The reactivity of an atom arises from:

• A. the total number of neutrons in the atom
• B. the presence of unpaired electrons in its valence shell
• C. the mass of the atom
• D. the number of protons in its nucleus

Answer: (B)

Verified step by step guidance

1. Understand that the reactivity of an atom is primarily determined by its electrons, especially those in the outermost shell, called the valence shell.
2. Recall that neutrons and protons are located in the nucleus and mainly affect the atom's mass and identity, but do not directly influence chemical reactivity.
3. Focus on the concept of unpaired electrons: these are electrons in the valence shell that are not paired with another electron in the same orbital.
4. Recognize that unpaired electrons tend to be more reactive because they seek to pair up by forming chemical bonds with other atoms.
5. Conclude that the presence of unpaired electrons in the valence shell is the key factor that determines an atom's chemical reactivity.