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1. Atoms of the same element but different by mass (number of neutrons).
Carbon-12, C-12 or 126C.
2. #neutrons = Mass number - Atomic number or also can write in symbols, #n = A - Z
3. Atomic number (Z)
4. The "mass" of the "Element". " " indicates that it does not really exist.
5. For the following #p - number of protons, #e - number of electrons, #n = number of neutrons.
a. Ca - 42, 20p, 20e, 22n
b. C - 13, 6p, 6e, 7n
c. U - 238, 92p, 92e, 146n
6. See Isotope Information Table for actual Atomic masses (last column)