Question

An element has atomic number 17 and mass number 37. There are 17 electrons outside the nucleus. Find the number of neutrons in it.
a.) 20
b.) 21
c.) 17
d.) 37

Answer 1

Hint: Atomic number indicates numbers of protons present in the nucleus or number of electrons outside the nucleus. Mass number indicates total number of protons and neutrons present in the nucleus. Number of neutrons can be calculated by taking the difference between mass number and atomic number.

Complete step by step answer:
-Atomic number indicates number of protons present in the nucleus.
-Atomic number also indicates the number of electrons revolving outside the nucleus.
-In a neutral atom, the number of protons are equal to the number of electrons.
-Mass number represents total number of protons and neutrons present in the nucleus.
-Mass number can be obtained by taking the sum of the number of protons and neutrons present in the nucleus.
As given in data, Atomic number of elements is 17 which indicates the number of protons present in the nucleus is 17.
Also, the number of protons is equal to the number of electrons.
Mass number is 37 which indicates the sum of the number of protons and neutrons in the nucleus is 37.
So, Mass number=number of protons + number of neutrons
          37 = 17 + number of neutrons
          Number of neutrons = 20
So, the correct answer is “Option A”.

Note: Mass number indicates the sum of protons and neutrons present in the nucleus which is also called nucleons. Atomic number indicates number of electrons or number of protons. So, the number of neutrons can be calculated by taking the difference of mass number and atomic number.