Question

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? atomic mass is the average mass of different samples of the same element; different elements can have similar chemical properties even if their atomic masses are different; the number of protons in an atom is different than the atom’s total mass; protons strongly affect an element’s atomic mass, while the electrons in atoms do not.

Explanation:

To determine the fact discovered between Mendeleev's and Moseley's time that helped Moseley, we analyze each option:

• Option 1: Atomic mass being the average of different samples of the same element was known earlier and doesn't directly relate to Moseley's work on the periodic table based on atomic number.
• Option 2: Mendeleev already arranged elements by atomic mass and found anomalies where elements with similar properties had different atomic masses (like iodine and tellurium), but this was known during Mendeleev's time, not between the two periods.
• Option 3: The number of protons (atomic number) being different for each atom is the key. Moseley discovered that elements should be arranged by atomic number (number of protons) rather than atomic mass. This was a new discovery between Mendeleev's (arranged by atomic mass) and Moseley's time, and it helped Moseley develop his version of the periodic table.
• Option 4: Protons affecting atomic mass and electrons not is not the key discovery that helped Moseley; the focus was on the number of protons (atomic number) for arrangement.

Answer:

C. The number of protons in an atom is different than the atom’s total mass. (Note: The option description might have a slight wording issue, but the core idea is that the number of protons (atomic number) is distinct and is the basis for Moseley's periodic table, which was discovered between Mendeleev's and Moseley's time.)