Atomic

1
Atomic Nuclear Physics

• Richard Feynmann, a Nobel Prize winning physcist
  and noted bongo player was once asked, If you
  could only pass along one sentence to future
  generations, in the event that all present
  knowledge was destroyed, what would it be?
• He replied, Everything is made of atoms.

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Atomic Nuclear Physics

• Everything is made of atoms contains the kernel
  of all scientific knowledge.
• We intend to explore the details of the atom
• Naturally occurring in nature are 92 distinct
  atoms
• Each differs systematically from the others

3
Elements

• Each of these distinct kinds of atoms is called
  an element
• Everything in nature that we can observe and
  measure is composed of pure elements or
  combinations of elements
• We will examine more about the elements later
  when we explore the periodic table

4
Atoms

• Atoms are very tiny things and arose some 14
  billion years ago in the Big Bang
• 90 of all atoms are Hydrogen
• The next most abundant atom is Helium
• We will talk later about production of heavier
  atoms

5
Atoms

• It is tough to draw realistic pictures of atoms
  because they are mostly empty space
• The typical atom is spherically shaped with a
  radius of about 10-10 meters
• This size is due to the position of electrons in
  the atom
• At the center of the sphere is a nucleus whose
  radius is about 10-14 meters

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Atoms

• To put these relative sizes in perspective if the
  nucleus were the size of a golf ball, the atom
  would have a diameter of about 2.5 miles
• You can see that the atom is mostly empty since
  the electrons are measured to be apparent point
  particles

7
Atoms

• The electrons are whizzing around at very high
  speed on the order of 2 x 106 m/sec
• So an electron can orbit the nucleus some
  1015-1016 times per second
• This picture is a bit phony as we shall see
  later, but it helps gain perspective on what the
  atom is like

8
Atoms

• So why are atoms so big compared to the size of
  the constituent particles?
• The enormous electric forces that the electrons
  exert on each other and between the nucleus and
  the electrons
• These forces have to be balanced correctly to
  keep the whole system stable
• Well explore this balance in detail later

9
Atoms

• Atoms just recycle in the universe
• Consider the oxygen in the air
• You are breathing some of the same oxygen
  molecules that Shakespeare breathed
• All the hydrogen is from original big bang
• All the helium is from original big bang
• We look at creating others later in the term

10
The Nucleus

• The nucleus is composed of protons and neutrons
• Compared to electrons, these are heavyweight guys
• Electron mass 9.1 x 10-31 kg
• Proton mass 1.673 x 10-27 kg
• Neutron mass 1.675 x 10-27 kg

11
The Nucleus

• The proton has a positive electric charge that is
  exactly the same size as the negative charge on
  the electron
• The neutron has zero charge
• In any atom, the number of protons is the same as
  the number of orbiting electrons
• The number of neutrons can vary

12
The Nucleus

• We identify each of the 92 elemental atoms by
  counting the number of protons in the nucleus
• This count is called the atomic number
• If you look at the masses in the periodic table,
  you will see that there are neutrons in every
  atom heavier than simple hydrogen

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The Nucleus

• We call the protons and neutrons that live
  together in the nucleus, nucleons
• Later, well look at the forces that hold the
  protons and nucleons together in very cramped
  quarters in the nucleus

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Atomic Mass

• For any element, there is no fixed number of
  neutrons in the nucleus
• For the lighter atoms past hydrogen, there are
  usually equal numbers of protons and neutrons,
  but they dont have to be equal
• Remember, the number of protons determines which
  element we are talking about

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Atomic Mass

• There are three forms of hydrogen in nature

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Isotopes

• We refer to the different variants of an element
  as the isotopes
• The isotopes of an element differ only in the
  number of neutrons in the nucleus
• We identify the isotope by mass number
• This is the total count of nucleons (protons
  neutrons)

17
Isotopes

• Once we know the element we are dealing with, we
  know the number of protons
• To find the number of neutrons, we subtract the
  atomic number from the mass number
• Chemistry is based on the electrical interactions
  of atoms and depends on the electrons which are
  outside the nucleus
• Thus, chemistry doesnt depend on the number of
  neutrons in the nucleus except for very minor
  effects

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Atomic Mass

• The total mass of an atom is the sum of the
  masses of all the protons and neutrons and
  electrons and is called the atomic mass
• Electrons are very tiny and dont have much
  effect on the total mass
• We define a new unit call the atomic mass unit
  which is 1.661 x 10-27 kg

19
Atomic Mass

• When you look in a periodic table, the masses are
  given in atomic mass units (amu)
• What you see in such a table is an average mass
  based on the relative abundance of the different
  isotopes of an element found on earth
• Chlorine is about 3/4 Cl35 and 1/4 Cl37

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Symbols for Isotopes

• We will write the symbol for an isotope in an
  easy-to-remember form

Mass Number
Atomic Symbol
Atomic Number
Neutron number is mass number minus atomic number