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-the basic building block of all matter
- made up of protons, neutrons, and electrons.


substance made entirely of one type of atom


a group of two or more atoms that are joined together.


Special type of molecule
- Made up of two or more elements.

atomic number

tells you how many protons and electrons are in an atom

atomic weight

-the number of protons + the number of neutrons


Found inside the nucleus of an atom.
# of neutrons = (atomic number) - (number of protons)

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