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Atomic Number & Mass Number
- Used to help determine the number of subatomic particles found in an atom.
Atomic Number – order of periodic table
o The atomic number is the same for all atoms of an element
o Because each element has a unique number of protons in its atoms, no two elements
have the same atomic number
o Atomic numbers are always whole numbers.
o The atomic number also reveals the number of electrons in an atom of an element.
Mass Number – sum of protons and neutrons in atom
o You can calculate the number of neutrons in an atom by subtracting the atomic number
(the number of protons) from the mass number (the number of protons and neutrons).
mass number – atomic number = number of neutrons
o Unlike the atomic number, the mass number can vary among atoms of a single element
(CALLED ISOTOPES)
•
Atomic number always appears on the lower left side of the symbol.
•
Mass numbers are written on the upper left side of the symbol & AFTER NAME!
EXAMPLE #1
Name of Element
Symbol
Mass Number
Atomic Number
# of Neutrons
# of Protons
# of Electrons
Avg. Atomic Mass
EXAMPLE #2
Name of Element
Symbol
Mass Number
Atomic Number
# of Neutrons
# of Protons
# of Electrons
Avg. Atomic Mass
3
2
4
Copper – 63
5
Copper – 65