RBSE Guide
Science ·Class 9 Science

An atom has 18 neutrons and an atomic number of 17. What is its mass number? Class 9

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· Jul 04, 2026 · Reviewed & updated Sep 17, 2026 · 1 min read

An atom has 18 neutrons and an atomic number of 17. What is its mass number? Class 9


Question 1.

An atom has 18 neutrons and an atomic number of 17. What is its mass number? (Page 150) Class 9

Answer:

  1. Number of neutrons = 18, Atomic number (protons) =17.
  2. Mass number (A) = number of protons + number of neutrons = 17 + 18 = 35.

Answer: The mass number is 35. (This element is Chlorine, Cl.)


Question 2.

An atom 23A has 11 electrons. Find the number of neutrons in it. (Page 150) Class 9

Answer:

  1. Mass number (A) = 23. number of electrons = 11.
  2. Since the atom is neutral, number of protons = number of electrons =11.
  3. Number of neutrons = mass number - number of protons = 23 - 11 = 12.

Answer: The atom has 12 neutrons. (This element is Sodium, Na.)