Unit 2 Quiz 1 Review - Atomic theory & Structure

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• Also called the Planetary Model

• Discovered by Niels Bohr

• Describes electrons existing in different levels or Shells around the nucleus of an atom​

  • Each Level holds a different number of electrons

  • Properties depend on where electrons are​

​The Bohr Model of the atom

• The dense central core of an atom

  • Contains both protons and neutrons

  • Neutrons stop protons from repelling each other

• Contains most of the mass of the atom​

  • Mass is the sum of protons and neutrons​

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​The Nucleus

• We distinguish between different elements based on their atomic number

  • The number of protons they have

• The Mass number is the number of neutrons + number of protons in an atom

  • Mass No. is measured in atomic mass units (amu)​

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​Atomic number and mass number

​All carbon atoms have 6 protons in the nucleus, but there may be 6, 7, or 8 neutrons. These are called isotopes of carbon. There are isotopes of every element!

An atom's mass is its total of protons and neutrons . The electron cloud mass is small enough to ignore.

Carbon-12, carbon-13, & carbon-14 are carbon's isotopes. 12, 13, and 14 are their masses.

• charged atoms formed by an atom gaining or losing electrons

Isotope Abundance and Atomic Mass

• In nature, most elements occur as a combination of two or more isotopes

  • Each isotope has a natural percent abundance​

• The average atomic mass is the weighted average of the masses of its isotopes

  • a weighted average reflects the relative abundance of each isotope found in nature

• Calculated Atomic Mass: The more common an isotope is, the more important it is in atomic mass calculations

  • Step 1: ​multiply the mass of each isotope by its natural abundance

  • Step 2: add the products of all the weighted isotopes masses

Chem 1 | Unit 1 Lessons 2 & 3

Atomic Mass Estimations Practice Problem 2

• Consider a hypothetical element, X, that has three isotopes. Calculate the weighted- average atomic mass of the element from the following data:

  • ​Step 1: Multiply each atomic mass by it's abundance

    • 85.32 x 0.10

    • 87.51 x 0.70

    • 88.10 x 0.20

  • Add the sum of the relative masses together

Chem 1 | Unit 1 Lessons 2 & 3