Atoms and their structure

1
Atoms and their structure
2
The Modern Model

• The modern atom is composed of two regions
• 1. Electron cloud- Most of the volume of an
  atom and is the region where the electron can be
  found (extranuclear)
• 2. The nucleus with protons and neutrons so
  that it has a Positive charge and almost all the
  mass

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Major Subatomic Particles
Name Symbol Charge Relative Mass (amu) Actual Mass (g)
Electron e- -1 1/1840 9.11x10-28
Proton p 1 1 1.67x10-24
Neutron no 0 1 1.67x10-24

• Atoms are measured in picometers, 10-12 meters
• Hydrogen atom, 32 pm radius
• Nucleus is tiny compared to atom
• If the atom were a stadium, the nucleus would be
  a marble
• Radius of the nucleus is on the order of 10-15 m
• Density within the atom is near 1014 g/cm3

4
Nuclear Symbols

• Every element is given a corresponding symbol
  which is composed of 1 or 2 letters (first letter
  upper case, second lower), as well as the mass
  number and atomic number

5
Locations on periodic table

• Atomic Number number of protons (p) in the
  nucleus
• Determines the type of atom
• Li atoms always have 3 protons in the nucleus,
  Hg always 80
• Mass Number number of protons neutrons
• Electrons have a negligible contribution to
  overall mass
• In a neutral atom there is the same number of
  electrons (e-) and protons (atomic number)

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• Find the
• number of protons
• number of neutrons
• number of electrons
• atomic number
• mass number

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• If an element has an atomic number of 34 and a
  mass number of 78 what is the
• number of protons in the atom?
• number of neutrons in the atom?
• number of electrons in the atom?
• complete symbol of the atom?
• If an element has 91 protons and 140 neutrons
  what is the
• atomic number?
• mass number?
• number of electrons?
• complete symbol?

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Isotopes

• Atoms of the same element can have different
  numbers of neutrons and therefore have different
  mass numbers
• The atoms of the same element that differ in the
  number of neutrons are called isotopes of that
  element

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Naming Isotopes

• When naming, write the mass number after the name
  of the element
• Examples
• carbon- 12
• carbon -14
• uranium-235

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Average Atomic Mass

• Did you ever notice that the mass number on the
  periodic table is not a whole number?
• There are the decimal numbers on the periodic
  table.
• This is because it is an average mass for all
  the isotopes of each atom

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How heavy is an atom of oxygen?

• There are different kinds of oxygen atoms
  (different isotopes)
• 16O, 17O, 18O
• We are more concerned with average atomic
  masses, rather than exact ones
• Based on abundance of each isotope found in
  nature
• We cant use grams as the unit of measure
  because the numbers would be too small
• Instead we use Atomic Mass Units (amu)
• Standard amu is 1/12 the mass of a carbon-12
  atom
• Each isotope has its own atomic mass

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Calculating Averages

• You have five rocks, four with a mass of 50 g,
  and one with a mass of 60 g. What is the average
  mass of the rocks?
• Total mass 4 x 50 g 1 x 60 g 260 g
• Average mass (4 x 50 g 1 x 60 g)/5 (260
  g)/5
• 52 g

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Calculating Averages when given
80 of the rocks were 50 grams 20 of the rocks
were 60 grams What is the average mass of the
rocks? Average mass 0.8 x 50 0.2 x 60
40 12 52 g Average ( as decimal) x
(mass1) ( as decimal) x (mass2)
( as decimal) x (mass3)
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EXAMPLES

• Calculate the av atomic mass of copper if copper
  has two isotopes
• 69.1 has a mass of 62.93 amu
• The rest (30.9) has a mass of 64.93 amu
• Magnesium has three isotopes
• 78.99 magnesium 24 with a mass of 23.9850 amu
• 10.00 magnesium 25 with a mass of 24.9858 amu
• The rest magnesium 26 with a mass of 25.9826 amu
• What is the atomic mass of magnesium?

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Average Atomic Masses

• If not told otherwise, the mass of the isotope is
  the mass number in amu
• The average atomic masses are not whole numbers
  because they are an average mass value
• Remember, the atomic masses the decimal numbers
  on the periodic table

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To do ?

• Page 45
• Questions 1, 2, 3, 4
• Page 46
• Questions 2, 3, 4, 5a