Chapter 3 Atomic Structure 3-1 Early Models of the Atom 3-2 Discovering Atomic Structure 3-3 Modern Atomic Theory 3-4 Changes in the Nucleus

3-1 Early Models of the Atom What are atoms? What are the postulates of Daltons atomic theory?

Democritus Ancient Greek 450 BC Proposed that all matter is composed of tiny, invisible particles called atoms No one believed him during his lifetime Including Aristotle His beliefs were not accepted until the 17 th and 18 th centuries

Acceptance Was not accepted until 2 discoveries were made Lavoisiers law of conservation of matter Joseph Louis Prousts law of constant composition A compound will always contain the same proportions by mass of elements Water will always have 88.9% oxygen (O) and 11.1% hydrogen (H)

John Dalton (1766 1844) English school teacher Studied past theories of atoms and laws of matter Formed an atomic theory of matter

Daltons Atomic Theory of Matter Ea element is composed of extremely small particles called atoms All atoms of a given element are identical, but they differ from those of any other element Atoms are neither created/destroyed in any chem rxn A given compound always has the same relative #s and kinds of atoms

Atoms The smallest particle of an element that retains the chemical identity of that element There are 118 elements wh means there are 118 different kinds of atoms.

Atoms Atoms are like the words in these slides. If we broke it all apart, separated and organized the letters, you would find only 26 piles. But by taking letters from different piles we can create millions of very different words Just like words can be separated into letters, matter can be separated into atoms. These separated atoms are called elements Think of all the words you could make with the letters A, D, and M.

Scanning Tunneling Microscope (STM) Produces images of atoms Created in 1981 NickelPlatinum

Chemistry In Action (p93) Consumer Tip 100 Percent Natural

Macroscopic vs Microscopic Macroscopic looking a the whole picture A tree It is made of the leaves, branches, trunk, roots Microscopic the more detailed vision of an object and what makes it function A leaf off a tree and the little veins that carry the nutrients through it

Macroscopic vs Microscopic

Chemists make their observations in the macroscopic world It is the world in wh we all live In order to understand that world, the goal is to understand the atoms that the world is made of Discoveries/Possibilities b/c of the study of atoms Deciphering the genetic code Designing plastics Understanding the hole in the ozone Imprinting data on silicon chips

3-2 Discovering Atomic Structure How is atomic structure related to electricity? What did cathode rays indicate about atoms? What did Rutherford conclude from his alpha-scattering experiment?

Electric Charges Scientists couldnt figure out why atoms of one element acted differently than another elements atoms Michael Faraday (1791-1867) said that the structure of an atom was directly related to electricity

Electric Charges Atoms contain particles that have electrical charges

Benjamin Franklin An object will either have a positive or negative charge 2 like charges will repel Positive w/ positive Negative w/ negative 2 opposite charges will attract Positive w/ negative Franklin didnt know where these charges came from

Cathode Rays and Electrons Electric current - A moving stream of electrical charges Electricity from wall socket or battery Studying electrical currents provide keys to understanding electrical charges Mid-1800s, began studying electric currents in glass tubes w/ little air

Cathode Rays and Electrons Tube attached on ea end to a battery Positive and negative Negative = cathode Positive = anode Radiation travels from cathode to anode b/c radiation came from cathode end, called cathode ray and the tube a cathode ray tube

Cathode Rays and Electrons Cathode Ray tube being effected by a magnet http://www.youtube.com/watch?v=7YHwMWcx eX8&feature=relatedhttp://www.youtube.com/watch?v=7YHwMWcx eX8&feature=related Battery - +

Electrons Negative particles within the atom JJ Thompson (1856-1940) Mass of 9.11 x 10 -28 gram 0.000000000000000000000000000911 gram Robert Millikan (1868-1953)

Radioactivity Henry Becquerel (1852- 1908) Placed uranium on photo paper and an image appeared Uranium was emitting radiation Radioactivity: spontaneous emission of radiation from an element Marie Curie and husband Pierre discovered the elements of radium and polonium were also radioactive

The Nuclear Atom Thompson said there were electrons in the atom (neg charge) Why is the atom neutral then? Rutherfords Gold Foil Experiment http://www.youtube.com/watch?v=5pZj0u_XMbchttp://www.youtube.com/watch?v=5pZj0u_XMbc Called this center the nucleus Has a positive charge Very small If the atom was the size of a football stadium, the nucleus would be smaller than a dime sitting in the middle Electrons would be smaller than Franklin Roosevelts eye on the dime

The Nuclear Atom http://www.ndt- ed.org/EducationResources/Hi ghSchool/Magnetism/reviewat om.htm

How far are the electrons from the nucleus? If the earth was the nucleus, the electrons would cover an area as large as the distance b/w the earth and nearest stars

3-3 Modern Atomic Theory What are the names and properties of the 3 subatomic particles? How can you determine the # of protons, neutrons, and electrons in an atom/ion? What is an isotope? What is atomic mass?

Subatomic Particles We know atoms are made from protons, neutrons, and electrons Recently scientists have found even smaller particles Quarks, Gluons, Mesons, Muons, and others They dont seem to impact any Chemistry so chemists ignore Physicists study them

The Structure of the Atom Nucleus Contains the protons and neutrons Protons = positive p + Have the same but opposite charge as electrons Neutrons = neutral/no charge n 0 Electrons Negatively charged e - Move in the space outside nucleus e - cloud Very small compared to p + 2000 e - = 1 p +

Size of Subatomic Particles Mass Too small for normal measurements Has own unit - atomic mass unit (amu) P + and n 0 = 1 amu, e - = 0 amu b/c so small Length Diameter = 0.100 0.500 nanometer Nanometer = nm = 10 -9 meter If you drew a line across a penny (1.9 cm), you would touch 810 million copper atoms If you lined up all 810 million nuclei, you would only have a line 4 x 10 -6 meter long 4 millionths of a meter

Atomic Numbers Henry Moseley (1887-1915) Student of Rutherford Discovered atoms of ea element contained differ positive charges Lead to the idea that an atoms identity comes from the # of p + in nucleus Call this # atomic number

Atomic Number The # of protons Ea element has a unique atomic # Can tell an elements atomic # from periodic table

Neutral Atom The p + are positive The e - are negative The atom is neutral This means, the p + must equal the e - For N, atomic # = 7 Means p + = 7 Means e - = 7

Examples How many protons and electrons in: Oxygen (O) 8 p + and e - Magnesium (Mg) 12 p + and e - Silicon (Si) 14 p + and e - What element has 11 protons? Sodium

Ions When an atom gains/loses e -, it will have a charge When an atom has a charge, called ion Charge of ion = #p + - #e - If a magnesium atom loses 2 e -, ionic form has a charge of: #p + - #e - = 12 10 = +2 It is important to add the plus (+) sign into the answer Also possible to have a negative (-) Some people write the charge with the +/- after the # (2+) After you have calculated the charge, to write it with the element symbol, add it as a subscript For our magnesium example: Mg +2

Examples Write the chemical symbol for the ion w/: 9 p + and 10 e - F-F- 13 p + and 10 e - Al +3 7 p + and 10 e - N -3 How many p + and e - are present in: S -2 ion 16 p + and 18 e - Li + ion 3 p + and 2 e - Write the chemical symbol for the ion w/: 12 p + and 10 e - Mg +2 74 p + and 68 e - W +6

Isotopes All atoms of the same element, have the same # of p + They may not have the same # of n 0 If atoms have the same # of p + but different # of n 0, we call them isotopes Most elements have at least 1 isotope 1 usually more frequent than another In nature, it is usually a mixture To tell isotopes apart, we use the mass #

Mass Number Mass # = #p + + #n 0 An atom w/ 17p + and 18n 0 would have an mass # of 35 Mass # = 17 + 18 = 35 b/c 17 p +, tells us it is a chlorine atom Chlorine 35 A way to write the element symbol w/ atomic and mass #s would be: Cl 37 17 element symbol mass # atomic #

Examples How many protons, neutrons, and electrons are in the following ions? Fe +2 26 p +, 24 e -, and 30 n 0 Al +3 13 p +, 10 e -, and 14 n 0 Se -2 34 p +, 36 e -, and 45 n 0 Write the complete chemical symbol for the ion w/ 21 p +, 24 n 0, and 18 e - Sc +3 53 p +, 74 n 0, and 54 e - I - 26 56 13 27 34 79 21 45 53 127

Atomic Mass The average mass of all the isotopes of an element Listed in the periodic table

Practice Problems # 1-30

3-4 Changes in the Nucleus What changes accompany nuclear reactions? What is radioactivity?

Nuclear Reactions Change the composition of an atoms nucleus Produces alpha, beta, or gamma radiation Alpha and beta radiation comes from radiation emitted from the nucleus

Nuclear Stability Almost all atoms have stable nuclei Not radioactive Radioactivity could have harmful effects good its rare to find in nature Why are some more stable than others? # of p + and n 0 in the nucleus Some combinations cause instability

Nuclear Stability In nucleus, p + and n 0 are packed together in a very small space How do p + stay together in the small space if like charges repel? Held there by strong nuclear force Can only be found in this situation Neutrons act like a net to hold the p + in along with the strong nuclear force

Nuclear Stability Pattern of stability Atomic # 1-20 nuclei stable, = # of p + and n 0 Beyond 20 p + - more n 0 needed to keep stable Atomic # above 83 radioactive nuclei No # of n 0 will make it stable Atoms unstable if too many or too few neutrons Atoms w/ too many emit beta radiation

Types of Radioactive Decay Alpha () Alpha particles have 2 p + and 2 n 0 Identical to Helium 4 nucleus Travel only a few cm Easily stopped by paper or clothing Usually doesnt pose a health threat unless actually enters the body He 4 2 +2 He 4 2 4 2

Types of Radioactive Decay Beta () High speed electrons (not the ones around the nucleus) Comes from charges inside a nucleus A neutron changes into a p + and e- p+ stays in nucleus e- (beta particle) is propelled out of nucleus at high speed 100 times more penetrating than alpha Able to penetrate 1-2 mm of solid material Able to pass through clothing and damage skin e 0 - e 0 0

Types of Radioactive Decay Gamma () Very energetic form of light our eyes cant see Doesnt have any particles More penetrating than others Able to penetrate deep into solid material Body tissue Stopped only by heavy shielding Concrete or lead 0 0

When an atom emits radiation, it undergoes radioactive decay Called decay b/c nucleus is decomposing to form a new nucleus The best way to understand the decay is w/ a nuclear equation RaRn 226 88 222 86 4 2 + Types of Radioactive Decay

Partner Activity Look at Figure 3-30 on p115 and answer the questions Would this protective suit protect the worker from alpha radiation? Why would a person working w/ alpha radiation also need to be concerned w/ gamma radiation? Would protective clothing such as this stop gamma radiation from penetrating the workers skin?

Beta decay equation 131 53 I Xe+ 131 54 0

Practice Problems Alpha decay of 79 Au Alpha decay of 92 U Beta decay of 11 Na Alpha decay of 94 Pu Alpha decay of 91 Pa Beta decay of 87 Fr 185 238 24 242 231 233

Chapter 3 Review Multiple Choice all True/False all Concept Mastery (20-22, 25) Critical Thinking and Problem Solving 29, 31-33

Documents

Chapter 3 Atomic Structure 3-1 Early Models of the Atom 3-2 Discovering Atomic Structure 3-3 Modern Atomic Theory 3-4 Changes in the Nucleus