• A = mass number : # of protons + # of neutrons
• Z = atomic number : # of protons
• X = atomic symbol
Which pair of atoms constitutes a pair of isotopes of the same element?
A) 158X 157X
B) 136X 126X
C) 189X 178X
D) 1910X 199X
E) 2010X 2111X
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