They have the same number of protons and electrons, but they (only) differ in the number of neutrons (and so they differ in atomic massaccordingly)
Mass number is a property specific to a particular isotope or nuclide of an element, while the usual periodic table include average properties for all the stable, naturally occurring isotopes of each element.
Isotopes of each element.
The average of all the naturally occurring isotopes of a particular element are an element's atomic Mass.
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the answer is Technetium
Isotopes of one particular element (atom) only differ in mass and stability (decay), not in chemical properties.
There is not an isotope number on the Periodic Table of Elements. There is an atomic number (# of protons) and the average atomic mass. The average atomic mass is the weighted average of all the naturally occurring isotopes for each element. This number is not a whole number, because it is an average of all the isotopes for a particular element.
Isotopes are atoms of same element having different atomic mass, but the same atomic number. Isotopes have not been given separate place in the modern periodic table.
Electrons
The periodic table would be disturbed only if isotopes of a new element are discovered, because a periodic table is based on order of atomic number, not atomic mass. If new isotopes of a previously known element were discovered, the atomic mass shown in the periodic table might be changed, but this is very unlikely because the atomic masses shown in a periodic table are based on the naturally occurring distribution of isotopes, and any newly discovered isotopes would probably occur only in very small fractions of the total.
All isotopes of an element have the same number of protons in the atomic nucleus, which is its atomic number on the periodic table. All isotopes of an element contain different numbers of neutrons in their atomic nuclei, which causes the isotopes of an element to vary in mass number (protons + neutrons).
Isotopes of a chemical element have a similar number of protons but a different number of neutrons.