3-3 Modern Atomic Theory

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3-3 Modern Atomic Theory
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The Structure of the Atom

• The atom has a positively changed central core
• Contains Protons and Neutrons
• Protons are positive, equal and opposite to
  electrons
• Neutrons do not carry a charge and are slightly
  more massive

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• 1 proton has the mass of about 2000 electrons

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• Electrons move in space around the nucleus
• Rutherford visualized it as a mini solar system.

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Atomic Numbers

• Henry Moseley found that atoms contain unique
  positive charge in their nucleus.
• The number of protons is called the atomic number.

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• The atomic number indicates protons.
• Chlorine has 17 protons atomic number

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Practice

• How many protons and electrons are in a magnesium
  atom?
• What is the name of the element that has atoms
  that contain 11 protons.

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Ions

• When an atom gains or loses electrons it acquires
  a charge
• Fewer electrons means positive charge
• More electrons means negative charge
• Charge of ion protons - electrons

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Sample

• Write the chemical symbol for the ion with 9
  protons and 10 electrons
• Answer F-
• What is the symbol of the ion with 13 protons and
  10 electrons?
• Answer Al3
• 7 Protons and 10 electrons?
• N 3-

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Isotopes

• Dalton said all atoms of an element are the same.
• Not quite true, ISOTOPES have a different number
  of neutrons

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• In nature, elements are almost always found as a
  mixture of isotopes
• Isotopes are usually in the same percentages.

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• To identify isotopes more specifically
• Use the Mass Number
• Mass Number ( protons) ( neutrons)

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• To identify an isotope chemists write the mass
  number behind the element symbol for example
  Cl-37 indicates that this chlorine has 20
  neutrons, it is written in symbol form as 3717Cl
• Cl-35 has 18 neutrons and is written as 3517Cl

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The Mass of an Atom

• Measured by Atomic Mass Units (AMU)
• The atomic mass is approximately the same as the
  sum of protons and neutrons
• This is not very precise so Scientists define it
  more precisely.

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• 1 amu is equal to 1/12 the mass of a carbon-12
  atom.
• 1.66 x 10 24 grams
• Carbon 12 is the only element with an AMU equal
  to protons and neutrons, because of isotopes

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• The average mass of an elements atoms is called
  the atomic mass.
• AM (mass isotope x abundance)(mass isotope x
  abundance)...

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Fundamental Subatomic Particles
Particle Location Charge (C) Mass (g) Mass (AMU)
Proton Inside nucleus 1.602 x 10-19 1.673 x 10-24 1
Neutron Inside nucleus 0 1.675 x 10-24 1
Electron Outside nucleus -1.602 x 10-19 9.109 x 10 28 0