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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. Z=#p
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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. A=#p + #n
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Notation
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Other ways to write elements: 36 17 ClCl-36 Atomic Number Mass 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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Example: Isotopes of Carbon and Hydrogen protium deuteriumtritium HH H Isotopes of Carbon Isotopes of Hydrogen
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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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