Which of the following have the largest ionic size ; a.FLOURINE ION , OXYGEN ION , LITHIUM ION ,CHL?

chloride ion has the largest size because all of the rest belongs to the second grp while chlorine belongs to the third grp.when we look at the energy levels, chlorine almost has every energy level filled. What this means is that as we added electrons non ionically and move to the right of the periodic table, the atom size will actually shrink. This is due to the addition of a proton on each element. For example, Carbon is smaller than nitrogen, which is smaller than oxygen, which is smaller then fluorine, etc... This is purely because we add an additional proton, and there is more coulomb force on each electron when we add a proton.

So what happens when we add an electron and not a proton (an anion)? The electrons already around the nucleus are very tightly packed, while the outer electrons are also packed closely to the nucleus, meaning that when we bring that electron close to the outmost energy level, two like charges repel, and this forces the electrons to repel more and more until the radius increases. So, if we simple add an electron, there are more electrons than protons, and there is more of a repulsion form the electrons already in the atom, and in physics you learn that two negative charges repel. So, when we add a negative charge, the electrons cannot maintain their density around the nucleus because there is now an additional pushing forces outward on every electron due to the addition of the new electron.

You also may ask why do the electrons not just crash into the nucleus? We don't know, they should all attract and collide and game over, but we have yet to discover this. It is due to the wave like properties of electrons.