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Science ·Class 9 Science

An atom 70X has 31 electrons. How many neutrons are there in its nucleus? Class 9

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· Jul 06, 2026 · Reviewed & updated Sep 17, 2026 · 1 min read

An atom 70X has 31 electrons. How many neutrons are there in its nucleus? Class 9


Question 1.

An atom 70X has 31 electrons. How many neutrons are there in its nucleus? Class 9

Answer:

  1. Mass number (A) = 70 (given as superscript, so the atom is 70X).
  2. Number of electrons = 31.
  3. Since the atom is neutral, number of protons = number of electrons = 31. So atomic number (Z) = 31.
  4. Number of neutrons = Mass number - Atomic number = 70 - 31 = 39.

Answer: There are 39 neutrons in its nucleus. (This element is Gallium, Ga.)


Question 2.

An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons. Class 9

Answer:

  1. Number of protons (atomic number Z) = 79.
  2. Mass number (A) = 197.

(i) Number of neutrons = Mass number - Number of protons = 197 - 79 = 118.

(ii) Since the atom is electrically neutral, number of electrons = number of protons = 79.

Answer: (i) Number of neutrons =118. (ii) Number of electrons = 79. (This element is Gold, Au.)