Structure of the Atom

1
Structure of the Atom

• CHEM

2
Basic Parts of the Atom

• nucleus the positively charged center of the
  atom
• protons positive charged particle with a
  relative mass of 1
• neutrons neutrally charged particle in the
  nucleus with a relative mass of 1
• electron cloud negatively charged, mostly empty
  space
• electrons negatively charged particles with a
  relative mass of 1 / 1840 of a hydrogen atom

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Atomic Structure
4
Forces in Atoms

• Electromagnetic Force
• Like charges repel, opposite charges attract.
• Strong Force
• Opposes the electromagnetic force between protons
  in the nucleus.
• The strongest of the four forces, but only acts
  over short distances.
• Weak Force
• The force responsible for radioactive decay.
• Gravity
• The force of attraction between all objects in
  the universe.
• The weakest of the four forces.

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Atomic Number

• The atomic number is equal to the number of
  protons in the nucleus of an atom
• This number is expressed as a whole number on the
  periodic table.
• Since and - charges are equal in atoms, it is
  also the number of electrons

6
Mass Number

• The mass number is equal to the sum of the
  protons and neutrons in the nucleus of an atom.
• This number is expressed as a
• Number of neutrons in a atom (mass number, as a
  whole number) - (atomic number)
• How many neutrons are in an atom of zinc? . . . .
  65 - 30 35 neutrons in Zn

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Atomic Number and Mass Number
8
Isotopes

• Atoms which have the same number of protons with
  different amounts of neutrons.
• Carbon-12 has 6 neutrons, while Carbon-14 has 8
  neutrons.
• The mass number for an element on the periodic
  table represents the average for all the isotopes
  in a sample of the element.

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Hydrogen has 3 common isotopes
1 2 3

• Carbon also has 3 common isotopes
• nuclide name
• 6p, 6n mass 12 carbon - 12
• 6p, 7n mass 13 carbon - 13
• 6p, 8n mass 14 carbon - 14

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Isotopes
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Atomic Mass

• The average mass of the mixture of an elements
  isotopes
• Atomic mass is measured in Atomic Mass Units
  where 1amu (1/12) mass of carbon measured in
  grams.
• In order to find this average you need to know
• number of stable isotopes of the atom
• mass of isotopes
• percent abundance in nature of isotopes

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Atomic Mass

• Example A sample of cesium is 75133Cs, 20
  132Cs, and 5 134Cs.
• What is its average atomic mass?
• Answer .75 x 133 99.75
• .20 x 132 26.4
• .05 x 134 6.7
• Total 132.85 amu