What are atoms

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What are atoms?

• Smallest particle of element that retains
  properties of the element
• Possible to see individual atoms with scanning
  tunneling microscopes
• Original model Democritus
• Atomos smallest unit of matter
• Indivisible and indestructible

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History of Atomic Theory - Daltons Theory

• 1) Elements composed of tiny indivisible (not
  invisible) particles called atoms
• 2) Atoms of the same element are identical
• 3) Atoms of different elements can chemically
  combine with one another in simple whole number
  ratios (compounds)
• 4) In chemical reactions, atoms are separated,
  joined, and rearranged. Atoms of one element are
  never changed.

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How did the structure of the atom develop? -
Electrons

• J.J. Thomson, cathode ray tubes
• Electric current flowing through gases
• Beam cathode ray travels from cathode (-) to
  the anode ()
• Metal plates beam attracted to positive plate
• Electrons negative charge
• Plum Pudding Model

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Atomic Structure

• Millikan oil drop experiments
• determined charge/mass ratio of electron
• Mass of electron 1/1840 mass of hydrogen atom

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Structure cont.

• Protons positively charged particles
• Needed to balance charge of electrons in atom
• Goldstein CRT with loss of electrons canal
  rays from cathode traveling in opposite direction
  as electrons
• Neutrons no charge, mass - proton
• Chadwick knew some mass was unaccounted for

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Rutherfords Gold Foil Experiment

• Flash Animation
• Led to discovery that most of the atom is empty
  space
• All of the positive charge and most of the atoms
  mass found in a small region
• Nucleus of the atom
• Core that contains protons and neutrons

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Summary of Subatomic particles
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Atomic Number

• Number of protons in nucleus of atom of element
• Also equals number of electrons if electrically
  neutral
• Determines which element you have
• Examples
• C
• 27
• Br

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Mass number

• Total number of protons and neutrons
• Not given on the periodic table
• Examples
• C-14
• 11B

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Calculating Number of Protons, Neutrons, and
Electrons

• Number of neutrons
• Mass Number Atomic Number (12-6 6)
• Shorthand Notation
• Mass/Atomic, Element (126C)
• OR Element and Mass number (C-12)
• Practice
• Determine number of protons, electrons, and
  neutrons
• Mg-24, Ba-137

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Practice Table
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What are Isotopes?

• Atoms that have the same number of protons but
  different numbers of neutrons
• Different mass numbers
• Atoms are chemically alike behave similarly
• Common misconception radically different NO!
• Similar physical properties

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Examples 3 isotopes of hydrogen

• Tritium Spiderman 2 errors
• Gas, not solid
• More than 25 pounds in world
• Chemical symbols, Subatomic particles examples
  for isotopes (O-16,17,18)
• Symbolic Notation isotopes
• Heavy Water

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If mass number is not on the P-Table, what is?

• Expressing Mass
• Masses of subatomic particles small needed
  something more convenient
• Arbitrary amount set Mass of a single
  proton/neutron 1 amu
• 1 amu (atomic mass unit) 1/12 mass of C-12 atom
• Why are the atomic masses on the table decimals?
• Most elements exist as a mixture of 2 or more
  isotopes

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How do you calculate average atomic mass?

• Average atomic mass weighted average mass of
  atoms in naturally occurring sample
• Number on Periodic Table explains why we could
  not use as mass number
• Reflects both the mass and the relative abundance
  of the isotopes as they occur in nature
• Practice
• Which is more abundant? S-32 or S-34?

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Practice Calc. Average Atomic Mass

• Next, an example comparing to weighted average in
  class (grades)
• Tests 85, HW 98, Labs 90, Final 96
• Calculate the average atomic mass of Chlorine if
  the two known isotopes have the following
  abundances
• 1) Cl-35 75.77
• 2) Cl-37 24.23

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Origin of Periodic Table

• Mendeleev
• Listed elements in columns and in order of
  increasing atomic mass
• Elements in groups with similar properties
• Moseley
• Determined atomic numbers
• Rearranged periodic table to current state
• P-Tables Online
• HTML Table
• Interactive Periodic Table
• Comic Book P-Table

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Organization of Periodic Table - Overview

• Originally ordered in increasing atomic mass, now
  increasing atomic number
• Periods
• Horizontal rows
• Groups
• Vertical columns of elements
• Periodic law
• Repetition of chemical and physical properties
• Representative elements
• Metals (alkali, alkaline earth, transition)
• Nonmetals (halogens, noble gases)
• Metalloids

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END
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Models of Atoms