Tabla de Contenidos
The atomic radius and the ionic radius are two concepts that are similar but not the same. Both are measures of the actual size of atoms and ions, respectively. The same element can have both atomic radius and ionic radius, and can even have several of the latter, depending on the different valences that it can assume in the different chemical compounds that it forms.
Next, we will see what these two concepts refer to and how they differ from each other.
What is atomic radius?
The atomic radius is a property of chemical elements that is defined as half the average distance between the nuclei of two identical atoms that are bonded together .
It is a concept that provides us with an idea of the size of atoms in their elementary state. However, care must be taken not to interpret atomic radius as proof that atoms are spheres having a definite radius. In fact, atoms are made up of a nucleus surrounded by a cloud of electrons and this cloud is generally anything but spherical; neither does it have sharp boundaries as most illustrations showing a representation of atomic radius suggest.
Having said this, there is no doubt that some atoms are larger than others and the concept of atomic radius is very useful to get an idea of which are larger and which are smaller.
How is the atomic radius determined?
The atomic radius can be obtained from the crystalline structure of the elements in the solid state. In turn, the crystalline structure can be obtained through the technique of X-ray, neutron or electron diffraction, a technique through which we can find how the atoms are packed in the unit cell of the crystal and what are the dimensions of said cell. Once the structure is solved and the positions of all the atoms in the unit cell are known, the atomic radius is calculated as half the distance between the nuclei of two adjacent atoms.
Factors Affecting Atomic Radius
There are many factors that affect atomic radius and give rise to the periodic variation of this property. The most important factor is the effective nuclear charge, which is nothing more than the actual electrical charge felt by the outermost electrons as a result of the shielding of the innermost electrons.
Since as we move from left to right through a period of the periodic table the effective nuclear charge increases, the valence electrons are more attracted to the nucleus, so the outermost electron cloud becomes smaller. contract. The consequence of this is that the atomic radius decreases.
On the other hand, as we go down a group in the table we go from one energy level to a higher one, so the average distance between the electrons and the nucleus increases. Consequently, the atomic radius increases from top to bottom.
What is ionic radius?
The ionic radius is defined in a similar way to the atomic radius, except that in this case it is the distance between the nuclei of two monatomic ions, a cation and an anion. The ionic radius is the distance between the nucleus of an ion and its outermost electrons, that is, its valence electrons . Unlike atomic radius, ionic radius cannot be calculated as half the distance between two ions in a crystal, since ions of the same charge do not bond with each other but with ions of opposite charge. However, the total distance between the nuclei of two counterions is the sum of both ionic radii.
How is the ionic radius determined?
The ionic radius is determined in the same way as the atomic radii, that is, through the shape and dimensions of the crystalline structure of ionic solids. In turn, this structure can be obtained through techniques such as X-ray diffraction, neutron diffraction and electron diffraction, to name a few. However, since there is no direct way to determine the radius of a particular isolated ion, then the best thing to do is to estimate the ionic radius of one and by comparison find those of the other ions with which it is associated.
The ionic radius is more variable than the atomic radius, since the nature of the ionic bond varies depending on the atoms that are bonded. In addition, the ionic bond is never 100% ionic and always has a variable covalent character, which causes the ionic radius to vary from one compound to another. Thus, when the value of the ionic radius of a certain ion is reported, it is actually an average among a large number of experimental determinations, which is why the ionic radii rarely add up to the actual dimensions of the crystalline cell.
Factors Affecting Ionic Radius
In addition to also being influenced by the effective nuclear charge felt by the valence electrons, the most determining factor of an element’s ionic radius is the charge on the ion.
Anions, that is, those ions that have an excess of electrons and therefore have a net negative charge, usually have a larger ionic radius than cations with a similar number of electrons. Furthermore, the greater the charge carried by the ion, the greater the ionic radius for the same element.
On the other hand, positively charged ions, that is, cations, are formed by the loss of electrons from the neutral element. This reduces the repulsion between the electrons and increases the effective nuclear charge, so the electron cloud contracts, giving rise to a smaller ion. The greater the charge on the ion, the more the electron cloud can contract, so the smaller the ionic radius.
Summary of the differences between atomic radius and ionic radius
The following table summarizes the most important differences between atomic and ionic radii based on different criteria:
| Criterion | Atomic radio | ionic radius |
| Definition | Half the average distance between two bonded atomic nuclei in a pure element. | The average distance between the nucleus of an ion and its outermost, or valence, electrons. |
| serves for | Estimate the size of atoms. | Estimate the size of the ions. |
| Species to which it applies | Neutral atoms. | Ions both positive and negative and of different charges. |
| Determination | Through diffraction techniques. It is calculated as half the distance between two linked nuclei. | Through diffraction techniques. The radius of an ion is estimated and based on it all the others are determined by comparison. |
| Accuracy | It can be determined with good accuracy. | It cannot be determined with good accuracy. It can only be estimated. |
| periodic trend | It decreases over a period and increases over a group. | It decreases with positive charge and increases with negative charge. Between isoelectronic ions it decreases with atomic number. |
| Variability | It is an essentially fixed value for each element. | It varies for the same ion depending on the counterion to which it is bound in the ionic compound. |
References
Chang, R. (2002). Physicochemistry (1st ed .). MCGRAW HILL EDUCATION.
Chang, R. (2021). Chemistry (11th ed .). MCGRAW HILL EDUCATION.
Educaplus.org. (2021). Element properties . http://www.educaplus.org/elementos-quimicos/propiedades/radio-atomico.html
Atomic Radius and Ionic Radius . (nd). Pure chemistry. https://es-puraquimica.weebly.com/radios-atomico-e-ionico.html
Term – Ionic Radius . (nd). EHU.EUS. http://www.ehu.eus/imacris/PIE05/web/terminos/RadioIonico.htm
