CBSE Class 9 Answered

The atomic number of Chlorine is 17 and mass number is 35. (a) What would be the electronic configuration of negatively charged chlorine ion, Cl^-? (b) What would be the atomic number and mass number of Cl^-? (c) Define valency and calculate the valency of Cl^-. OR (a) Consider the atom $begin mathsize 12px style straight B presubscript 5 presuperscript 10 end style$ . (i) What does the figures 5 and 10 indicate? (ii) How many number of electrons does this element contain? (iii) The relative atomic mass of Boron is 10.8 u. Calculate the percentage of its isotopes $begin mathsize 12px style straight B presubscript 5 presuperscript 10 end style$ and $begin mathsize 12px style straight B presubscript 5 presuperscript 11 end style$ , occurring in nature.

Asked by Topperlearning User | 04 Apr, 2016, 10:37: AM

Expert Answer

(a) Chlorine atom (Cl) has atomic number 17. It contains 17 protons and 17 electrons.

Chlorine ion (Cl^-) is formed when Cl gains one electron.

So, Cl^- has 18 electrons and 17 protons.

Therefore, the electronic configuration of Cl^- = 2, 8, 8.

(b) Atomic number of Cl^- = Number of protons = 17

Mass number of Cl^- will be the same as Cl i.e. 35.

For a non-metallic element, it is equal to eight minus the number of electrons present in outermost shell.

Here, Cl^- has 8 electrons in the outermost shell, therefore valency of Cl^- = 8 - 8 = 0

(a) (i) 5 is the atomic number.

10 is the atomic mass number.

(ii) Number of neutrons = Mass number - Atomic number

= 10 - 5

= 5

(b) Let the percentage of $begin mathsize 12px style straight B presubscript 5 presuperscript 10 end style$ be y in the sample.

Let the percentage of $begin mathsize 12px style straight B presubscript 5 presuperscript 11 end style$ will be (100 - y)

$begin mathsize 12px style Average space mass space equals space left parenthesis 10 space straight x space straight y over 100 space right parenthesis space plus space left square bracket 11 space straight x space fraction numerator open parentheses 100 space minus space straight y close parentheses over denominator 100 end fraction right square bracket 10.8 space space space equals space space 10 straight y space plus space space left square bracket 11 space straight x space fraction numerator open parentheses 100 space minus space straight y close parentheses over denominator 100 end fraction right square bracket 1080 space space equals space space 10 straight y space plus space 1100 space minus space 11 straight y space space space space space space space space space space space space space space space space space space space space space space space space space straight y space space equals space 20 end style$