Download The Atom

Elements, Atoms,
and Ions
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Element names and symbols
Atomic structure, history and nucleus
Isotopes and average atomic mass
Formation of ions
Nuclear
Radioactivity and nuclear transformation
The Atom – A History
• Ancient Greece
• Democritus
• All matter consists of tiny particles or combinations of
particles of four types.
The Atom- A History
• Alchemists
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Quest for gold
Some sincere scientists
Lacked a quantitative approach
Resulted in progress none the less
• Mercury, sulfur, antimony, acids
• Robert Boyle (1627-1691)
• Quantitative experimental approach
• Gases
• Defined element as a substance that could not be broken down
into two or more simpler substances
The Atom- A History
• John Dalton (1766-1844)
• What was already clear?
• Most natural materials are mixtures of pure substances.
• Pure substances are either elements or combinations of elements
called compounds.
• A given compound always contains the same proportions by mass
of the elements which constitute it.
• ie, water always contains 8g of oxygen for every 1g of hydrogen
The Atom- A History
• Dalton’s Atomic Theory
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Elements are made of tiny particles called atoms
All atoms of a given element are identical
The atoms of a given element differ from those of other
elements
Atoms of one element can combine with those of other
elements to form compounds, and a given compound always
has the same relative numbers (ratios) of types of atoms.
• ie, H2O is a 2:1 ratio by mass and that ratio will not change
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Atoms are indivisible in chemical processes, but the way in
which they may be grouped can change
• Allowed prediction of formation of multiple compounds
between two elements and simple formulas
The Atom- A History
• Billiard Ball Model
• What was still not clear?
• What does an atom look like?
• Can it be broken into small pieces?
• J.J. Thomson
• 1897
• Discovery of electron
• Cathode ray tube
The Atom- A History
• J.J. Thomson
• Cathode ray was deflected away from a negatively charged
plate.
• What conclusion could he draw from this?
• Plum pudding model
The Atom- A History
• Ernest Rutherford
• Gold Foil Experiment (1911)
• Emission of positively charged particles from radioactive
elements was somewhat clear already. A radioactive element
served as Rutherford’s source of alpha particles.
The Atom- A History
• Gold Foil Experiment
Rutherford Scattering
What conclusions could be
drawn from this experiment?
The Atom- A History
• Rutherford’s Nuclear Atom
• Other conclusions
• Proton – same size charge as
electron but positive
• Most of the atom actually consists
of what?
• Neutron
• Rutherford later discovered
existence of neutral particle with
slightly larger mass than the
proton.
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Defining the Atom
• Atom – smallest particle of an element that
retains its identity in a chemical reaction.
• Protons
p+
(+)
• Mass 1840 times larger than electron
• Neutrons
n0
(no charge)
• Mass nearly equal to that of a proton
• Electrons
e(-)
• Can be shared or exchanged between atoms
• Mass is 1/1840 the mass of a hydrogen atom
• occupy almost all the volume of the atom.
Distinguishing Among Atoms
• How are atoms of hydrogen different than atoms of oxygen?
• Elements are different because they contain different numbers of
protons.
• Atomic number = number of protons = electrons in a neutral
atom
• Mass number = the total number of protons and neutrons in an
atom
• Number of neutrons = mass # - atomic #
Isotopes
• Isotopes – atoms that have
the same number of protons
but different numbers of
neutrons.
C
C
Atomic # = 6 Atomic # = 6
Mass # = 12 Mass # = 13
 Mass # = protons + neutrons
 Estimation of actual mass or isotopic mass
 Slightly different than mass number due to a
small difference in the masses of protons and
neutrons.
• In nature, most elements are mixtures of isotopes.
• So what about the mass on the periodic table?
Average Atomic Mass
• Atomic mass – weighted average mass of the atoms in a
naturally occurring sample of the element.
• Which method(s) would result in the correct average atomic mass?
Average Atomic Mass
• Avg Atomic Mass= (mass1)(abundance1) + (mass2)(abundance2) …
100
Both valid
Periodic Table
• Groups and Periods
• Metals, Nonmetals, Metalloids
• Electrons and Ions
The Modern Periodic Table
• The Periodic Table is a listing of all the known elements.
• The elements are organized by:
• Atomic number
• Chemical Properties
• Group( or Family): a vertical column. Elements in groups have
similar chemical properties.
• Period: a horizontal row.
Metals, Nonmetals and Metalloids
• Elements can be classified into:
• Metals
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Good conductors of electricity and heat.
Malleable (can be hammered into sheets).
Ductile (can be drawn into wires).
Lustrous.
Easily lose electrons.
Form positive (+) ions. (cations)
• Nonmetals
• They do not have the properties of metals.
• Gain electrons.
• Form negative ions. (anions)
• Metalloids (semimetals)
• Display properties of both
Nonmetals
Metals
Metalloids
Actinides and Lanthanides
Actinides and Lanthanides
Electrons and Ions
Bohr’s Model – Energy Levels
Electrons and Ions
• The periods (rows) of the periodic table indicate the highest
energy level occupied by one or more electrons.
• The “A” groups are numbered 18.
• The number of the “A” groups correspond to the number of
valence electrons.
• Valence electrons – electrons in outermost, occupied energy level
Charges- Noble Gas Configuration
Nuclear Chemistry
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Radioactive Decay - types
Nuclear Equations
Half-life calculations
Nuclear Energy-fission and fusion
Radioactive Decay
What affects nuclear
stability?
• Neutron:Proton ratio
A/Z
• Even or odd number of
protons
• "magic" numbers 2, 8,
20, 28, 50, 82, and 126
• Unknown factors
Radioactive Decay
Radioactive Decay
• Types
Radioactive Decay
Positron Decay
• Antiparticles
• Every particle has a corresponding antiparticle
Positron Decay
• Antimatter
• Hydrogen and anti-hydrogen
Positron Decay
• “Going beyond the basics, we can say that an antiparticle is related to the
particle by charge conjugation. This includes more than just electric
charge; it inverts all internal quantum numbers such as baryon
number, lepton number, and strangeness.”
• http://hyperphysics.phy-astr.gsu.edu/hbase/particles/antimatter.html
Positron Decay
• Beta Decay = neutron  electron + proton
• Positron Decay = proton  neutron +positron
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What is the effect of positron decay on the mass and atomic
numbers of the parent isotope?
Electron Capture
Electron Capture
• A proton absorbs an electron to become a neutron.
no
p+
What is the effect of electron capture on the
mass and atomic numbers of the parent
isotope?
Predicting Decay Type
Radioactive Decay
Nuclear Equations
Uranium-238 decays by alpha emission into thorium-234
• Carbon-11 produces a betaparticle
• Neptunium-237 produces an
alpha particle
Half-Life
• How long does it take for unstable
nuclei to become stable?
• Rate of Decay
• Measured in half-lives
• Half-life (t1/2) – amount of time it
takes for half of the radioactive
nuclei to decay
Half-Life