Chapter 4 Atomic Structure p. 101

Section 4.1 Defining the Atom Greek philosopher Democritus Greek philosopher Democritus suggested atoms (Greek “atomos”) suggested atoms (Greek “atomos”) Believed indivisible & indestructible Believed indivisible & indestructible His ideas His ideas Based on philosophy Based on philosophy didn’t explain chem behavior didn’t explain chem behavior Lacked experimental support Lacked experimental support

Dalton’s Atomic Theory (experiment based!) 3)Atoms of diff elements combine in simple whole # ratios (chem cmpds) 4)In chem rxns, atoms combine, separate, or rearrange – never change into atoms of another element 1)All elements composed of indivisible particles (atoms) 2)Atoms of same element identical. Atoms of 1 different from other elements John Dalton (1766 – 1844)

Sizing up the Atom  Elements subdivided into smaller particles – atoms,… still have properties of that element  Line up 1.0 x 10 8 copper atoms = 1 cm long  individual atoms observed w/ scanning tunneling (electron) microscopes The atom 1:15

Section 4.2 Structure of the Nuclear Atom One change to Dalton’s atomic theory - atoms ARE divisible One change to Dalton’s atomic theory - atoms ARE divisible Into subatomic particles: Into subatomic particles: Electrons, protons, and neutrons Electrons, protons, and neutrons

Discovery of Electron electron J.J. Thomson used cathode ray tube to deduce presence of negatively charged particle……. electron JJ Thomson’s Cathode Ray 2:49

Mass of Electron 1916 – Robert Millikan determines e- mass: 1/1840 mass of H atom; one unit of - charge The oil drop apparatus Mass of the electron is 9.11 x g

Conclusions from the Study of the Electron: a)All elements must contain identically charged e-s. b)Atoms r neutral, must be + particles to balance – e-s c)e-s have little mass - atoms must contain other more massive particles

Conclusions from Electron Study:  1932 – James Chadwick confirmed “neutron” existence  particle w/ no charge  n 0 mass = p + mass

Subatomic Particles ParticleCharge Mass (g) Location Electron (e - ) (e - ) x x Electron cloud Proton (p + ) x x Nucleus Neutron (n o ) (n o ) x x Nucleus

Thomson’s Atomic Model Believed e - s like plums in + charged “pudding,” “plum pudding” model. J. J. Thomson

Ernest Rutherford’s Gold Foil Experiment  massive alpha particles (He atom w/o e-’s) fired at Au foil  Thought most would pass thru w/ slight deflection  MOST passed straight thru! Rutherford’s Gold Foil Experiment 4:06

Rutherford’s problem: In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target? Target #1 Target #2

The Answers: Target #1 Target #2

Rutherford’s Findings  Most particles passed thru  Few deflected  VERY FEW greatly deflected The Atom Song The Atom Song Atoms song - Mark Rosengarten “As incredible as howitzer shells bouncing off of tissue paper!”

The Rutherford Atomic Model New theory: New theory: atom mostly empty space atom mostly empty space Nucleus Nucleus p + & n 0 make nucleus! p + & n 0 make nucleus! all + charge all + charge almost all mass almost all mass e-s around nucleus…occupy most volume e-s around nucleus…occupy most volume called “nuclear model” called “nuclear model” Rutherford’s Atom 3:08Quarks 2:56

Section 4.3 Distinguishing Among Atoms p. 110 How are elements different from one another? different # p+’s! different # p+’s! ID’s elements (like fingerprint/DNA) ID’s elements (like fingerprint/DNA)

Atomic Number of that element. Atomic number (Z) of element = # of p+’s in nucleus of each atom of that element. Element # of protons Atomic # (Z) Carbon6 Phosphorus15 Gold

Mass Number Mass number is # of p + ’s and n 0 ’s in nucleus of isotope: n 0 = mass # - atomic # p+p+p+p+ n0n0n0n0 e-e-e-e- Mass # Oxygen Arsenic Phosphorus

Complete Symbols X Mass number Atomic number Subscript → Superscript →

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

Practice….. 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) complete symbol

More practice……. n If an element has 91 protons and 140 neutrons what is the a) Atomic number b) Mass number c) number of electrons d) complete symbol

Isotopes Dalton was wrong! Dalton was wrong! Atoms of same elements NOT identical… Atoms of same elements NOT identical… can have different # n 0 ’s. can have different # n 0 ’s. different mass # different mass # Isotopes Isotopes (flavors) (flavors)

Isotopes Frederick Soddy proposed idea of isotopes in 1912 Frederick Soddy proposed idea of isotopes in Nobel Prize in Chemistry 1921 Nobel Prize in Chemistry crater w/ his name on far side of Moon crater w/ his name on far side of Moon Isotopes - atoms of same element w/ diff masses b/c varying #s of n 0 Elements occur in nature as mixtures of isotopes. Elements occur in nature as mixtures of isotopes.

Naming Isotopes put mass # after element name: put mass # after element name: carbon-12 carbon-12 fluorine-19 fluorine-19 uranium-235 uranium-235

IsotopeProtonsElectronsNeutronsNucleus Hydrogen–1 (protium) (protium)110 Hydrogen-2(deuterium)111 Hydrogen-3(tritium)112

Atomic Mass  How heavy is an oxygen atom?  Depends, b/c different kinds of oxygen atoms exist.  average atomic mass  Based on abundance (%) of each variety of that element in nature.  not in grams - #’s tooooo small

Measuring Atomic Mass Atomic Mass Unit (amu) Atomic Mass Unit (amu) 1/12 mass of C-12 atom 1/12 mass of C-12 atom C-12 b/c isotope purity C-12 b/c isotope purity Each isotope has own atomic mass Each isotope has own atomic mass determine average from % abundance determine average from % abundance

To calculate weighted average mass: Atomic mass x isotope abundance (change % to decimal) Atomic mass x isotope abundance (change % to decimal) Add all results Add all results Isotope mass usually in amu ’s Isotope mass usually in amu ’s

Atomic Masses IsotopeSymbol Composition of the nucleus % in nature Carbon C 6 protons 6 neutrons 98.89% Carbon C 6 protons 7 neutrons 1.11% Carbon C 6 protons 8 neutrons <0.01% Atomic mass - avg of all naturally occurring isotopes of that element What is the average atomic mass of Carbon? What is the average atomic mass of Carbon? Atomic mass (amu)

- Page 117 Question Solution Answer Knowns and Unknown