Question

1. the basis for ordering elements in the modern periodic table is increasing

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• atomic mass, as originally used by mendeleev when arranging elements into his periodic table.
• number of neutrons, since isotopes of an element differ in neutron count.
• average atomic mass, which accounts for isotopic abundance but does not define periodic placement.
• atomic number, which reflects the number of protons and defines each element.

Explanation:

The modern periodic table arranges elements by increasing atomic number, which is the number of protons in an element's nucleus. This number defines each element uniquely. Average atomic mass is affected by isotopes and was the basis in Mendeleev's original table, but not for the modern one. Neutron count varies among isotopes and doesn't define periodic placement.

Answer:

atomic number, which reflects the number of protons for each element.