How many neutrons are present in atom with mass number and atomic number 37 and 17 respectively?

Step 1: Analyze the question You're given the mass number (37) and the atomic number (17), and you need the number of neutrons.

Step 2: The concept

• Atomic number (Z) = number of protons (and electrons in a neutral atom)
• Mass number (A) = protons + neutrons

So: Neutrons = Mass number − Atomic number

Step 3: Solution Neutrons = A − Z Neutrons = 37 − 17 Neutrons = 20

(This atom is chlorine-37, an isotope of chlorine. It has 17 protons, 17 electrons, and 20 neutrons.)

Step 4: Answer The correct option is B. 20.

Why the other options are wrong:

• A (18): This is the number of neutrons in chlorine-35 (35 − 17), a different isotope.
• C (37): This is the mass number, which counts protons and neutrons together.
• D (17): This is the number of protons, not neutrons.

Exam tip: Remember "N = A − Z", or think "Neutrons are the Number left over after you remove the protons." Always check which isotope the question gives, because chlorine-35 and chlorine-37 give different answers.