Atoms and Elements

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Atoms and Elements

• Chemical Activity
• Ions
• Chemical Composition of the Earths Crust
• Crystallinity
• The Silicon-Oxygen Tetrahedron  

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Minerals

• Crystalline Solids
• Natural and Inorganic Substances
• Definite Chemical Composition
• The Important Minerals

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Rocks versus Minerals
Granite rock type
Rock an aggregate of one or more minerals in
this case there are three minerals present.
Feldspar NaAlSi3O8 - KAlSi3O8
Quartz SiO2
Biotite KMg3AlSi3O10(OH)2 -
KFe3AlSi3O10(OH)2
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Mineral Structures and Atoms
1 angstrom 1.0 10-10 meters
Mineral A naturally occurring inorganic element
or compound having orderly internal structure and
characteristic chemical composition, crystal
form, and physical properties.
From Plummer et al.
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Atomic Structure and the Composition of Matter

• The atom is a basic building block of minerals.
• Matter is a special form of energy it has mass
  and occupies space. Neither matter nor energy may
  be created or destroyed - they may only be
  converted from one form to the other.
• Energy is the ability to do work and it occurs in
  a number of forms, including
• Potential Kinetic Electrical Heat Chemical
  Nuclear
• Radiant (the only form in which there is an
  absence of matter)
• Atoms are the smallest division of matter that
  retain the characteristics of the elements.

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Elements and the Periodic Table

• Approximately 300 different kinds of atoms that
  are capable of independent, prolonged existence.
  These are called nuclides.
• If nuclides are grouped by chemical
  characteristics, about 100 sets result and these
  are referred to as elements.
• The modern Periodic Table was devised in 1869 by
  Julius Meyer and Dmitri Mendeleev. It organizes
  the elements into groups and families with
  similar chemical and physical properties.

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Periodic Table
Periods
Groups or Families
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Crustal Abundance
Crustal volumelt1 of Earth Mantle volume 83 of
Earth Core volume 16 of Earth
Crustal masslt1 of Earth Mantle mass 68 of
Earth Core mass 31 of Earth
What element is most abundant for the entire
Earth?
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Atomic Particles Basics

• Atoms are composed of electrons and two large
  nuclear particles called protons and neutrons.
• Protons and neutrons are approximately equal in
  mass and are 1800 times more massive than the
  electron. Both nuclear particles are composed of
  quarks, smaller fundamental particles.
• Protons have unit positive charge (1), while
  electrons have unit negative charge (-1).
  Neutrons carry no charge.
• Atoms are electrically neutral and thus the
  number of electrons must equal the number of
  protons.

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Basic Terminology

• Atomic number (Z) The atomic number represents
  the number of unit positive charges on the
  nucleus and is equal to the number of protons
  within the nucleus, since each proton carries
  unit positive charge. In electrically neutral
  atoms, it also represents the number of
  electrons, which carry unit negative charge.
• Mass number (A) The mass number is equal to the
  total number of nucleons, which is the sum of the
  number of protons and neutrons. A does not equal
  the total mass of the atom rather, it represents
  a whole number approximation of the mass, as
  expressed in amu.
• The number of neutrons is simply defined as the A
  - Z.

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Isotopes and Isobars

• A specific type of atom is designated by using
  its chemical symbol, which is an abbreviation of
  its name in German, Latin, or English, with the
  A, the mass number, placed in the upper left and
  Z, the atomic number, placed in the lower left
  corner. For example, 23Na11, has a mass number
  of 23 and an atomic number of 11.
• Isotopes are atoms of the same element that
  differ in mass. For example, 87Sr and 86Sr or
  238U and 235U. Isotopes have similar chemical
  characteristics and are studied using a mass
  spectrograph or spectrometer. Most elements have
  at least two naturally occurring isotopes.
• Isobars are nuclides that have the same mass
  number but different atomic numbers. For
  example, 36S and 36Ar are isobars they both
  contain a total of 36 nucleons (protons plus
  neutrons), but the sulfur isotope has 16 protons
  and 20 neutrons, while the argon isotope has 18
  protons and 18 neutrons. Isobars do not have
  similar chemical characteristics!

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Atomic Weight

• Atomic weight is the weighted average of the
  atomic masses of the naturally occurring
  isotopes. For example, a natural sample of the
  element chlorine contains a mixture of 75.53
  35Cl and 24.47 37Cl. Thus the atomic weight is
  obtained by multiplying the mass of each isotope
  (in amu) times its fractional abundance
• 0.7553 (34.97 amu) 0.2447 (36.95 amu)
  35.45 amu

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Atomic Models

• Bohr Model
• Electron shells
• Quantum Mechanics
• Orbitals
• Afbau Filling Order
• Quantum numbers and superposition of states

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Bohr Atomic Model
Oxygen Atom
The Bohr model for the atom envisioned these
electrons in stable orbits of specified radius
and energy, where we could exactly pinpoint the
position of any individual electron. Each energy
level was permitted to have a specified number of
electrons, and was called a shell. We know now
that this simple view is not correct it is
impossible to exactly determine the position of
an electron in space.
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The Quantum Mechanical View

• Using the theory of quantum or wave mechanics we
  can calculate the probabilities of various
  electron configurations, and thus show that
  specified regions near the nucleus have higher
  probabilities for finding an electron than
  others. Each electron does, however, have a
  specific energy. Must solve wave equation for
  specific states!
• The combination of the energy and probability
  gives rise to the current understanding for
  electron distributions, which are referred to as
  electron orbitals these orbitals are referred to
  as s (sharp), p (principal), d (diffuse), and f
  (fundamental).
• With increasing atomic number, each new element
  has an additional electron also added to it
  extra-nuclear cloud. From theory and experiment,
  we know that these electrons are added in a
  systematic fashion, with the lowest energy
  orbitals being filled first.
• This process is called aufbau filling (1s -gt 2s
  -gt 2p -gt 3s -gt 3p -gt 4s -gt 3d -gt 4p -gt 5s, etc.
  ). Combine with Pauli exclusion principle to
  yield filling order.

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Quantum Numbers

• n principal
• l orbital (0 -gt (n-1))
• ml magnetic ( l)
• ms spin (1/2)
• l 0 -gt s orbital
• 1 -gt p orbital
• 2 -gt d orbital
• 3 -gt f orbital

n 6 l 4 m 1
From http//www.daugerresearch.com/orbitals/
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Animated Orbital Simulations
Superposition of States
4, 3, 3 and 4, 1, 0
3, 2, 2 and 3, 1, -1
From http//www.daugerresearch.com/orbitals/
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Water, Ice and Snow

• Most important substance on Earth
• Essential for biological life as we know it
• Unique volumetric property
• Molecular symmetry and its relationship to
  crystal morphology
• Crystal growth and its effect on crystal
  morphology

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Water Atomic Structure
Chemical Formula H2O
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Snowflake Morphology
Hexagonal Symmetry
1 - 5 mm
Oddly, ice is less dense than liquid water, hence
it floats and lakes freeze from the top down!
From http//www.its.caltech.edu/atomic/snowcryst
als
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Snowflake Growth
From http//www.its.caltech.edu/atomic/snowcryst
als
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LT-SEM Images of Snow Crystals
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Compounds and Bonding

• Chemical compounds
• Ionic bonding
• NaCl as a type example
• Electron transfer and shell completion
• Covalent bonding
• Diamond - pure C example
• Electron sharing
• Silicon Tetrahedron
• Structure (strong sp3 covalent bonds)
• Building block of silicate minerals

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Halite Atomic Structure
Chemical compound Two or more elements joined
together by a chemical bond. Most minerals are
composed of at least two elements.
Chemical Formula NaCl
Note Cubic Symmetry and Closest Packing
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Outer Shell Filling
1s2
1s22s22p6
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Ionic Bonding and Electron Exchange
11p 10e- 1
1s22s22p63s1
1e-
1s22s22p63s23p5
17p 18e- -1
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Ionic BondingElectron Transfer
Cations () are always smaller than the neutral
atom Anions (-) are always larger than the
neutral atom.
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Covalent Bonding Electron Sharing
Diamond Example - Pure Carbon in complex 3D
network
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Silicon Tetrahedron SiO4 (net -4 charge)
1 angstrom 1.0 10-10 meters
1.30 Å
0.34 Å
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Bridging Oxygen (BO)
Silicate Mineral Structures I
Silcon Tetrahedron strong sp3 hybrid covalent
bonds - 50 ionic 50 covalent character These
structures are the basic building blocks
of silicate minerals.
Silicate structures may be Characterized by the
the number of BOs per Si. The higher the
BO/Si Ratio, the more complex and polymerized the
structure.
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Silicate Structures
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Olivine Structure (Mg,Fe)2SiO4
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Models of Chain Silicates Pyroxenes
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A
B
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Positive ion
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Positive ion