Atomic Structure The Idea of the Atom

Early Models Greek philosophers – 450 B.C. what is the smallest particle? Democritus - Particles are atomos An atom is the smallest particle of an element that retains the chemical identity of that element.

Aristotle Wins Greek society - slave based Beneath them to work with hands – no experiments Settled disagreements by arguments Aristotle more famous than Democritus Aristotle wins Didn’t believe or look into atoms until…

Late 1700s Chemists believed elements: couldn’t be broken down by ordinary means Combine to form compounds Advancements in balances  better experiments

Laws Discovered Law of Conservation of mass: mass is neither created nor destroyed during chemical or physical changes Then discovered no matter where or how a compound is made it is made of fixed proportions of elements  Law of Definite Proportions Ex: Sodium Chloride, table salt, always has 39.34% Na and 60.66% Cl by mass

Laws Discovered Elements combine to form more than one compound Ex: carbon monoxide, CO, and carbon dioxide CO2 Law of Multiple Proportions

John Dalton 1808 –DALTON’S ATOMIC THEORY All matter is composed of small particles called atoms. *All atoms of a given element are identical, but they differ from those of any other element.

Dalton’s Atomic Theory *Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in whole-# ratios to form compounds In a chemical reaction, atoms are combined, separated, or rearranged.

1800s – The atom IS divisible Atom has two regions: Nucleus: center of atom Protons: (+) charged Neutrons: no charge Electron cloud: region around nucleus containing electrons, (-) charged Subatomic particles: p+, no, and e-

- + J.J. Thomson’s Experiment Cathode - Ray Tube Voltage source Moving electrical current (Cathode Ray) through glass tube with inert gas (Noble Gas)

J.J. Thomson’s Experiment Voltage source + - By adding magnets, found the moving (-) particles

Thomson’s Model Same properties no matter what element used All atoms had ELECTRONS!! Thomson’s Model Plum pudding: bunch of positive stuff, with the electrons able to be removed.

Ernest Rutherford - 1911 Atoms have e- = (-) charge Atoms are neutral  need (+) to balance Must be other particles to account for mass of atom… Rutherford used alpha particles to test. Alpha particle: (+) charge, (2 p+ and 2 no) Experiment: beam of high speed alpha particles into thin gold foil

Lead block Uranium Gold Foil Florescent Screen Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

His prediction…. Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

What he expected Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

What he happened Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

Results… Most particles passed through 1 in 8000 bounced back Some particles completely reversed path!!!

Small dense, positive piece at center, nucleus Plum-pudding is WRONG! Atom is mostly empty Small dense, positive piece at center, nucleus Alpha particles are deflected by nucleus if they get close. + Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

+ Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

Electron Cloud Model Current model of atom Nucleus surrounded by negative cloud “electron cloud”

Size of Atom Atom = size of football stadium Nucleus = smaller than a dime in center Electron = each one smaller than Roosevelt’s eye on the dime Atom is mostly empty space!

Composition of Atom Proton has (+) charge equal in magnitude to (-) charge of electron Atoms neutral  # p+ = # e- p+ and no about same mass e- = 1836 times smaller than p+ Nucleus has majority of mass Nuclear Forces: short-range forces that hold nucleus together p+-p+, p+-no, no-no

Counting Atoms Atomic Number (Z): number of p+ of each atom of the element Identifies Elements!!

Atomic Number What is the atomic number for… Hg What is the element with … 20 p+ Neutral atoms  # p+ = # e- In neutral atoms, atomic number = # e-

Isotopes Def: atoms of same element with different # of no (different masses) 3 H isotopes: Protium: 1 p+ 99.9885% Deuterium: 1 p+ 1 no 0.0115% Tritium: 1 p+ 2 no

Mass Number Use mass number to determine isotopes Mass #: # of p+ and n0 Atom with 17 p+ and 20 no has mass # of… Chlorine-37 – Hyphen Notation

Cl Writing Isotopes 37 17 Nuclear symbol: Mass number Elemental symbol Atomic number

Finding # of Neutrons 27Al # of neutrons = mass # - atomic # How many p+, no and e- are in… Selenium – 79 27Al Oxygen - 18 13

Isotopes Write the complete chemical/nuclear symbol for the isotope with 21 protons, 24 neutrons, and 21 electrons.

Relative Mass of Atom Mass of Oxygen-16 = 2.656 x 10-23 g Use relative atomic masses Pick standard  other masses are expressed in relation to standard Standard: Carbon –12 atom

Carbon - 12 Has mass of 12 atomic mass units (amu) 1 amu: is exactly 1/12 the mass of a carbon-12 atom Carbon-12: how many p+ and no? 1 p+  1 amu (1.007276 amu) 1 no  1 amu (1.008665 amu) 1 e-  0 amu (0.005486 amu)

Carbon - 12 What element is 4 x mass of Carbon –12? What element is 1/3 mass of Carbon –12?

Average Atomic Masses Most elements are mixture of isotopes Mass of element is average mass of isotopes We need the % of each isotope Average Atomic Mass: weighted average of the atomic masses of the naturally occurring isotopes of an element.

Weighted Average Box with 100 marbles of 2 sizes: 25% have mass of 2.00g 75% have mass of 3.00g What is the average mass of a marble? 2.75g (sig figs!!)

Average Atomic Mass Or (atomic weight) found on periodic table (ROUND TO 2 DECIMAL PLACES!!) Average Atomic Mass = (% Iso. #1)(Mass Iso.#1) + (%Iso.#2)(Mass Iso.#2) + … 100

Average Atomic Mass Find the atomic mass of Li if… 7.5% is Lithium – 6 = 6.015 amu 92.5% is Lithium – 7 = 7.016 amu

Example – AAM = 16.00 amu Isotope Fractional Abundance Oxygen – 16 (15.99 amu) 99.757% Oxygen – 17 (17.00 amu) .038% Oxygen – x .235%

The Mole

Quantities 1 dozen = 1 gross = 1 ream of paper = 12 1 gross = 144 1 ream of paper = 500 In chemistry: 1 mole = 6.022 x 1023

The Mole SI unit for amount of substance (mol) Def: the number of particles in exactly 12 g of carbon – 12. 12 g of carbon –12 has 6.022 x 1023 atoms Avogadro’s #, after Amadeo Avogadro The number is HUGE!!

Molar Mass Def: mass of 1 mole of a pure substance 1 mole Carbon –12 = 12 g 1 atom Carbon – 12 = 12 amu Mass of 1 mole of He atoms? 4.00g/mol Same as atomic mass from periodic table, different units

Molar Mass So: 4.00g He, 6.94 g Li and 200.59 g Hg all have 6.022 x 1023 atoms = 1 mole!!! Molar mass (g/mol)  conversion factor!!

Gram/Mole Conversions Mass, in g, of 3.6 mol of C? 43g How many moles are in 23.5 g S? 0.733 mol p. 85 problems

Conversions with Avogadro’s # Diatomic Molecules: elements that exist as 2-atom molecules in natural state 7 diatomic molecules H2, O2, N2, F2, Cl2, Br2, I2 Remember!!! HONClBrIF

Conversions with Avogadro’s # How many moles of oxygen are in 2.00x1022 molecules of oxygen? How many atoms of sodium are in 3.80 mol of sodium?

Conversions with Avogadro’s # How many molecules of hydrogen are in 0.020 g of hydrogen? Mass of 5.0x109 atoms of neon? p.86 problems