In general ionic radius decreases from group 1 to 3 as the ions formed have a higher nuclear charge, more protons meaning the electrons are pulled closer to the nucleus and the ionic radius is smaller . However this is different for elements in group 5 to 7 as the ionic radius decreases from group 5 to 7. This is because ions formed in group 5 would gain 3 electrons and have a -3 charge compared to ions in group 7 which gain 1 electron and have a -1 charge. The larger charge of -3 on elements in group 5 means there is greater repulsion of the electrons in the orbitals causing a larger ionic radius. For example the ion of nitrogen N3- has a radius of 146 x10-12 m. and the ion of oxygen, O2- has an ionic radius of 140 x10-12 showing the electron repulsion is greater in nitrogen despite both having the same electron configuration and the same number of orbitals being filled.
In general down a group the ionic radius increases in size due to the increasing number of shells of electrons and increased shielding of the nuclear charge means the repulsion of electrons can be greater when more electrons are added. For example looking at the ionic radius of the halide ions going down the group fluoride has an ionic radius of 464 x10-12 m where as Bromine has an ionic radius of 598x10-12 m.
Determining ionic radius can be a difficult science as ions only exist in lattice structures and to measure the ionic radius we need to isolate the ion which is often influenced by the presence of other ions. The technique of X-ray crystallography can be used to determine the length of radii between ions and is a powerful technique for working out the distances between ions and bonds. When measuring the ionic radius of an ion it is difficult to define the radius, for example when measuring the radius of sodium in sodium chloride we can measure the length of the sodium chloride crystal unit cell (the distance from one sodium to chloride to another sodium ion, Na-Cl-Na) which can be measured as 564 x10-12 m but if we want to estimate the distance between the sodium and chloride nucleus we can take half of this value which is 282x10-12m. However this does not give the distance from the nucleus to the valence electron of the sodium ion. Through a series of experiments on different ion pairs it can be averaged out the ionic radius of an ion.
Shielding is the effect of inner shell electrons close to the nucleus reducing the nuclear charge on the valence electron.
The ionic radius is a measure of the radius of a positive or negative ion formed from the gain or loss of electrons usually measured using xray crystallography
Nuclear charge is the attraction exerted by the nucleus on electrons due to the positive charge of the protons and negative charge of the electron
The valence electron is the outermost electron of an atom
A lattice is a term given to an ordered arrangement of points in a 3D shape creating a regular arrangement of atoms and ions
An ion forms when an atom loses or gains electrons to form a positive or negative particles due to the unbalanced number of protons
A proton is a positive particle that makes up the atom in the nucleus with a positive charge
The electron configuration is the numerical layout of electrons in the orbitals around the atom. Can be done simply for example lithium has 3 electrons and has 2 in the first shell (as this is the maximum) and 1 in the second shell giving it the configuration 2,1.
The electron is the smallest sub atomic particle that make up the atom. Has a negative charge and is located in shells that orbit the nucleus
Xray crystallography is a technique to image atoms and ions by bombarding them with Xrays and creating images of the nucleus and electrons
Repulsion is when two charges that are the same come into contact they prefer to be further away from each other so push each other away. For example when two electrons come close to each other the negative charges cause repulsion