Atomic Structure

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Atomic Structure

• The Idea of the Atom

2
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.

3
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
• Didnt believe or look into atoms until

4
Late 1700s

• Chemists believed elements
• couldnt be broken down by ordinary means
• Combine to form compounds
• Advancements in balances ? better experiments

5
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

6
Laws Discovered

• Elements combine to form more than one compound
  Ex carbon monoxide, CO, and carbon dioxide CO2

7
John Dalton

• 1808 DALTONS 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.

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Daltons 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.

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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-

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J.J. Thomsons ExperimentCathode - Ray Tube

-
Moving electrical current (Cathode Ray) through
glass tube with inert gas (Noble Gas)
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J.J. Thomsons Experiment

-

• By adding magnets, found the moving (-) particles

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Thomsons Model

• Same properties no matter what element used
• All atoms had ELECTRONS!!
• Thomsons Model ?Plum pudding bunch of positive
  stuff, with the electrons able to be removed.

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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

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Florescent Screen
Lead block
Uranium
Gold Foil
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His prediction.
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What he expected
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What he happened
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Results

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

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Plum-pudding is WRONG!

• Atom is mostly empty
• Small dense, positive pieceat center,
  nucleus
• Alpha particlesare deflected by nucleus if
  they get close.

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Electron Cloud Model

• Current model of atom
• Nucleus surrounded by negative cloud electron
  cloud

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Size of Atom

• Atom size of football stadium
• Nucleus smaller than a dime in center
• Electron each one smaller than Roosevelts eye
  on the dime
• Atom is mostly empty space!

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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

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Counting Atoms

• Atomic Number (Z) number of p of each atom of
  the element
• Identifies Elements!!

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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-

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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

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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

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Writing Isotopes

• Nuclear symbol

37
Mass number
Cl
Elemental symbol
17
Atomic number
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Finding of Neutrons

• of neutrons mass - atomic
• How many p, no and e- are in
• Selenium 79
• 27Al
• Oxygen - 18

13
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Isotopes

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

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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

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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)

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Carbon - 12

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

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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.

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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!!)

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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

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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

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Example
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The Mole
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Groups

• 1 dozen
• 12
• 1 gross
• 144
• 1 ream of paper
• 500
• In chemistry 1 mole 6.022 x 1023

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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
• Avogadros , after Amadeo Avogadro
• The number is HUGE!!

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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

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Molar Mass

• Contains one mole of atoms
• So 4.00g He, 6.94 g Li and 200.59 g Hg all have
  6.022 x 1023 atoms!!!
• Molar mass (g/mol) is used as a conversion
  factor!!

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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

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Conversions with Avogadros

• Diatomic Molecules elements that exist as 2-atom
  molecules in natural state
• 7 diatomic molecules
• H2, O2, N2, F2, Cl2, Br2, I2
• Remember!!! HONClBrIF

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Conversions with Avogadros

• How many moles of oxygen are in 2.00x1022
  molecules of oxygen?
• How many atoms of sodium are in 3.80 mol of
  sodium?

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Conversions with Avogadros

• How many molecules of hydrogen are in 0.020 g of
  hydrogen?
• Mass of 5.0x109 atoms of neon?
• p.86 problems