# Atomic Structure

A sample of chlorine is found to have 75% of 17Cl35 and 25% of 17Cl37.
a. Find the average atomic mass of chlorine
b. Give the number of neutrons in both the isotopes.

26
Asmita Chatterjee ·

Both the isotopes of Chlorine in accordance to their relative abundance in nature are found in the form 3;1.

hence,Average atomic mass of Chlorine=

(3x35 + 1x37)/3+1 =35.5.....(a)
Also,Atomic weight=no. of protons + no. of neutrons.

no. of neutrons in Cl-35= 35 - 17
=18.
no. of neutrons in Cl-37= 37 - 17
=20. ......(b)

11

Gven % of Cl-35 is 75%
and that of Cl-37 is 25%
(a) Average atomic mass of chlorine is[((75*35)/100)+((25*37)/100)]
=26.25+9.25=35.50(Ans)
(b) No of neutrons:-
in Cl-35=35-17=18(Ans)
in Cl-37=37-17=20(Ans)
AVERAGE ATOMIC MASS OF CHLORINE IS 35.50g
NUMBER OF NEUTRONS IN Cl-35 IS 18
NUMBER OF NEUTRONS IN Cl-37 IS 20....

283
Sahil Jain ·

1. a.Cl-35 and Cl-37

They are found in the approximate ratio of 3 Cl-35s to 1 Cl-37s.

We multiply each by its % (75% = 0.75 and 25% - 0.25):

0.75 x 35 = 26.25
0.25 x 37 = 9.25

Add them together: 26.25 + 9.25 = 35.5
b. Cl-35(no. of neutrons = 18)
Cl-37(no . of neutrons= 19 )

• Bitan Chakraborty The first answer is correct. But not the second. Hint: Atomic No and Atomic Weight are two different quantities. Identify them and learn their significance.
• Sahil Jain Cl-35: 17 protons, 18 neutrons Cl-37: 17 protons, 20 neutrons
• Bitan Chakraborty Mass No. - Atomic no. = Neutron no. So answer is a. 35.5 b. 18 and 20
• Jagrat Wadhwa U HAVE NOT GIVEN THE UNIT FOR THE ATOMIC MASS.............