Between F- and Cl-, which one has
bigger size? Explain.

The order in decreasing size would be Cl- > F- > Na+ > Li+.

This can be explained by the following logic.

Cl- would be the largest because it has the largest number of electrons, i.e. 18. So, the electron- electron repulsion would be maximum in the chloride ion.

Similarly Li+ would be the smallest because it has only 2 electrons and therefore the least electron- electron repulsion.

When comparing F- and Na +, we have to be careful, as both of them have 10 electrons. Experimental results show that F- is slightly larger than Na+(119 pm as compared to 116 pm).

One way of explaining this is that in the case of fluoride ion, there are only 9 protons holding the 10 electrons in orbit whereas the sodium ion has 11 protons holding the same number of electrons. So Na+ is smaller due to the larger attractive force of the protons.

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