NEET Chemistry Classification of Elements and Periodicity in Properties / तत्त्वों का वर्गीकरण एवं गुणों में आवर्ति NEET PYQ-Classification Elements In Properties

The species \[Ar,{{K}^{+}}\] and \[C{{a}^{2+}}\] contain the same number of electrons. In which order do their radii increase?                                                                                                                        [NEET 2015 ]

A) \[Ar+{{K}^{+}}<C{{a}^{2+}}\]

B)    \[C{{a}^{2+}}<Ar<{{K}^{+}}\]

C) \[C{{a}^{2+}}<{{K}^{+}}<Ar\]

D)    \[{{K}^{+}}<Ar<C{{a}^{2+}}\]

Correct Answer: C

Solution :
\[C{{a}^{2+}}\,<{{K}^{+}}\,<Ar\]

\[Ar,{{K}^{+}}\] and \[C{{a}^{2+}}\] are isoelectronic i.e. with same number of electrons, 18. For isoelectronic species ionic radii decreases with increase in effective (relative) positive charge. Also Ar, K and Ca belong to the same period (3rd period).