Atomic Radii

The atomic radius of a chemical element is a measure of the size of its atoms, generally the mean or typical distance from the center of the nucleus to the border of the surrounding shells of electrons. There are several non-equivalent definitions of atomic radius since the boundaries are not well-defined physical objects. Van der Waals radius, ionic radius, metallic radius, and covalent radius are four commonly used definitions of atomic radius. Typically, due to the difficulties of isolating atoms in order to measure their radii independently, atomic radius is measured in a chemically bonded state; but theoretical calculations are simpler when considering atoms in isolation. The dependence on the environment, probes, and state leads to a multiplicity of definitions.

Depending on the definition, the word may apply to atoms in condensed matter, covalently bonded in molecules, or in ionized and excited states, and its value may be derived from actual measurements, or estimated from theoretical models. The value of the radius may depend on the atom’s status and circumstances.

Electrons do not have definite orbits or clearly defined ranges. Rather, their positions must be defined as probability distributions that taper off gradually as one gets away from the nucleus, without a sharp cut-off; these are referred to as atomic orbitals or electron clouds. Moreover, in condensed matter and molecules, the electron clouds of the atoms frequently overlap to some extent, and some of the electrons may roam across a large region containing two or more atoms.

Types of Radius

The atomic radii are difficult to calculate because the position of the outermost electron is unclear — we don’t know exactly where the electron is. The Heisenberg Uncertainty Principle helps explain this phenomenon. We determine the radius based on the distance between the nuclei of two bonded atoms in order to acquire an accurate, but still inadequate, measurement of the radius. Atoms’ radii are thus determined by the bonds that they form. An atom’s radius varies depending on the bond it forms, hence there is no such thing as an atom’s fixed radius.

Covalent Radius

The covalent radius can be determined when two atoms have formed a covalent bond. When two atoms of the same element are covalently bonded, the radius of each atom is half the distance between the nuclei because the electrons are equally attracted to the two nuclei. The diameter of an atom is determined by the distance between two nuclei, but we want the radius, which is half the diameter.

Ionic Radius

The radius of an atom creating an ionic bond or an ion is called the ionic radius. In an ionic bond, each atom has a different radius than in a covalent bond. This is a crucial concept. The fact that the atoms in an ionic bond are of vastly different sizes accounts for the variation in radius. One of the atoms is a cation, which is smaller than the other, while the other is an anion, which is much bigger. To account for this disparity, one must first calculate the entire distance between the two nuclei and divide it by the atomic size. The larger the atomic size, the greater the radius.

The cation, which is a positively-charged ion, contains fewer electrons than protons. Because there are fewer electrons for the protons to pull towards the nucleus, the atomic size will reduce, resulting in a greater attraction of the electrons towards the nucleus. It will also decrease since there are fewer electrons in the outer shell, resulting in a smaller radius.

An anion, on the other hand, will be larger than the atom from which it was formed due to the gain of an electron. The addition of an electron to the outermost shell increases the radius because there are now more electrons further away from the nucleus and more electrons to pull towards the nucleus, causing the pull to become slightly weaker than that of a neutral atom, resulting in an increase in atomic radius.

Metallic Radius

The radius of an atom joined by a metallic bond is known as the metallic radius. In a metallic cluster, the metallic radius is half of the total distance between the nuclei of two adjacent atoms. The distance between each atom in a metal will be the same since it is made up of atoms of the same element.

Atomic Radii Trends

Periodic trend: Atom’s atomic radius decreases with time as they move from left to right. Some minor exceptions exist, such as the oxygen radius being slightly greater than the nitrogen radius. Protons are added to the nucleus as electrons are added to the same principal energy level over a period of time. Because of the increasing positive charge on the nucleus, these electrons are gradually drawn closer to it. The size of the atoms shrinks as the force of attraction between nuclei and electrons rises. The impact reduces as one gets further to the right in a period because of electron-electron repulsions that would otherwise cause the atom’s size to increase.

Group Trend- Within a group, the atomic radius of atoms normally rises from top to bottom. The positive nuclear charge grows as the atomic number increases in a group. However, there is an increase in the number of occupied principal energy levels. Higher principal energy levels have bigger orbitals than lower energy levels. The impact of the increased number of primary energy levels overcomes the rise in nuclear charge, and therefore, the atomic radius grows down a group.

Related Terms

Van der Waals Radius

When the electrostatic forces between two unbonded atoms are balanced, the Van der Waals radius is equal to half the distance between them. In other words, it’s half the distance between two non-bonded atoms or inside the same molecule.

Bohr Radius

The radius of the lowest-energy electron orbit was predicted by the Bohr model of the atom. Only atoms and ions having a single electron, such as hydrogen, are affected by Bohr’s radius. The Bohr radius for the hydrogen atom is still considered an important scientific constant, despite the fact that the model is now obsolete.

Conclusion

Different elements in the periodic table behave differently. Various elements have some points of difference among them. Each individual atom has different properties in relation to valency, atomic radius, etc. The atomic radius of a chemical element is a measure of the size of its atoms and is of various types, like covalent, ionic, and metallic radius.