Question

Which element has the equal number of electrons as ${Ca}^{2+}$?

• A. $Ar$
• B. $O$
• C. $S$
• D. $Ti$
• E. $U$

Answer 1

The correct option is A $Ar$

$Ca$ atom has $20$ electrons but after losing two electrons it gets $+2$ charge and forms cation with 18 electrons. Argon has atomic number 18 with $18$ electrons. So both calcium cation and $Ar$ atom have same number of electrons. Such species having same number of electrons are called isoelectronic species.