An atom of element Be has four protons, four electrons, and five neutrons. Which element could be an isotope of this atom?

Isotopes are atoms of the same element, so they must have the same number of protons, but can have different numbers of neutrons. Beryllium has four protons; the given atom has five neutrons, making its mass number 4 + 5 = 9. Beryllium-10 is still beryllium and therefore could be an isotope, with a different number of neutrons.

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