What statement about the outermost boundaries of an atom is true?

• A. They are not visible with any microscope
• B. They are the part visible under a microscope
• C. The nucleus is visible
• D. Electrons are concentrated in the center

Answer: (B)

The key idea is what we can actually observe at the atomic scale. The outer boundary of an atom defines its size—the edge where the atom ends and surrounding space begins. Imaging methods can reveal that edge, giving us a view of the atom’s boundary, even though the interior parts aren’t seen as separate features. The nucleus sits at the center and is far too small to resolve with typical microscopes, and electrons form a diffuse cloud around the nucleus rather than a tight central cluster. So the statement about the outer boundary being the part visible under a microscope matches what these tools reveal about the atom, while the other ideas misrepresent how visibility at that scale works.