The chemical characteristics or reactivity of an element depend mostly on the:

• A. mass number of the atom
• B. number of protons plus neutrons
• C. total number of neutrons in the nucleus
• D. number of electrons in its outermost shell

Answer: (D)

Verified step by step guidance

1. Understand that the chemical characteristics or reactivity of an element are primarily determined by how it interacts with other atoms, which depends on the electrons involved in bonding.
2. Recall that the mass number of an atom is the sum of protons and neutrons, which affects the atom's mass but not its chemical behavior directly.
3. Recognize that the total number of neutrons influences the isotope of the element but does not significantly affect chemical reactivity.
4. Focus on the electrons, especially those in the outermost shell (valence electrons), because these electrons participate in chemical bonds and reactions.
5. Conclude that the number of electrons in the outermost shell determines the element's chemical properties and reactivity.