So all atoms are neutral!

72 Rhenium Re Protons: 75 Neutrons: 111 Electrons: 75

73 Nuclear Symbols n Find each of these: a) number of protons b) number of neutrons c) number of electrons d) Atomic number e) Mass Number Br 80 35

74 Nuclear Symbols n If an element has an atomic number of 34 and a mass number of 78, what is the: a) number of protons b) number of neutrons c) number of electrons d) Write the complete symbol Se

75 Naming Isotopes We can name isotopes by placing the mass number after the name of the element: We can name isotopes by placing the mass number after the name of the element: carbon-12 carbon-12 carbon-14 carbon-14 uranium-235 uranium-235 Mass numbers

76 ISOTOPES Isotope p+p+p+p+ n0n0n0n0 e-e-e-e- Mass # Oxygen Arsenic Phosphorus

77 IsotopeProtonsElectronsNeutronsNucleus Hydrogen–1 (protium) (protium)110 Hydrogen-2(deuterium)111 Hydrogen-3(tritium)112 The element hydrogen has 3 isotopes

78 Examples of Isotopes

79 Learning Check – Counting Naturally occurring carbon consists of three isotopes, 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of these carbon atoms. Naturally occurring carbon consists of three isotopes, 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12 C 13 C 14 C #p + _______ _______ _______ #n o _______ _______ _______ #e - _______ _______ _______

80 Answers 12 C 13 C 14 C #p #n o #e

81 Learning Check An atom has 14 protons and 20 neutrons. A.Its atomic number is 1) 142) 163) 34 B. Its mass number is 1) 142) 163) 34 C. The element is 1) Si2) Ca3) Se D.Another isotope of this element is 1) 34 X 2) 34 X 3) 36 X

82 Atomic Mass  How heavy is an atom of oxygen?  It depends, because there are different kinds of oxygen atoms.  We are more concerned with the average atomic mass.  This is based on the abundance (percentage) of each variety (isotope) of that element in nature.  We don’t use grams for this mass because the numbers would be too small –

83 Measuring Atomic Mass Instead of grams, the unit we use is the Atomic Mass Unit (amu) Instead of grams, the unit we use is the Atomic Mass Unit (amu) It is defined as one-twelfth the mass of a carbon-12 atom. It is defined as one-twelfth the mass of a carbon-12 atom. Carbon-12 chosen because of its isotope purity. Carbon-12 chosen because of its isotope purity. Each isotope has its own mass number, so we determine the average atomic mass from the element’s percent abundance. Each isotope has its own mass number, so we determine the average atomic mass from the element’s percent abundance.

84 To calculate the average atomic mass: Multiply the mass of each isotope by it’s abundance, then add the results. Multiply the mass of each isotope by it’s abundance, then add the results. Abundance may be expressed as a decimal or a %, (Divide by 100 if using %’s) Abundance may be expressed as a decimal or a %, (Divide by 100 if using %’s) Avg. Atomic Mass

85 IsotopeSymbol Composition of the nucleus % in nature Carbon-12C-12 6 protons 6 neutrons 98.89% Carbon-13C-13 6 protons 7 neutrons 1.11% Carbon-14C-14 6 protons 8 neutrons <0.01% on the Periodic Table Atomic Mass is the weighted average of all the naturally occurring isotopes of an element. on the Periodic Table = (98.89 x 12) + (1.11 x 13) + (0.01 x 14) 100

86 Avg. Atomic Mass D. Average Atomic Mass EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16 O, 0.04% 17 O, and 0.20% 18 O. EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16 O, 0.04% 17 O, and 0.20% 18 O amu

87 Sub-atomic Particles - Summary Protons p+ p+ - positive charge, in nucleus, mass of 1 amu Electrons - e - negative charge, orbiting nucleus, “no mass” Neutrons n0 n0 – no charge, in nucleus, mass of 1 amu


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