Which part of the atom has to stay the same for it to be the same element?

a.) number of neutrons
b.) number of protons
c.) number of electrons
d.) number of protons, neutrons, and electrons

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Suppose that three astronomical objects, 1, 2, and 3 are observed to lie on a line, and the distance from object 1 to object 3 is d. object 1 has four times the mass of object 3, and seven times the mass of object 2. find the distance between objects 1 and 2 for which the net force on object 2 is zero.