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?

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

Explanation:

Mendeleev had presented the first structured periodic table in 1869. But, in 1913, Henry Moseley developed the modern periodic table. The crucial difference between the two periodic tables is that Mendeleev's periodic table is generated on the basis of the atomic masses of chemical elements while the chemical element's atomic numbers were the main focus of Moseley's periodic table. There were just 56 chemical elements in Mendeleev's periodic table. On the other hand, 74 chemical elements are presented in Moseley's periodic table.

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.