Atoms: The Building Blocks of Matter Chapter 3

Objectives –Recognize discoveries from Dalton (atomic theory), Thomson (the electron), Rutherford (the nucleus), and Bohr (planetary model of atom) and understand how these discoveries lead to the modern theory. –Describe Rutherford’s “gold foil” experiment that led to the discovery of the nuclear atom. Identify the major components (protons, neutrons, and electrons) of the nuclear atom and explain how they interact.

Objectives  Interpret and apply the laws of conservation of mass, constant composition (definite proportions), and multiple proportions.  Describe how changes in the nucleus of an atom during a nuclear reaction result in emission of radiation.

History of the Atom Not the history of the atom itself, but the history of the idea of the atom.

Atom Definition Atom  Smallest particle of an element that retains the chemical identity of that element

Democritus Greek PhilosopherGreek Philosopher ~450 BC~450 BC Thought atoms were tiny, individual, indivisible atomsThought atoms were tiny, individual, indivisible atoms Used logic to formulate ideasUsed logic to formulate ideas One of the first to develop idea of atomsOne of the first to develop idea of atoms om/search/images/

Contributing Principles to Idea of Atom  Law of Definite Composition  A given compound always contains the same elements in the same proportion by mass  Joseph Louis Proust 

John Dalton’s Atomic Theory (1803) 1.Elements composed of small particles called atoms 2.All atoms of a given element are the same, but different from other elements 3.Atoms cannot be created or destroyed in a chemical reaction 4.Compounds are composed of atoms combined in simple whole number ratios ml - 2k

Studdy Buddy Review  Describe the contribution of each towards the historical development of the atom: –Proust –Democritus –Dalton

What is inside the atom?

J.J. Thomson (1897)  Cathode Ray Tube Experiments Conclusions: Stream of negative particles that have mass Named electrons Atoms are not indivisible Found ratio: (electrical charge of electron) (mass of electron) 1.76 x 10 8 coulombs = 1 gram of electrons

Thomson’s Experiment Voltage source +- Vacuum tube Metal Disks

Thomson’s Experiment Voltage source +-

Thomson’s Experiment Voltage source +-

Thomson’s Experiment Voltage source +-

n Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source +-

n Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source +-

n Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source +-

n Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source +-

Thomson’s Experiment  By adding an electric field

Voltage source Thomson’s Experiment n By adding an electric field + -

Voltage source Thomson’s Experiment n By adding an electric field + -

Voltage source Thomson’s Experiment n By adding an electric field + -

Voltage source Thomson’s Experiment n By adding an electric field + -

Voltage source Thomson’s Experiment n By adding an electric field + -

Voltage source Thomson’s Experiment n By adding an electric field he found that the moving pieces were negative + -

Robert Millikan (1909)  Oil Drop Experiment Measured charge of an electron  Charge of one electron = -1.6x C THUS…. Mass of e - = 9.11x g 9.11x g

Rutherford’s experiment  English physicist Ernest Rutherford (1911)  Shot alpha particles at gold foil which can be made a few atoms thick. –alpha particles: positively charged helium nuclei –A form of radioactivity  When an alpha particle hits a fluorescent screen, it glows.

Lead block Uranium Gold Foil Fluorescent Screen

What he got

How Rutherford explained results  Atom is mostly empty space.  Small dense,positive piece at center. (NUCLEUS)  Alpha particles are deflected by it if they get close enough.

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Credit for subatomic particles Credit for subatomic particles  1897 Thomson discovered the electron –Used cathode ray tube  (1918) Rutherford named positive charged particle the proton –Goldstein (1886) first discovered positively charged particle using cathode-ray tube with perforated cathode  (1932) James Chadwick discovers neutrons –Worked with cloud chambers to produced neutrons and determined their masses

Subatomic particles Electron Proton Neutron NameSymbolCharge Relative mass (amu) Actual mass (g) e-e- p+p+ n0n / x x

Studdy Buddy Review  Name three subatomic particles.  Who is credited with discovering each particle?  Describe the Rutherford Gold Foil Experiment.

Models of the Atom

Dalton Model of Atom  Small, indivisible spheres

J.J. Thompson’s Model of Atom  Plum Pudding Model, 1896  Thought an atom was like plum pudding –Dough was positively charged –Raisins scattered throughout the dough were negatively charged –Didn’t know about neutrons at this time

Rutherford’s Model of the Atom  Rutherford Model, 1911  Thought atom was mostly empty space –Nucleus in center is dense, positively charge –Electrons (negatively charged) are in empty space surrounding nucleus

Bohr’s Model of the Atom  Neils Bohr, 1913  Similar to Rutherford’s model  Thought atom was mostly empty space –Nucleus in center is dense, positively charge –Electrons move in orbits around the nucleus

(Modern) Quantum Mechanical Model of the Atom  Heisenberg, Schrodinger, many others, ~1926  Think atom is mostly empty space –Nucleus in center is dense, positively charge –Electrons are around the nucleus –Cannot locate location of electron at specific time

Information about Atom from Periodic Table

Atomic Number Avg Atomic Mass

Atomic Number and Atomic Mass  Chemical Symbol: abbreviation for element name  Atomic Number (Z): number of protons in nucleus of atom (and electrons if neutral)  Mass Number: number of protons and number of neutrons in nucleus (whole number)

Isotopes  Isotopes: atoms with the same number of protons but different number of neutrons  Hyphen Notation: –oxygen-16 and oxygen-17  Nuclear Symbol: 16 8 O 17 8 O

Average Atomic Mass  Average Atomic Mass: weighted average mass of atoms found in nature (decimal number on periodic table)  Can calculate average atomic mass of elements if know percent abundance in nature  (WS Isotopes and Average Atomic Mass)

Ch. 25 Nuclear

Radioactivity

Objectives  Describe how changes in the nucleus of an atom during a nuclear reaction results in the emission of radiation Describe alpha, beta, and gamma particles; discuss the properties of alpha, beta, and gamma radiation; and write balanced nuclear reactions.Describe alpha, beta, and gamma particles; discuss the properties of alpha, beta, and gamma radiation; and write balanced nuclear reactions. Compare nuclear fission and nuclear fusion.Compare nuclear fission and nuclear fusion.

Objectives Explain the difference between stable and unstable isotopes.Explain the difference between stable and unstable isotopes. Explain the concept of half-life of a radioactive element, e.g., explain why the half-life of C-14 has made carbon dating a powerful tool in determining the age of very old objects.Explain the concept of half-life of a radioactive element, e.g., explain why the half-life of C-14 has made carbon dating a powerful tool in determining the age of very old objects.

Radioactivity

Strong Nuclear Force  Opposites attract, like charges repel  So why do protons stay together in nucleus?  Strong Nuclear Force holds nucleus together and is stronger than electrostatic repulsion between protons –Only works over small diameter –Neutrons help keep protons separated slightly to reduce repulsion between protons

Mass Defect  You’d expect the mass of an atom to be the sum of the individual subatomic particles 4 2 He2 ( amu) = ( amu) = ( amu) = Total = amu Actual mass helium atom = amu  The difference between the calculated mass and the actual mass is called mass defect.

Binding Energy  In Einstein’s equation: E=mc 2 the “lost” mass can be converted into energy  Binding energy: energy released when a nucleus is formed from protons and neutrons  Could be considered as the amount of energy to break apart the nucleus  Associated with the strong nuclear force holding particles together

Binding Energy per Nucleon

Radiation  Stable nuclei have large binding energies –High energy means it is hard for nucleus to break apart  Unstable nuclei can break apart and give off particles  Radiation: emission of energy as electromagnetic waves or subatomic particles

Discovery of Radiation  Henri Becquerel (1896) experiment with uranium found it was emitting particles  Marie Curie (1898) discovered radioactive element Polonium and Radium

Common Types of Radiation  Alpha    He  –Helium nucleus –Weak strength : can stop with paper  Beta  electron   e  –Electron –Medium strength: stop with clothing  Gamma  –High energy –High energy: stop with lead  mass #  4, Atomic #  2  Mass # stays same, atomic #  1  EM wave so mass doesn’t change

Other Types of Radiation  Positron    e   Neutron (n)   n   mass # stays the same,  Atomic #  1  Mass #  1, atomic # stays the same

Nuclear Equations  U  Th + _________  14 6 C  14 7 N + _________  9 4 Be + _________  12 6 C n Answers: alpha, beta, alpha

Study Buddy Review  What force holds the nucleus together?  What is binding energy?  What happens when a nucleus is unstable  What is an alpha particle? Beta particle? Gamma radiation?

Nuclear Decay and Half Life

Decay  Radioactive decay: spontaneous emission of radiation from nucleus of atom  Transmutation: change in the identity of an element due to the emission of particles from the nucleus

Half-Life  Half-life: time required for half of a sample of an element to decay into another element  Known as rate of radioactive decay  Different for each isotope A = A o (½) n

Half Life of Some Radioactive Isotopes

Half life of Potassium-40

Half-Life Problem  The half life of polonium-210 is days. How many milligrams of polonium-210 remain after days if you start with 2.0 mg of the isotope?  Answer: 0.25 mg

Nuclear Fission and Fusion

Fusion  Energy of our sun and other stars is produced from nuclear fusion reactions  Fusion: light massed nuclei combine to form a heavier, more stable nucleus  Produces a lot of energy, also nuclear waste H  4 2 He  ENERGY

Fission  Nuclear power plants create energy from fission reactions  nuclear fission: a heavy nucleus splits into a more stable nuclei of intermediate mass –energy produced –nuclear power plants –Nuclear waste produced U n  Kr Ba n + ENERGY

Study Buddy Review  What is half-life?  What is radioactive decay?  Compare and contrast fusion and fission.