Concept:The modern periodic table arranges elements in order of their atomic number, not their mass.
Explanation:The atomic number is the number of protons in the nucleus of an atom.
It is denoted by
Z and uniquely identifies each element.
When elements are arranged by increasing
Z, their chemical properties repeat periodically.
Mass number and nucleon number both mean the total of protons and neutrons, so they are not used for the basic arrangement.
Neutron number also varies among isotopes and is not the basis of ordering.
Answer:A. atomic number.