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 solve this, we analyze each option:

• Option 1: Atomic mass definition was known earlier, not between their table times.
• Option 2: Similar chemical properties with different atomic masses was part of Mendeleev's work (using atomic mass/chemical properties), not new for Moseley.
• Option 3: Moseley used atomic number (protons), so discovering protons' number vs. mass (atomic number concept) is key. This discovery (protons' role, atomic number) helped Moseley, as he arranged by atomic number (protons), unlike Mendeleev's atomic mass.
• Option 4: Protons do affect atomic mass (along with neutrons), and electrons have negligible mass, but this isn't the key discovery for Moseley's work.

So the correct fact is about protons' number vs. atomic mass, leading to atomic number concept.

Answer:

The number of protons in an atom is different than the atom’s total mass.