Question
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The order of size is:
A.\[{S^{2 - }}\]>\[C{l^ - }\]>\[{O^{2 - }}\]>\[{F^ - }\]
B.\[C{l^ - }\]>\[{S^{2 - }}\]>\[{O^{2 - }}\]>\[{F^ - }\]
C.\[{S^{2 - }}\]>\[{O^{2 - }}\]>\[C{l^ - }\]>\[{F^ - }\]
D.\[{S^{2 - }}\]>\[{O^{2 - }}\]>\[{F^ - }\]>\[C{l^ - }\]

Answer
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Hint:
The order of the size of the given ions is an iteration of the period patterns that can be observed across the periodic table. These periodic trends arise from the electronic configurations of the elements associated that are parent to these ions.

Complete step by step answer:
Within the same group in a periodic table, the number of valence electrons in the elements remain constant. Hence, as we move down a group, the size of the atoms goes on increasing because of the new electron shells that get added as we move down the group.
On the other hand, as we move across a period, the size of the atoms goes on decreasing. This is because within the same period, the base electronic configuration of all the atoms remains constant. It is just the valence electron shell, in which the number of electrons keeps on increasing across the period. Due to this, the net nuclear force of attraction on the valence shell keeps on increasing as we move from left to right in a period.
The ions given to us are \[{S^{2 - }}\],\[C{l^ - }\],\[{O^{2 - }}\] and \[{F^ - }\]
Out of this, \[{S^{2 - }}\]and \[C{l^ - }\]are in the same period and the atomic number of sulphur is 16 and that of chlorine is 17 Hence the size of\[{S^{2 - }}\]is greater than\[C{l^ - }\].
Also\[{O^{2 - }}\] and \[{F^ - }\]are in the same period and the atomic number of oxygen is 8 and that of fluorine is 9 Hence the size of \[{F^ - }\]is greater than \[{O^{2 - }}\]
Also, \[{O^{2 - }}\] and \[{F^ - }\] belong to period 2 while \[{S^{2 - }}\]and \[C{l^ - }\]belong to period 3. Hence the relation between all their atomic sizes can be determined as:
\[{S^{2 - }}\]>\[C{l^ - }\]>\[{O^{2 - }}\]>\[{F^ - }\]

Hence, Option A is the correct option.

Note:
Cations are the ions which lose electrons and hence are smaller than the parent ion. On the other hand, anions accept electrons and hence their sizes are larger than their parent ion.