The following table may be used for reference and better understanding of the trend. A maximum radius is obtained in between for the anion with the maximum negative charge. Thus the ionic radius initially decreases and later increases, followed by another decrease. So, the elements in the left typically form cations while those in the later periods towards the right form anions. This implies that it becomes difficult to lose electrons as we move across a period. Hence the effective nuclear charge increases on moving across a period and the ionization potential (The energy required to remove the loosely bound outermost electron from an isolated gaseous atom) increases. The number of electrons in the outermost shell keeps increasing until the last element (noble gas) is reached. While moving across any period in the periodic table, the nature of elements gradually changes from metallic to non-metallic. As a result, the ionic radius of elements also increases on moving down a group. If we move down along a group in the periodic table, then the number of electronic shells keeps increasing by one with every group. In this context, it may be interesting to note an alternative definition for ionic radius, which states that it is half the distance between two ions hardly in contact with one another, placed in a crystal lattice. Important analytical conclusions about chemical reactivity of elements can be drawn from this. Ionic radius trends refer to a predictable pattern change in the ionic radius of elements on moving down or across in the modern periodic table. It follows the above trend, and hence, F - has a larger ionic radius compared to Na +. However, their atomic sizes differ due to the difference in effective nuclear charge. For instance, F - and Na +, both have 10 electrons. Isoelectronic species are those having the same number of electrons in total. An evident conclusion that can be drawn from here is that anionic radius> cationic radius. Similarly, for an anion, the repulsive force existent among the electrons is dominant over the nuclear attractive force (As the electrons are more in number), and as a result, anions are larger in size compared to parent atoms. The atomic size of a cation is smaller than the parent atom as the attractive force exerted by the positively charged nucleus on the electrons in the outer electron shell is unbalanced and greater than that of the electrons (they are less in number and the atom is not electrically neutral anymore). There will be an obvious change in atomic properties due to the loss or gain of electrons. The typical range of values is from 31 pm (0.3 Å) to over 200 pm (2 Å). Picometers (pm) or angstroms (Å), with 1 = 100 pm, are the most used units for ionic radii. Despite the fact that neither atoms nor ions have sharp boundaries, they are regarded as if they were hard spheres with radii such that the distance between ions in a crystal lattice is equal to the sum of the ionic radii of the cation and anion. The radius of a monatomic ion in an ionic crystal structure is called the ionic radius, or r. Hence, the ionic radius can be defined as the radial distance measured between the centre of the nucleus of an ion to the outermost electronic orbital where the electron cloud is still under the influence of the positive electric field of the nucleus. The loss of electrons results in a positively charged cation, whereas gain forms a negatively charged species called an anion. An ion is formed when an atom loses or gains electrons from its valence electronic orbital to attain a stable electronic configuration. Before delving into the definition of ionic radius, it might be necessary to brush up on the basics and discuss the formation of an ion.
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