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The size is an important characteristic of the atoms that make up the different elements present in the periodic table. It allows us to understand many of their characteristics, such as the tendency of hydrogen and helium to escape from the containers that contain them, or the inability of certain ions to pass through some ion channels in the cell wall.
However, when we imagine an atom as consisting of a very dense and small nucleus surrounded by a cloud of even smaller electrons moving around it, it is difficult to understand what “size” means in the case of an atom. This is because atoms are made almost entirely of empty space and we are used to understanding size as something associated with solid bodies that we can see and manipulate with our hands.
In view of the above, in order to explain the relative size of the atoms of the chemical elements, we must begin by defining said size from the chemical point of view.
Several ways to see the size of atoms
Defining the size of something starts from knowing its shape and its dimensions. In the case of atoms , we generally assume that they have the shape of a sphere, although this is not strictly true. However, it is practical to assume it that way.
Considering them as spheres, the size of the atoms is determined by their radius or their diameter. When we think of the radius of an atom, the first thing that comes to mind is the distance between the center of the atom, or its nucleus, and the outer edge of its electron cloud. The problem is that the electron cloud does not have a sharp edge (just as clouds do not have a sharp outer surface).
This implies that defining the radius is complicated and somewhat ambiguous. In addition, it also means that measuring the radius of an individual atom is practically impossible. So, some ways have been developed to determine or estimate the radii of atoms based on experimental data.
There are three main ways to express the size of atoms:
- The atomic radius or metallic radius.
- The covalent radius .
- The ionic radius.
The three concepts are different from each other and apply to different cases. For this reason, it is not always possible to directly compare the size of two atoms with each other. In addition, the size changes depending on whether it is a neutral atom or an ion. In the latter case, the size also varies depending on the value and the sign of the electric charge.
Atomic radius or metallic radius
The simplest concept to understand is that of atomic radius. The atomic radius of an element is defined as half the average distance between two adjacent atoms in a crystal of the pure element. This distance can be easily determined by means of X-ray diffraction techniques.
The concept of atomic radius applies mainly to metals, which are the only elements that form crystalline structures in which each atom of the neutral metal is exactly the same as the one next to it. Nonmetals, on the other hand, do not generally form the same type of solids. It is for this reason that atomic radius is often called metallic radius.
covalent radius
With the exception of the noble gases, most nonmetals in their pure state form either discrete molecules or solids with extensive covalent network structures. For example, elemental oxygen is made up of diatomic oxygen molecules (O 2 ), so in a solid oxygen crystal, the covalently bonded oxygen atoms in each molecule will be closer to each other than to each other. atoms of adjacent molecules.
On the other hand, cases such as carbon, whose most stable allotrope is graphite, form layered structures in which atoms within one layer are covalently bonded to each other, while they are not bonded to atoms in adjacent layers.
This makes defining the radius as a function of the distance between two adjacent nuclei ambiguous. In these cases, the size is defined as half the distance between two identical atoms covalently bonded to each other. This radius is called the covalent radius, and it is the most commonly used to establish the size of non-metal atoms .
On the other hand, the covalent radius is a concept that has a greater applicability than the metallic radius, since it allows us to assign a radius to the atoms that are part of a molecule or a covalent compound. Furthermore, by knowing the covalent radius of one atom, we can estimate the covalent radius of another by measuring the length of a covalent bond formed between the two.
Usually, the covalent radius of an atom is slightly less than its respective metallic radius.
ionic radius
The two measures of atomic size mentioned in the previous sections can only be applied to neutral atoms or to atoms that are part of covalent molecules. However, many elements that have markedly different electronegativities combine to form ionic compounds in which they gain or lose electrons, thus becoming anions or cations, respectively.
In these cases, we can establish the relative size of the atoms by comparing the sizes of their ions, that is, their ionic radius.
When we have two different ions linked together and we know the distance that separates them, we assume that this distance will be the sum of the two ionic radii. However, how can we know what fraction of this distance corresponds to one or another ion? It is evident that, to determine the radius of either of the two ions, we need the value of the radius of the other. This means that we only need to determine the radius of any cation and any anion.
Then we can use the radius of the cation to determine the radius of any other anion we want, while we can use the radius of the anion to determine the radius of any other cation.
This was first achieved from crystallographic data for lithium iodide, an ionic compound made up of a very small cation and a very large anion.
In this compound, the crystalline structure is formed by a network of iodide ions (I – ) in which each anion is in direct contact with six other iodides, while lithium ions (Li + ) are located in the cavities that are formed. every four iodides, being in direct contact with all of these. Thus, the ionic radius of iodide can be determined as half the distance between two adjacent iodine nuclei, while the distance between the lithium and iodine nuclei makes it possible to determine the ionic radius of lithium by subtracting that of iodide.
Periodic trend of atomic radius
As mentioned at the beginning, atomic size is a periodic property of matter. That is, it varies in a predictable way over a period and across a group.
Over the period, both the atomic radius and the covalent radius decrease from left to right. The same happens with the ionic radii of ions that have the same electric charge. The reason behind this behavior is the effective nuclear charge, which increases as the atomic number increases.
On the other hand, as you move from one period to another within a group (i.e., moving down the length of a group), the effective nuclear charge also increases, but the outermost electrons (i.e., valence electrons) ) are located in electron shells of increasing energy levels. This implies that the valence shells are further and further away from the nucleus, so the radius of the atom also increases.
Variation of ionic radius with charge
In addition to the periodic variation of atomic, covalent, and ionic radii, ionic radii are also strongly dependent on electric charge. Each additional electron that is introduced into an atom to convert it into an anion and increase its negative charge increases the electrostatic repulsion between the electrons in the valence shell, causing the electron cloud to expand and increasing the ionic radius.
The opposite happens with cations. Each electron that is removed from an atom to convert it into a cation and increase the positive charge, reduces the repulsion between the electrons, increases the effective nuclear charge and therefore the electrons are more strongly attracted to the nucleus. The effect is a decrease in ionic radius with increasing positive charge.
Example
If we compare the radii of the different ions that chlorine can form, the order of the ionic radii will be:
Cl 7+ < Cl 5+ < Cl 3+ < Cl + < Cl < Cl –
References
Bodner Research Web. (nd). Size of Atoms . https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch7/index.php
Physics and Chemistry. (2019, June 15). Atom and ion sizes . Physics and chemistry. https://lafisicayquimica.com/7-3-tamanos-de-atomos-e-iones/
Socratic. (2016, January 3). How is atomic size measured? Socratic.org. https://socratic.org/questions/how-is-atomic-size-measured
Studynlearn. (2014, June 14). AtomicSize . Youtube. https://www.youtube.com/watch?v=HBIUnpU_vJA
Tome, C. (2020, February 4). Why are atoms the size they are? Scientific Culture Notebook. https://culturacientifica.com/2020/02/04/por-que-los-atomos-tienen-el-tamano-que-tienen/
