Counting Atoms

1
Counting Atoms
2
Atomic Number

• The atomic number of an element is the number of
  protons
• Atomic number determines the identity of the
  element.

3
Isotopes

• Isotopes are atoms of the same element that have
  a different number of neutrons, and therefore
  different masses.

4
Isotopes of Hydrogen

• Protium
• 0 neutrons
• Deuterium
• 1 neutron
• Tritium
• 2 neutrons

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Mass Number

• The mass number is the total number of protons
  and neutrons in the nucleus.

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Calculating Neutrons

• Mass number atomic number of neutrons
• A nuclide is the term that refers to a particular
  isotope.

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Designating Isotopes

• Hyphen notation
• Element name mass number
• EX Carbon-14
• Nuclear symbols
• element symbol with superscripted mass number and
  subscripted atomic number to the left.

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Relative Atomic Mass

• Masses of atoms are very small, so a relative
  scale of mass is used.
• The standard unit is the atomic mass unit, or amu
• One amu is equal to the mass of 1/12th the mass
  of a carbon-12 atom.
• Protons and neutrons have masses approximately
  equal to 1 amu

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Average Atomic Mass

• Is the weighted average of the atomic masses of
  the naturally occurring isotopes of an element.
• This is the value commonly listed on Periodic
  tables.

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Calculating Average Atomic Mass

• Copper has 2 natural isotopes, Copper-63 (69.17)
  and Copper-65 (30.83)
• Take the abundance() x mass for the first
  isotope abundance() x mass for the second
  isotope.
• 69.17(62.929599) 30.83(64.927793) 63.546 amu

11
The Mole

• A mole (mol) is the amount of a substance that
  has as many particles as there are atoms in
  exactly 12 g of Carbon-12.
• 1 mole 6.02 x 1023 (Avogadros number)

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Molar Mass

• The mass of one mole of a pure substance.
• Units are g/mol
• Use the average atomic mass from the periodic
  table.
• Use molar mass to convert grams to moles, and
  vice versa.

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Atomic Mass vs. Molar Mass

• Atomic mass is the mass, in a.m.u, of 1 atom or
  molecule.
• Molar mass is the mass, in grams, of 1 mole of
  atoms or molecules.
• EX Oxygen gas (O2)
• 1 molecule 32.0 a.m.u.
• 6.02 x 1023 molecules 32.0 grams

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Converting Grams Moles

• The molar mass of the element or compound is used
  to make this conversion.
• 35 g O2 1 mol O2
• 32.0 g O2

1.09 mol O2
Conversion Factor
15
Converting Moles Atoms/Molecules

• Avogadros number is used for this conversion.
• How many molecules are in 2.30 moles of Nitrogen
  gas?
• 2.30 moles N2 6.02 x 1023 molecules
• 1 mole

1.38 x 1024 Molecules N2
Conversion Factor