Atomic Structure Chapter 4

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

Subatomic particles Actual mass (g) Name Symbol Charge Electron e- -1 Relative mass (amu) Actual mass (g) Name Symbol Charge Electron e- -1 1/1840 9.11 x 10-28 Proton p+ +1 1 1.67 x 10-24 Neutron n0 1 1.67 x 10-24

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: 168O 178O

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)

Models of the Atom

Dalton Model of Atom Small, indivisible spheres http://images.search.yahoo.com/search/images/

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 http://images.search.yahoo.com/search/images/

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 http://images.search.yahoo.com/search/images/

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 http://images.search.yahoo.com/search/images/

(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 http://particleadventure.org/particleadventure/frameless/modern_atom.html

Ch. 25 Nuclear Ch. 22 Nuclear General Chemistry

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. Compare nuclear fission and nuclear fusion.

Objectives 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.

Radioactivity

Discovery of Radiation Henri Becquerel (1896) experiment with uranium found it was emitting particles Marie Curie (1898) discovered radioactive element Polonium and Radium Ch. 22 Nuclear General Chemistry

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 Ch. 22 Nuclear General Chemistry

Mass Defect You’d expect the mass of an atom to be the sum of the individual subatomic particles 42He 2 (1.007276 amu) = 2.014552 2 (1.008665 amu) = 2.017330 2 (0.0005486 amu) = 0.001097 Total = 4.032979 amu Actual mass helium atom = 4.00268 amu The difference between the calculated mass and the actual mass is called mass defect. Ch. 22 Nuclear General Chemistry

Binding Energy In Einstein’s equation: E=mc2 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 Ch. 22 Nuclear General Chemistry

Binding Energy per Nucleon Ch. 22 Nuclear General Chemistry

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 Ch. 22 Nuclear General Chemistry

Common Types of Radiation Alpha (a) 42 He) Helium nucleus Weak strength : can stop with paper Beta (b) electron 0-1 e) Electron Medium strength: stop with clothing Gamma (g) High energy High energy: stop with lead mass #  4, Atomic #  2 Mass # stays same, atomic #  1 EM wave so mass doesn’t change Ch. 22 Nuclear General Chemistry

Other Types of Radiation Positron (0+1 e) Neutron (n) 10 n) mass # stays the same, Atomic #  1 Mass #  1, atomic # stays the same Ch. 22 Nuclear General Chemistry

Nuclear Equations 23892 U  23490 Th + _________ 146 C  147 N + _________ 94 Be + _________  126 C + 10 n Answers: alpha, beta, alpha Ch. 22 Nuclear General Chemistry

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? Ch. 22 Nuclear General Chemistry

Nuclear Decay and Half Life Ch. 22 Nuclear General Chemistry

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 Ch. 22 Nuclear General Chemistry

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 = Ao(½)n Ch. 22 Nuclear General Chemistry

Half Life of Some Radioactive Isotopes Ch. 22 Nuclear General Chemistry

Half life of Potassium-40 Ch. 22 Nuclear General Chemistry

Half-Life Problem The half life of polonium-210 is 138.4 days. How many milligrams of polonium-210 remain after 415.2 days if you start with 2.0 mg of the isotope? Answer: 0.25 mg Ch. 22 Nuclear General Chemistry

Nuclear Fission and Fusion Ch. 22 Nuclear General Chemistry

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 4 11 H  42 He + 2 0-1 b + ENERGY Ch. 22 Nuclear General Chemistry

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 23592 U + 10 n  9336 Kr + 14056 Ba + 3 10 n + ENERGY Ch. 22 Nuclear General Chemistry

Study Buddy Review What is half-life? What is radioactive decay? Compare and contrast fusion and fission. Ch. 22 Nuclear General Chemistry

Ch. 22 Nuclear General Chemistry

Ch. 22 Nuclear General Chemistry

Ch. 22 Nuclear General Chemistry

Ch. 22 Nuclear General Chemistry

Ch. 22 Nuclear General Chemistry

Ch. 22 Nuclear General Chemistry

Ch. 22 Nuclear General Chemistry

Ch. 22 Nuclear General Chemistry