Origin of the Chemical Elements

1
Origin of the Chemical Elements

• Reminder - the basis for accepting a theory is
  not its validity but its utility - it explains
  observations already made and correctly predicts
  future observations

2
Big Bang Theory

• From the discipline of Cosmology - study of
  origin, evolution and ultimate fate of the
  universe
• Theory first proposed by Belgian priest, Father
  George Lemaitre, in 1927
• Lemaitre later became a bishop and served as head
  of the Pontifical Academy of Science at the
  Vatican

3
Description of Big Bang

• About 12-14 billion years ago, all the energy of
  the universe was concentrated in a small volume
• Universe began with tremendous explosion which
  created matter out of energy (E mc2 , where E
  is energy, m is mass of matter created, and c is
  velocity of light)

4
Cont'd

• In 0.001 s, temperature dropped from 1030 K to
  1012 K and neutrons were formed (373 K 212 oF)
• Neutrons decayed to produce protons and electrons
  (the components of the hydrogen atom) n p e

5
Continued...

• Over the first three minutes the temperature
  dropped from 109 K to 107 K and nuclear reactions
  occurred forming cations of H-2, H-3, He-3, He-4
• After three minutes, the temperature was too low
  to sustain further reactions

6
Evidence Supporting Theory

• The Universe is currently expanding (i.e.
  galaxies are "red-shifted")
• The ratio of helium to hydrogen is consistent
  with theory
• The Universe is filled with Cosmic Background
  Radiation at a temperature of 3 Kelvin

http//www.oceansonline.com/universe.htm
7
Representing Nuclear Changes
8
Nuclear Equation

• Nuclear equation is a symbolic representation of
  a nuclear change
• Nuclide symbols (usually without charges) are
  used to represent nuclei of atoms
• Symbols used for other particles frequently
  involved in nuclear changes are shown on next
  slide

9
Symbols for Selected Particles
http//www.lcc.edu/science/nuclearchem/index.html
10
Writing a Nuclear Equation

• First write the skeleton equation - translation
  of a fact into symbols
• Insert plus signs between symbols of particles
  originally present (reactants) and particles
  finally present (products) and then insert an
  arrow between reactants and products

11
Contd

• Fact H-1 nuclides combine at high temperature to
  produce H-2 and a positron
• Skeleton equation 11H gt 21H 01e
• Balance equation insert coefficients to make
  sums of mass numbers, A, and sums of charge
  numbers, Z, equal for reactants and products
• 2 11H gt 21H 01e

12
Fact H-1 produces H-2 and a positron
Equation 2 11H gt 21H 01e
http//wine1.sb.fsu.edu/chm1046/notes/Nucleo/Nucle
o.htm
13
Fact H-1 and H-2 combine to produce He-3
Equation 11H 21H gt 32He
http//wine1.sb.fsu.edu/chm1046/notes/Nucleo/Nucle
o.htm
14
Fact He-3 produces He-4 and H-1
Equation 2 32He gt 42He 2 11H
http//wine1.sb.fsu.edu/chm1046/notes/Nucleo/Nucle
o.htm
15
Formation of First Atoms

• Following the Big Bang, nuclei of atoms and
  electrons existed as a gas (plasma), primarily
  H, He2, and e-
• When Universe cooled sufficiently, 300000-500000
  years, e- became associated with nuclei to form
  atoms of H and He having electrons moving in
  circles about nucleus

16
Galaxies and Stars

• Gravity brought atoms of H and He together to
  form stars and galaxies
• Gravity initiated nuclear reactions in stars
  which produced heavier nuclei
• When resulting plasma cooled, the neutral atoms
  of the heavier elements were formed

17
Overview
Stars and Galaxies
18
Nuclear Fusion Reactions in Stars

• Fusion reactions - small nuclides combine to
  produce a larger nuclide
• The larger the nuclide undergoing fusion, the
  greater the temperature required
• Fusion reactions in stars occur in order of
  increasing size of nuclide H first then He, etc
• When the supply of H runs out, the star collapses
  inward, the temperature increases, the fusion of
  He begins, and star expands

19
Synthesis of the Chemical Elements in Stars
Olmstead, Chemistry, Mosby, St. Louis, MO,
1994, p. 960.
Olmstead, Chemistry, Mosby, St. Louis, MO,
1994, p. 960.
Olmstead, Chemistry, Mosby, St. Louis, MO,
1994, p. 960.
20
Cosmic Dust
Coalescing
H-Burning
He-Burning
C-Burning
Silberberg, Chemistry Mosby,St. Louis, MO,
1996, p 965
Supernova
21
Collapse of Star and Supernova
If the core cools, unbalanced gravitational force
causes star to collapse and supernova occurs
Star is stable when these two forces are equal
http//wine1.sb.fsu.edu/chm1046/notes/Nucleo/Nucle
o.htm
22
Star Composition at Supernova
Accumulation of iron in the core causes the core
to cool since Fe does not undergo fusion reactions
Cox, The Elements, Oxford, New York, NY, 1989,
p. 80
23
Stability of Nuclei and Fusion Reactions
Nuclear fusion reactions are exergonic only when
products are more stable than reactants and thus
can only occur with elements lighter than Fe
Stability Increasing
Jones, "Chemistry, Man and Society", Saunders,
1972, 623
24
Formation of Nuclides Heavier Than Iron

• Collapse of the core of a star in a supernova
  produces many neutrons
• Neutrons can react with atoms one or more at a
  time and release electrons one or more at a time
  to produce elements heavier than iron - called
  neutron capture

25
Reactions that Produce Nuclides Heavier than Iron
26
http//www2.glos.ac.uk/gdn/origins/earth/ch2_4.htm
27
Abundances of Elements in Universe
Heavier elements formed only in a supernova are
less abundant due to lack of supernovas (only
0.05 of stars are large enough) and short time
frame for collisions with neutrons - only a small
fraction of lighter atoms converted in each
supernova.
As Z increases, abundances generally decreases
Radel/Navidi, "Chemistry", West, NY,1990, 987
28
Formation and Properties of Neutral Atoms
29
Formation of Neutral Atoms

• As the debris from a supernova cools, the nuclei
  will pick up electrons to form neutral atoms
• Electrons repel each other sufficiently to cause
  groups of electrons to occupy shells at
  ever-increasing distances from nucleus

30
Electron Structures

• As atomic number, Z, increases electrons either
  add to outermost shell (called valence shell) or
  to an inner shell resulting in a repeating
  pattern in which valence shell increases from 1e
  to 8e
• Elements having similar electron structures are
  observed to have similar properties

31
Arrangement of Elements by Electron Structure
32
Periodic Table

• Systematic Arrangement of the Chemical Elements

33
Terms Periods and Families
34
Terms Main Transition Elements
Electrons add to inner shell
35
Properties Electron Structures
36
Properties of Elements

• Properties of elements depend largely on the
  number of valence electrons in their atoms
• Elements in same family have same number of
  valence electrons so they have similar properties
• Intensity of a property varies with location of
  element within a family

37
Properties Types of Elements
38
Properties Sizes of Atoms
39
Properties Masses of Atoms
40
Comparing Masses of Average Atoms

• The relative masses of average atoms are given by
  their atomic weights
• Find the relative mass ratio between the average
  atoms of sulfur and oxygen
• S/O 32 amu/16 amu 2
• An average sulfur atom is twice as heavy as an
  average oxygen atom

41
Properties Chemical Reactions

• Elements in same family have similar reactions
  because they have the same number of e in valence
  shells and valence shells of approaching atoms
  come in proximity first
• Chemical reactions involve rearrangements of
  valence electrons

42
Reactions of Selected 1A Elements with Water
Real World all 1A elements react with water
Model all 1A elements have 1 valence e
Real World speeds of reactions of 1A elements
with water are different
Model The larger the 1A atom, the easier to lose
e, faster the reaction
http//www.chemicalelements.com/index.html