Exercise 9.7
Which of the following, in Fig. , correctly represents Cl number of chlorine = ). (i)
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
NCERT’s answer
(ii)
(ii) is the correct representation of the \(\displaystyle Cl^{-}\) ion.
Chlorine has atomic number $\displaystyle 17$, so a neutral chlorine atom has $\displaystyle 17$ protons and $\displaystyle 17$ electrons, filling as $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 7$.
A \(\displaystyle Cl^{-}\) ion is chlorine that has gained one electron, so it still has $\displaystyle 17$ protons but now $\displaystyle 18$ electrons: \(\displaystyle 2 + 8 + 8 = 18\), i.e. the shells fill as $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 8$.
Counting the electrons drawn on each of the four diagrams in Fig. $\displaystyle 9.18$:
(i) $\displaystyle 2$, $\displaystyle 7$, $\displaystyle 8$ — seventeen electrons in total, which is the right number for a neutral atom, but the shells are filled out of order: the second shell holds up to eight and must be completed before the third begins.
(ii) $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 8$ — eighteen electrons, one more than the seventeen protons, which is exactly a chloride ion. This is the answer.
(iii) $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 9$ — nineteen electrons, two more than the protons; the outermost shell also exceeds the eight that this shell can hold.
(iv) $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 7$ — the neutral chlorine atom, not the ion, and the commonest wrong choice here.
The distinction being tested is that gaining an electron changes the electron count and the outermost shell, never the number of protons — the nucleus is untouched, which is why the ion is still chlorine.