Ionic Radius

[Stevin1H]

Why is it that a Ca^2+ ion is smaller than a Na+ ion?

[Patrick Cai 1L]

The Z-effective of Ca 2+ is greater than that of Na+, which is to say the electrostatic attraction between the remaining electrons of Ca 2+ and protons of the nucleus eclipses the electrostatic interaction measured in Na+.

[Dayna Pham 1I]

Hi, Stevin!

I recall someone said that this is because of the proton to electron ratio. I don't understand it in full, but this is along the lines of what I remember...

Ca2+ has 22 electrons and 20 protons, while Na+ has 10 electrons and 11 protons.

I guess since calcium as a cation has the most charge and can pull in electrons stronger than the sodium cation?

[Douglas Nguyen 2J]

Why is it that a Ca^2+ ion is smaller than a Na+ ion?

An [Na]+ ion has 11 protons and 10 electrons whereas [Ca]2+ has 20 protons and 18 electrons. Although the periodic trends dictate that as you go down a group, the atomic radius increases due to increased shielding and energy levels, the Z-effective of [Ca]2+ denotes greater pull on the calcium electrons in comparison to sodium's electrostatic attraction to its own electrons.