Atomic number is used to identify what element an atom belong to

The atomic number is the number of protons in the nucleus

The mass number of an atom is the number of protons + neutrons

If atoms have the same atomic number but different mass number they are...

Isotopes were first suggested by Frederick Shoddy in 1913

The periodic table is a list of all known elements ordered by atomic number

The modern periodic table contains 118 elements