Atomic Structure

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Atomic Structure
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Stepwise Timeline of Atomic Theory
Rutherford 1911
Dalton 1803
Modern Theory
Thomson 1897
Bohr 1913
Chadwick 1932
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Democritus 460 B.C. to 360 B.C.

• Who
• Greek Philosopher
• What
• Atoms cannot be created, destroyed or divided.
• How
• Observing nature

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Daltons Atomic Theory

• Who
• John Dalton (1766-1844), an English schoolteacher
  and chemist
• What
• proposed his atomic theory of matter in 1803.
• Although his theory has been modified slightly to
  accommodate new discoveries, Daltons theory was
  so insightful that it has remained essentially
  intact up to the present time.

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Dalton

• What ( write this outside the text box)
• Cannot be created or destroyed.
• All atoms of one element are exactly alike, (same
  size, mass, properties) but different from atoms
  of other elements
• Combine in whole number ratios to form compounds.
• How-
• work with gases

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J.J. Thomson -1903

• Who
• a British physicist.
• What
• Plum Pudding model
• (or Chocolate Chip
• Cookie model)
• Discovered electrons
• Model
• Atom was a positively charged sphere with
  negative electrons in it like chips

POSITIVE CHARGE
ELECTRONS
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J. J. Thomson

• How
• discovered that cathode rays are made up of
  invisible, negatively charged particles referred
  to as electrons.
• http//tinyurl.com/cathodert

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Ernest Rutherford

• What
• Found nucleus (1911)
• Occupies a small volume of the atom
• Contains almost all the mass of the atom
• Electrons orbit around nucleus
• Discovered proton

• Who
• British chemist
• and physicist.

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Ernest Rutherford

• How
• Gold Foil Experiment

Alpha particles which are positively charged pass
through unmolested most of the time.
Occasionally they would bounce off when they hit
something (proton) that was also positively
charged.
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Niels Bohr - 1913

• What
• Planetary Model
• Electrons (e-) have definite path around the
  nucleus (orbit)
• e- arranged around the nucleus according to
  energy level
• e- with lowest energy level are closest to
  nucleus
• How
• Spectral emission lines

• Who
• Danish Physicist

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Chadwick

• What
• Discovered the neutron in 1932
• How
• Used alpha particles

• Who
• British Scientist

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Modern Atomic Theory

• 1. All matter is made up of very tiny particles
  called atoms.
• 2. Atoms of the same element are chemically
  alike.
• 3. Individual atoms of an element may not all
  have the same mass. However, the atoms of an
  element have a definite average mass that is
  characteristic of the element.
• 4. Atoms of different elements have different
  average masses.
• 5. Atoms are not subdivided, created, or
  destroyed in chemical reactions.

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Atom and Elements

• Element - a substance that is composed of a
  single type of atom.
• Atom - the smallest particle of an element that
  retains the properties of that element.
• The diameter of an atom is measured in nanometers
  
• 1 nm 1 x 10-9 m 0.000000001 m
• Atoms are composed of sub-atomic particles.

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Proton

• Discovered by Ernest Rutherford in early 1900s
• Determines the identity of an atom
• Relative mass of 1 atomic mass unit
• Part of the nucleus of an atom
• Positive charge
• If you change only the of protons, you change
  the element being described.

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Neutron

• Discovered by James Chadwick in 1932
• Determines the isotope of an atom
• Relative mass of 1 atomic mass unit
• Part of the nucleus of an atom
• No charge (neutral)
• If you change only the of neutrons, you have a
  new isotope (variety) of the element .

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Changing the number of neutrons

• Creates ISOTOPES -
• Atoms of the same element but with a different
  number of neutrons.
• Isotopes of an element have nearly identical
  chemical properties

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Electron

• Discovered by J. J. Thomson in 1903
• Determines the charge of an atom (charged atoms
  are called ions)
• Relative mass of 0 (1/1836) atomic mass unit
• Make up the electron cloud of an atom
• Negative charge

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Changing the number of electrons

• When an atom loses electrons, it results in a net
  positive charge and is called a CATION

• ions are I itive

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Example of a cation

• Neutral potassium (K) has 19 protons and 19
  electrons.
• 19 protons 19
• 19 electrons -19
• 0

• If potassium (K) loses an electron, it only has
  18 electrons.
• 19 protons 19
• 18electrons -18 1
• This is written as K1 and is called a cation

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Changing the number of electrons

• When an atom gains electrons, it results in a net
  negative charge and is called an ANION

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Example of an anion

• Neutral bromine (Br) has 35 protons and 35
  electrons.
• 35 protons 35
• 35 electrons -35 0

• If bromine (Br) gains an electron, it has 36
  electrons.
• 35 protons 35
• 36 electrons -36 -1
• This is written as Br -1 and is called an anion

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Gained electron
Lost electron
anion
cation
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Describing an atom

• ATOMIC NUMBER
• Equals the number of protons in an element.
• In a neutral atom, the atomic number also equals
  the number of electrons.
• - All atoms of the same element have the same
  number of protons.
• The smaller of the two numbers in the periodic
  table square, always a
• whole number

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Describing an atom

• ATOMIC MASS
• A weighted average of the mass of all the
  isotopes (varieties) of an atom
• Each element has only one atomic mass
• Also called average atomic mass
• The larger of the two numbers in the periodic
  table square
• Always a decimal number

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Describing an atom

• MASS NUMBER
• Equals the protons neutrons in an atom
• Not always the same for atoms of an element ?
  isotopes
• Not listed on the periodic table
• Always a whole number

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APE MAN

• A Atomic Number
• P Number of Protons
• E Number of Electrons
• M Mass number
• A Atomic Number (again)
• N Number of Neutrons

Always the same number in a neutral atom
Mass Number minus Atomic number equals Number of
neutrons
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Isotope Notation
mass number
element symbol
atomic number
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Isotope Name

• name of the element dash mass number
• Example Carbon -14 is the isotope name for a
  carbon atom with a mass number of 14

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Isotope Notation
mass number
element symbol
atomic number
number of neutrons mass number atomic number.
How many protons and neutrons in this isotope?
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Practice
Isotope notation Isotope name Atomic number Mass of p of no of e-
Silicon - 14 18
Helium - 4
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Determining (Average)Atomic Mass

• To determine the atomic mass you must know what
  percent of each isotope of the element is found
  in nature. This is called the relative
  abundance.
• Example There are 2 common isotopes of
  Chlorine.
• 25 is chlorine - 37
• 75 is chlorine - 35
• Calculate the average atomic mass of chlorine.

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Average Atomic Mass

• Neon in nature is 90.5 Neon-20,
• 0.3 Neon-21, and 9.2 Neon-22.
• What is the average atomic mass of Neon?

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REACTIONS

• CHEMICAL

• NUCLEAR

• involve the transfer or sharing of electrons

• involve the absorption or emission of particles
  by the nucleus of an atom

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Nuclear Chemistry Vocabulary

• Nuclide- General name given to the nucleus of an
  atom
• Parent nuclide- initial nucleus
• Daughter nuclide- the nucleus after the decay has
  occurred

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Nuclear Chemistry Vocabulary

• Radiation - energy that is emitted from a source
  and travels through space.
• Ionizing Radiation- Has enough energy to change
  atoms and molecules into ions examples X-rays
  and gamma rays.
• Nonionizing Radiation- Does not have enough
  energy to ionize matter examples radio waves,
  microwaves
• Accidentally discovered by Henri Becquerel in
  1896 when he was performing a lab with
  fluorescent screens.
• Radioactivity is the spontaneous emission of
  radiation from the nucleus of an atom.

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Types of Ionizing Radiation


Symbol Charge Penetrating Ability Size
Alpha He or ? positive (deflected by a magnet) Limited ability to pass through matter. (can be stopped by paper) Big
Beta formed when a neutron splits e or ?-1 negative (deflected by a magnet) Can penetrate better than alpha (can be stopped by a few mm of Al) Small
Gamma ? Neutral (is not deflected by a magnet) Penetrates the farthest. (several cm of lead or a larger layer of concrete will block ) nothing
4 2




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penetrating ability
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Why decay happens

• To become more stable.
• Large atoms are stable when the neutron proton
  ratio is 1.51
• Decay happens when the neutron proton ratio is
  too high.

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Alpha decay
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Alpha Decay

• Occurs when an alpha particle leaves the nucleus
• - alpha particle Helium nucleus
• Parent ? daughter mass decreases by 4 and
  atomic number decreases by 2
• Example Thorium-230 undergoes alpha decay.
  Write the decay reaction.
• Th ------gt He Ra

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Alpha decay practice

• Write the decay reaction for alpha decay of
  Uranium-238.

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Beta decay
Ac
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Beta Decay

• occurs when a beta particle e is emitted from
  the nucleus
• Parent ? daughter equal mass but atomic number
  increases by 1.
• a neutron becomes a proton.
• Example Carbon-14 undergoes beta decay. Write
  the decay reaction.
• C -----gt e N

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Beta decay practice

• Write the decay reaction showing beta decay of
  Thorium-234.

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Fission and Fusion
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Fission Reaction

• Nuclear reaction
• Splitting an atoms nucleus
• Releases energy
• Alpha, beta are examples
• Used in nuclear reactors
• Causes a chain reaction
• Problem produce radioactive waste storage of
  fuel is dangerous

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chain reaction
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Fusion

• Nuclear reaction
• Two light nuclei are combined to form one heavier
  more stable nuclei
• Energy is released
• this is how stars are fueled
• Problem with using on Earth requires EXTREMELY
  high temps and high pressure

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fusion in the sun
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How come the protons hang out with each other?

• The charge of a proton is positive. It is
  repelled by other protons. So, how do the
  protons stay in the nucleus? Shouldnt they want
  to avoid each other?
• The answer is that a Strong Nuclear Force exists,
  which is a very strong, but short range, force
  between quarks that keep the nucleus together by
  overcoming the repulsion between the protons.