Isotopes are atoms that have the same ________ but differing ________.

• A. number of neutrons; number of protons
• B. number of protons; number of neutrons
• C. atomic mass; atomic number
• D. number of electrons; number of protons

Answer: (B)

Verified step by step guidance

1. Recall the definition of isotopes: isotopes are atoms of the same element that have the same number of protons but differ in the number of neutrons.
2. Understand that the number of protons defines the element (atomic number), so isotopes must have the same atomic number.
3. Recognize that the difference in neutrons causes isotopes to have different atomic masses.
4. Therefore, the correct completion of the sentence is: 'Isotopes are atoms that have the same number of protons but differing number of neutrons.'
5. Review the options and identify that the choice 'number of protons; number of neutrons' correctly fills the blanks.