What is Carbon's ionic radius?
Lattice energy decreases as ionic radius increases. The lattice energy is the amount of energy required to completely separate a mole of the solid ionic compound into its gaseous ions. As an example, if the ionic radius decreases and the ionic charges increase, the ions will be held closer and tighter. As a result of this, more energy will be needed to separate the ions.
Why are size of cations always smaller and size of anions always bigger than the corresponding atom?
The larger the ionic radius, the more reactive the cation because the outermost electron is held more loosely and therefore more easily lost. In case of anions, the larger the ionic radius, the smaller the force of attraction between the nucleus and the coming -ve electron, so it becomes less reactive.
The elements that do not have quoted ionic radii are the noble gases. These elements do not form ionic compounds and therefore there is no crystallographic data. All of the other elements have ionic radii, these are either an "estimate" from the apparent size of the ion in different compounds, or a calculated value. Ionic radius is a useful idea but it should not be taken to literally, comparatively few compounds are truly ionic, most…
Both are ions and we should consider the ionic radius, not the atomic radius. Rubidium ion (Rb+) will have a larger IONIC radius than strontium ion (Sr2+). Both ions have the same electronic configuration. But strontium ion has more number of protons in the nucleus. Hence it will have greater force of attraction between the nucleus and the valence electrons, resulting in smaller size.