Which is the best predictor of the radioactive nature of an isotope?

The neutron-to-proton ratio of the atom.

Explanation:

An isotope is an atoms of elements with the same number of protons, but different number of neutrons. Thus, the different values of their mass number.

The neutron-proton ratio could make an isotope either unstable, thus predicting the radioactive nature of the isotope.

a.) travels faster in solids because the particles are closer together.

air is a gas water is a liquid

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