What causes some atoms of the same element to have different atomic masses?

Isotopes are atoms of the same element that have different numbers of neutrons but the same number of protons and electrons. The difference in the number of neutrons between the various isotopes of an element means that the various isotopes have different masses.

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answer:

technetium is the most reactive. this is followed by silver. the least reactive out of these three elements is strontium.