CEC 2006 lecture 1

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What is an Atom? Dr Graeme Jones
LECTURE 1
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Background and reference material

• Main reference L Jones P W Atkins,
  Chemistry Molecules, Matter and Change, 4th
  edition, Freeman 1999
• However there are many suitable textbooks at the
  College Chemistry level
• An interesting alternative perspective can be
  found in Bill Brysons A Short History of Nearly
  Everything, chapter 9!

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Dalton
Democritus
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Atoms and their fundamental particles

• Chemistry is the study of matter and the changes
  it undergoes
• Matter is defined as being composed of atoms
  (first proposed by Democritus in 400 BC)
• Daltons Atomic Theory (1808) all matter
  consists of atoms which cannot be created,
  destroyed or split

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Elements and atoms

• Elements are substances composed of only one type
  of atom
• The composition of atoms was established by a
  series of experiments
• The plum pudding model had suggested that atoms
  were made up of positive sponge material with
  the negative electrons distributed throughout
  like currants

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The composition of atoms

• However, this was all changed in 1909, when
  Rutherfords students, Geiger and Marsden,
  performed an experiment where they fired alpha
  particles at a sheet of gold foil, and found
  that some immediately bounced back!
• This led to Rutherford proposing that atoms are
  made up of a positive nucleus with the electrons
  at some distance away.

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Rutherford Nobel Prize for Chemistry 1908
Geiger
Marsden
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The Geiger-Marsden Experimenthttp//www.physics.
nmt.edu/raymond/classes/ph13xbook/node193.html
Schematic of Geiger-Marsden experiment. The
radioactive source produces alpha particles which
are collimated into a beam and directed at a gold
foil. The alpha particles scatter off the foil
and are detected by a flash of light when they
hit the scintillation screen.
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The Rutherford model of the atom

• Atoms are made up of a positive nucleus
• Electrons adopt planetary orbits around the
  nucleus
• Atoms are mainly space this has been
  illustrated in several ways
• Jones Atkins fly in a baseball pitch (p8)
• Bryson fly in a cathedral

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The size of atoms

• The traditional unit for measuring the size of an
  atom is the Angstrom (Å), nowadays we use
  picometers (pm 10-9 m)
• 1 Å 1.0 x 1010 m 0.1 nm 100 pm
• 1.0 x 1010 0.0000000001
• Atoms are considered to be spherical in nature
  and therfor atomic radii are measured e.g. helium
  31 pm, caesium 298 pm (calculated radii).
• By comparison the size of the nucleus is measured
  in femtometers (fm 10-15 m) e.g 1.6 fm for
  hydrogen, 1.9 fm for helium, 6.1 fm for caesium
  and 7.4 fm for uranium
• By comparison if you are the nucleus of an atom
  the outer edge of the atom is about 2 km away.
• See http//hyperphysics.phy-astr.gsu.edu/Hbase/nuc
  lear/nucuni.html to calculate radii

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Elementary particles

• Initially the nucleus was assumed to be made up
  of protons, positively charged particles
• Later, neutrons were discovered, particles with
  no charge but the same mass as protons
• Protons and neutrons have mass 1.67 x 10-27 kg
  and electrons have mass 9.09 x 10-31 kg

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Elementary particles summary
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Isotopes nuclei with varying numbers of neutrons

• Elements are defined by the number of protons in
  the nucleus, but in a given element the number of
  neutrons can vary
• An isotope is the term used to describe an
  element with a particular number of neutrons. For
  example hydrogen has three isotopes with varying
  numbers of neutrons
• 1H (0 neutrons), 2H (1 neutron), 3H (2 neutrons)

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Atomic number and atomic mass

• Atomic number, Z number of protons ( number of
  electrons) in the nucleus
• Atomic mass, A number of protons neutrons in
  the nucleus
• Notation
• Examples

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Average atomic masses - 1

• Most elements have several isotopes, so their
  atomic mass is expressed as an average, taking
  relative abundances into account
• For example, chlorine exists as two isotopes,
  35Cl and 37Cl, with relative abundances of 75
  and 25 respectively
• The average atomic mass is then calculated as
  follows

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Average atomic masses - 2

• Average atomic mass
• 35 x 75/100 37 x 25/100 35.5 a.m.u.
• Where 1 a.m.u. (atomic mass unit) 1.660 x 10-27
  kg
• Other examples will be given in the lecture (and
  you can find more in the textbooks)

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Mendeleev
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The Periodic Table - 1

• The modern Periodic Table is a classification of
  all known elements by their ATOMIC NUMBER (Z)
• Elements with the same properties are found in
  groups (columns) there is a direct
  relationship between an elements properties and
  its position in the Periodic Table, and trends
  can be observed going along rows and up/down
  columns

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The Periodic Table - 2

• It is useful to have in mind a simple block
  representation of the Periodic Table (to be drawn
  in the lecture)
• You can download different versions of the
  Periodic Table from the web site, and various
  versions will be shown
• The ultimate online version is probably
  webelements http//www.webelements.com/

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What is an Element?

• A substance that consists of atoms having the
  same chemical properties
• A substance that cannot be broken down into
  simpler components using chemical techniques
• Examples
• Hydrogen H
• Helium He
• Oxygen O
• Copper Cu
• Gold Au

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What is an Atom?

• The smallest particle of an element that has the
  chemical properties of that element
• Atoms are the building blocks of elements

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What is a Compound?

• A substance consisting of atoms of two or more
  elements in a definite ratio i.e. a specific
  combination of atoms
• A specific combination of elements that can be
  broken down by chemical techniques
• Examples
• Sodium Chloride NaCl
• Water H2O
• Ethanol C2H6O
• Viagra C22H30N6O4S

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What is a Molecule?

• A substance consisting of two or more atoms
  combined together
• Examples
• Hydrogen gas H2
• Nitrogen gas N2
• Ethanol C2H6O
• Viagra C22H30N6O4S

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What is an ion? (I)

• When an atom loses or gains an electron it is
  called an ion e.g.
• Sodium readily loses an electron to make a cation
• Chlorine readily accepts an electron to make an
  anion

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What is an ion? (II)

• Ionic compounds are compounds resulting from a
  reaction between ions e.g. Sodium Chloride (salt)
• As the compound contains equal numbers of anions
  and cations then the compounds themselves have no
  overall charge.

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Relative Molecular Mass, Mr

• This is the average mass of a single molecule and
  is sometimes termed RMM
• To calculate Mr add up the relative atomic masses
  of the different atoms in the molecule.
• H2 Mr 1x2 2
• H2O Mr (1x2) 16 18
• HCl Mr 1 35.5 36.5
• C6H12O6 Mr (12x6) (1x12) (16x6) 180