An Introduction to Ionic Radius Trends
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. An ion is formed when an atom loses or gains electrons from its valence electronic orbital to attain a stable electronic configuration.
The loss of electrons results in a positively charged cation, whereas gain forms a negatively charged species called an anion. 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 radius of a monatomic ion in an ionic crystal structure is called the ionic radius, or r. 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. Picometers (pm) or angstroms (Å), with 1 = 100 pm, are the most used units for ionic radii. The typical range of values is from 31 pm (0.3 Å) to over 200 pm (2 Å).
Atomic Size
There will be an obvious change in atomic properties due to the loss or gain of electrons. Let us consider atomic size first. 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).
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. An evident conclusion that can be drawn from here is that anionic radius> cationic radius.
For example,
Radius of Sodium atom = 227 pm.
Radius of Sodium cation = 186 pm.
Isoelectronic species are those having the same number of electrons in total. For instance, F- and Na+, both have 10 electrons. However, their atomic sizes differ due to the difference in effective nuclear charge. It follows the above trend, and hence, F- has a larger ionic radius compared to Na+.
Ionic Radius Trends
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. Important analytical conclusions about chemical reactivity of elements can be drawn from this. 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.
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Trends In Ionic Radius 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. As a result, the ionic radius of elements also increases on moving down a group.
Trends in Ionic Radius Across a Period
While moving across any period in the periodic table, the nature of elements gradually changes from metallic to non-metallic. The number of electrons in the outermost shell keeps increasing until the last element (noble gas) is reached. 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.
This implies that it becomes difficult to lose electrons as we move across a period. So, the elements in the left typically form cations while those in the later periods towards the right form anions. Thus the ionic radius initially decreases and later increases, followed by another decrease. A maximum radius is obtained in between for the anion with the maximum negative charge. The following table may be used for reference and better understanding of the trend.
The ionic radius trend can be observed to decrease, with increasing positive charge and, to increase with increasing negative charge.
Determination of Ions
When an atom loses one electron to become a cation, the remaining electrons become more attracted to the nucleus, and the ion's radius shrinks. Similarly, when one electron is added to an atom to produce an anion, interelectronic repulsion increases the size of the electron cloud.
The ionic radius is a variable attribute of an ion that fluctuates depending on coordination number, spin state, and other factors. Ionic radius values, on the other hand, are sufficiently transferrable to allow for the detection of periodic patterns. Ionic radii, like other sorts of atomic radius, grow as you go down a group. Ionic size rises with increasing coordination number for the same ion, and an ion in a high-spin state is bigger than an ion in a low-spin state. In general, ionic radius reduces as positive charge increases and increases as negative charge decreases.
Difference Between Ionic And Atomic Radius
Atomic Radius
The atomic radius of a neutral atom is the distance between the nucleus and the outermost stable electron. In practice, the diameter of an atom is measured and then divided in half to get the result. Neutral atoms have radii ranging from 30 to 300 pm, or trillionths of a metre.
The atomic radius is a measurement of the size of an atom. This number, however, has no accepted meaning. The ionic radius, as well as the covalent radius, metallic radius, and van der Waals radius, are all examples of atomic radius.
Ionic Radius
The ionic radius is equal to half the distance between two gas atoms that are contacting. The time ranges from 30 p.m. to more than 200 p.m. The atomic and ionic radius is the same in a neutral atom, although many elements exist as anions or cations. Because the atom loses an electron energy shell when it loses its outermost electron (positively charged or cation), the ionic radius is lower than the atomic radius. When an atom gets an electron (negatively charged or anion), the electron generally falls into an existing energy shell, making the ionic and atomic radiuses equivalent.
Ionic Radii Trends of Transition Elements
For ions having the same or closely similar charges, the ionic radii decrease slowly with an increase in atomic number across the period for transition elements positioned in Groups 3-12 of the modern periodic table. The reason behind this behavioral trend of ionic radius can be attributed to the increase in effective nuclear charge on moving across the period.
Solved Example
Q: Arrange the following in order of increasing ionic radius:
N3-, Li+, Be2+, O2-
A: Be2+<Li+<N3-<O2-
Following the periodic trend, cations have smaller radii than anions.
Hence, we may conclude that ionic radius is an important periodic property whose trends can be monitored and usefully put to application in predicting the properties of elements. Interestingly all periodic properties are interlinked and, to some extent, interdependent.
Fun Facts
Chemistry is all about exceptions, and the trends as mentioned earlier, although mostly generalized in the application, also show some exceptions.
The ionic radius of Oxide (O2-) is larger than Nitride (N3-).
The periodic table that we use in chemistry was first invented by Dmitri Mendeleev in the year 1869.
It was in 1886 when Antoine Becquerel first discovered the concept of radioactivity.
Conclusion
The radius of an atom in a crystal lattice is measured to calculate its ionic radius.
When electrons are removed, an ion is formed that is smaller than the parent element.
When electrons are added together, they form an ion that is bigger than the parent atom.
FAQs on Ionic Radius Trends in Modern Periodic Table
1. What are the Periodic Trends in Chemistry?
Periodic trends are patterns of change in properties (both chemical and physical) observed when we move across or down in the modern periodic table. The most common trends include atomic radius, ionic radius, electron affinity, electronegativity, ionization potential, etc. The root cause behind these trends is the change in the atomic structure of elements within their periods. The observable pattern helps us predict properties and derive conclusive ideas about chemical reactivity shown by different elements. These laws enable the chemical elements to be organized in the periodic table based on their atomic structures and properties. The unknown properties of newly discovered elements can also be predicted using these trends.
2. Is Ionic Radius the Same as Atomic Radius?
No, the atoms are electrically neutral substances, while ions have electrons less or more than the parent atom in order to attain stability. The size of ions is either greater or lesser than the parent atom. The trend in atomic radius across the periodic table also varies from the trend in ionic radius. The atomic radius decreases from left to right across a period because the number of shells remains constant, but the protonic charge keeps increasing, causing a greater attractive pull on the outermost shell electrons. The ionic radius trend across a period varies accordingly, as mentioned above in the article.
3. How is the Ionic Radius measured?
The ionic radius of an atom is determined in a crystal lattice, hence the compound must be solid. Depending on the method, these radii will vary slightly. X-ray crystallography is commonly used to calculate an ion's radius.
When electrons are removed from an atom, the outcome is invariably a cation that is much smaller than the parent atom. The valence electron(s) are eliminated, and the resultant ion has one less occupied primary energy level, resulting in a smaller electron cloud. Another explanation is that because protons now outweigh electrons, the remaining electrons are pulled closer to the nucleus. The number of electrons eliminated is also a factor. To make the equivalent ion, one electron is lost from the potassium atom, whereas calcium loses two electrons.
4. Why does the radius of an ion differ from that of an atom?
The ionic radius is equal to half the distance between two gas atoms that are contacting. The time ranges from 30 p.m. to more than 200 p.m. The atomic and ionic radius is the same in a neutral atom, although many elements exist as anions or cations. Because the atom loses an electron energy shell when it loses its outermost electron (positively charged or cation), the ionic radius is lower than the atomic radius. When an atom gets an electron (negatively charged or anion), the electron generally falls into an existing energy shell, making the ionic and atomic radiuses equivalent.
5. How does ionic radius vary across the period?
The nature of elements gradually shifts from metallic to non-metallic as you progress through the periodic table. The number of electrons in the outermost shell grows until it reaches the final element (noble gas). As a result, travelling over a period raises the effective nuclear charge and the ionisation potential (the energy required to remove the loosely bound outermost electron from an isolated gaseous atom). As we travel over a period, it gets more difficult to shed electrons. As a result, the components on the left tend to create cations, whilst those on the right tend to produce anions. As a result, the ionic radius lowers at first, then rises, then decreases again.