Na+ or Ca2+ . Which ion has the larger radius?

Which ion has the larger radius? Some sites say $r(\ce{Na+}) > r(\ce{Ca^2+})$. Any explanation would be appreciated. 




The thing is that you need to know the coordination environment in the first place as ionic radii are C.N.-dependent. From Shannon's canonical paper "*Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides*"[[1]](http://scripts.iucr.org/cgi-bin/paper?S0567739476001551): 


\begin{array}{cc}
\hline
\text{Ion} & \text{C.N.} & \text{C.R., Å} & \text{I.R., Å} \\
\hline
\ce{Na+} & 4 & 1.13 & 0.99 \\
& 5 & 1.14 & 1.00 \\
& 6 & 1.16 & 1.02 \\
& 7 & 1.26 & 1.12 \\
& 8 & 1.32 & 1.18 \\
& 9 & 1.38 & 1.24 \\
& 12 & 1.53 & 1.39 \\
\hline
\ce{Ca^2+} & 6 & 1.14 & 1.00 \\
& 7 & 1.20 & 1.06 \\
& 8 & 1.26 & 1.12 \\
& 9 & 1.32 & 1.18 \\
& 10 & 1.37 & 1.23 \\
& 12 & 1.48 & 1.34 \\
\hline
\end{array}


where C.N. - coordination number; C.R. - crystal radius; I.R. - ionic radius. The data is also [available online for free](http://abulafia.mt.ic.ac.uk/shannon/ptable.php). Comparing similar coordination environments, it's easier to notice that overall $\ce{Na+}$ is indeed larger than $\ce{Ca^2+}$:


[![enter image description here](https://i.stack.imgur.com/7YwiO.png)](https://i.stack.imgur.com/7YwiO.png)


### Reference


1. Shannon, R. D. Acta Cryst A, 1976, 32 (5), 751–767 DOI: [10.1107/S0567739476001551](http://scripts.iucr.org/cgi-bin/paper?S0567739476001551).


 15

According to the diagonal relation, $\ce{Na}$ and $\ce{Ca}$ have similar radii.


$\ce{Na+}$ and $\ce{Ca+}$ have thus similar radii as well but, when one more electron is removed from $\ce{Ca+}$, making it $\ce{Ca^{2+}}$, its effective nuclear charge increases, which will make its radius *decrease*.


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