what discovery led to the abandonment of the atomic mass as the determining factor for organizing elements in the periodic table? A. isotopes B. Positive nuclear charge C. Ions. D. Quantum mechanics

A. isotopes

Explanation:

The periodic table was organized by Dimitry Mendeleiev by 1869, as he showed the connection between the atomic mass and the properties of the elements.

Nevertheless, the order of some few elements had to be altered because the properties did not follow the order of the atomic masses.

Henry Mosely by 1913 determined that atoms of a same element had different number of neutrons, leading to different atomic masses. These atoms of a same element (with the same number of protons) with different number of neutrons are known as isotopes.

By arranging the elements in increasing order of the number or protons (atomic number), instead of the atomic masses, all the elements resulted ordered according with a repetitive (periodic) pattern of chemical properties.