Dmitri Mendeleev and Henry Moseley developed their periodic tables about forty years apart. During that time, many discoveries in chemistry were made. Which fact was most likely discovered between the time of Mendeleev’s table and the time of Moseley’s table that helped Moseley develop his version?
Answer
A
The number of protons in an atom is different than the atom’s total mass.
B
Different elements can have similar chemical properties even if their atomic masses are different.
C
Atomic mass is the average mass of different samples of the same element.
D
Protons strongly affect an element’s atomic mass, while the electrons in atoms do not.
Explanation
Mendeleev arranged elements mainly by atomic mass, while Moseley showed that the more useful ordering is by atomic number—the number of protons in an atom. Proton number is a count, not the atom’s total mass, so recognizing that distinction helped Moseley organize the periodic table. Similar chemical properties were already part of the pattern Mendeleev used.
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