Atomic Number, Mass Number, Atomic Mass and Isotopes

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Atomic Number, Mass Number, Atomic Mass and
Isotopes

• Mrs. Coyle
• Chemistry

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• Atomic Number (Z) is the number of protons
• in the nucleus of the atom.
• Zp

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• The number of protons (atomic number) determine
  the identity of an element.

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• Atoms have no overall electrical charge so,
• an atom must have as many
• electrons
• as there are
• protons
• in its nucleus.

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• The atomic number of an element also equals
• the number of electrons
• in a neutral atom of that element.

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Ex Sodium

• What is the atomic number of Sodium?
• How many protons does sodium have?
• How many electrons does sodium have?

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Mass Number (A)

• The sum of the protons and neutrons in the
  nucleus.
• Ap n

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Notation

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Other ways to write elements

• 36
• 17Cl Cl-36

Mass Number
Mass Number
Atomic Number
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Nucleons

• protons and neutrons

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Isotopes

• of an element have
• different mass numbers
• because
• they have
• different numbers of neutrons,
• but they have
• the same atomic number.

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ExampleIsotopes of Carbon and Hydrogen

• protium deuterium tritium
• H H H

Isotopes of Hydrogen
Isotopes of Carbon
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Atomic Mass Unit

• is a unit used to compare
• the masses of atoms
• and has the symbol
• u
• or
• amu.

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1 amu or u

• is approximately equal to
• the mass of
• a single
• proton or neutron.

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Carbon-12

• Chemists have defined
• the carbon-12 atom
• as having a mass of
• 12 atomic mass units.

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• 1 u 1/12 the mass of a Carbon-12 atom.

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Atomic Mass

• is the weighted average
• mass
• of all the naturally occurring
• isotopes
• of that element.

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