Grade 11 → Classification of elements and periodicity in properties → Periodic trends in properties ↓
Atomic and Ionic Radius
Introduction
In the fascinating world of chemistry, it is important to understand the size of atoms and ions. The terms "atomic radius" and "ionic radius" refer to the size of an atom and an ion, respectively. These concepts are essential for predicting and explaining the behavior of elements in chemical reactions and bonding. Let us learn about these properties in detail and understand their periodic trends.
Atomic radius
The atomic radius of an element is essentially the distance from the center of the nucleus to the outermost shell of the electron cloud in the atom. This measurement helps chemists understand the size of the atom and how it changes across the periodic table. Because electron clouds are not clearly defined, atomic radius is determined in a number of ways, usually related to how atoms bond in different situations.
Typically, atomic radii are measured in picometers (pm) or angstroms (Å), with 1 Å equal to 100 pm.
Covalent radius
It is half the distance between two identical atoms bonded together. It is generally used for non-metals where the atoms form covalent bonds.
d_A = 2 * r_c :r_c = Covalent radius :d_A = Distance between two nuclei of identical atoms
Metal radius
It is useful for metals, where the atoms are very close to each other. It is half the distance between two adjacent atoms in the metal lattice.
Periodic trends in atomic radius
Atomic radius varies as you move across a period and down a group in the periodic table. Understanding these trends is fundamental in predicting how the elements interact.
Trends over a period
As we move from left to right in a period, the atomic radius decreases. This happens because the nuclear charge increases due to the addition of protons to the atomic nucleus. The increased positive charge pulls the electrons closer to the nucleus, which reduces the size of the atomic radius.
Downward trend in the group
As one goes down a group in the periodic table, the atomic radius increases. This trend is due to the addition of a new electron shell with each successive element. Although the effective nuclear charge increases, its effect is reduced by the increase in the average distance of the electrons from the nucleus.
Ionic radius
Ionic radius refers to the radius of an atom's ion and measures the size of the atom after losing or gaining one or more electrons. Ions can be classified into two types: cations and anions.
Cations
Cations are positively charged ions formed by the loss of electrons. As electrons are removed, the electron-electron repulsion in the resulting ion decreases, allowing the remaining electrons to be drawn closer to the nucleus. As a result, the ionic radius of cations is smaller than that of their parent atoms.
For example, consider sodium:
Na → Na⁺ + e⁻
The radius of Na +
ion is smaller than that of the neutral sodium atom, since it has one less electron.
Anion
Anions are negatively charged ions formed by the gain of electrons. The addition of extra electrons increases electron-electron repulsion, causing the electron cloud to expand. Therefore, the ionic radius of anions is larger than that of their parent atoms.
For example, consider chlorine:
Cl + e⁻ → Cl⁻
In this case, the radius of Cl -
ion is larger than that of the neutral chlorine atom.
Periodic trends in ionic radii
The trends in ionic radii are very closely related to those in atomic radii, but they also involve the charges on the ions.
Trends over a period
As we move across a period from left to right, first the cations and then the anions show decreasing ionic radius. This trend exists due to the increase in nuclear charge, which makes the ionic radius pack more tightly. Between the cation and the anion, there is usually a significant jump in size, as the anions start with a larger volume due to the extra electrons. For a better understanding, analyze the size of the ions in the second period:
Downward trend in the group
Going down a group in the periodic table, the ionic radius increases. This pattern reflects the trend of atomic radius increasing due to the addition of a new electron shell with each element. As a result, even as ions, the radius of these atoms will be much wider than their predecessors.
Consider the alkali metal ions, such as Li +
, Na +
, and K +
, all of which show increasing ionic radius as we go down the group.
Visualization of periodic trends
Understanding periodic trends helps to predict and logically explain the behavior of an element. Here is a simple model using the atomic radius trend through a representative block of elements.
This diagram symbolically shows the trend in atomic radius along periods (horizontal) and groups (vertical). Moving to the right (across a period), the boxes shrink, indicating a decrease in atomic radius. Moving down the group, they increase in size.
Conclusion
In conclusion, understanding atomic and ionic radius trends is fundamental to the study of chemistry. These trends provide information about the properties of elements and help predict their chemical behavior. As you explore these concepts, remember that atomic and ionic radii go far beyond these generalizations, but serve as invaluable guides into the larger world of chemistry. Understanding and visualizing these trends gives us a deeper understanding of the nature of the elements in the periodic table.