Atomic Radii refer to the size of an atom, indicating how big or small an atom is. Atomic Radii or size, is defined as the shortest distance between the center of an atom’s nucleus and its outermost shell. This concept is similar to the radius of a circle, with the nucleus being analogous to the circle’s center and the outermost orbital of the electron corresponding to the circle’s outer edge. In this article, we will discuss various concepts related to atomic radii of various elements in detail.
What is Atomic Radii?
Atomic Radii, also known as atomic size or atomic radius, refer to the size of an atom.
Atoms are the fundamental building blocks of matter, and they consist of a central nucleus, composed of protons and neutrons, surrounded by a cloud of electrons.
In general, the atomic radii are defined as the total distance from the nucleus of an atom to the outermost electron orbital. Simplified, it can be described as something like the circle’s radius, with the electron’s outermost orbital at the circle’s outer edge and the nucleus in its center.
The net positive charge experienced by the valence electron in an atom is its effective nuclear charge or Zeff. The valence electron does not sense the entire positive charge because part of it is shielded by the core electrons. Zeff has a significant impact on an atom’s atomic size. Because there is a greater screening of the electrons from the nucleus, which lessens the attraction between the nucleus and the electron, the atomic radius will expand as the Zeff lowers. As Zeff experiences a drop when moving down a group and from right to left across the periodic table, the atomic radius will experience a rise in these directions.
Atomic Radii Definition
Atomic radii is defined as the distance from the center of the nucleus to the outermost shell of electrons in an atom. It is often measured in picometers (pm) or angstroms (Å).
We also define the atomic radii of a chemical element as:
The mean or typical distance from the center of the nucleus to the boundary of the surrounding shells of electrons.
Types of Atomic Radii
Based on the type of bond, atomic radii is divided into three types as follows:
- Covalent Radius
- Ionic Radius
- Metallic Radius
Covalent Radius
The covalent radius represents the distance from the center of an atom’s nucleus to the point at which the electron cloud effectively ends, where it overlaps with another atom’s electron cloud in a covalent bond.
The covalent radius can be calculated when there is a covalent link between two atoms. Because the electrons are drawn to the same atoms equally when they are covalently bound, the radius of each atom equals half the distance between the two nuclei. An atom’s diameter can be found by measuring the distance between its two nuclei, but you want the radius, which is half the diameter.
Ionic Radius
Ionic radius refers to the size of an ion, which is an atom or group of atoms that has gained or lost electrons, resulting in a net electrical charge. An ion can be positively charged (cation) or negatively charged (anion), and its ionic radius can vary depending on the type of ion formed.
The ion with a positive charge will have a smaller size than the ion with a negative charge if we consider the charge of the ion.
Metallic Radius
Metallic radius, also known as the metallic atomic radius, refers to the size of an atom in a metallic solid or the context of metallic bonding.
The radius of an atom connected by a metallic connection is known as the metallic radius. In a metallic cluster, the metallic radius is half of the total distance between the nuclei of two neighboring atoms. Each atom in a metal will be the same distance apart since it is a collection of atoms of the same element.
Read More,
Other Types of Atomic Radii
Some of the other types of atomic radii include
- Van Der Waals Radius
- Bohr Radius
Let’s discuss these in detail as follows:
Van Der Waals Radius
It is defined based on the attractive forces between atoms or ions nearby when they are not bonded in a covalent or ionic manner.
Bohr Radius
The Bohr radius is defined as the radius of the orbit in which the electron of a hydrogen atom revolves around the nucleus in the lowest energy state. In other words, it represents the average distance between the electron and the nucleus.
The formula for calculating the radii of any element is given as:
Atomic Radius = Distance between the two nuclei / 2
How to Determine Atomic Radii?
There are several ways to determine atomic radii of an element. Some important methods are discussed below:
- X-ray Crystallography: This method involves analyzing the diffraction patterns produced when X-rays pass through a crystal.
- Electron Diffraction: Electron diffraction involves firing electrons at a sample and analyzing the resulting diffraction pattern.
- Coulomb’s Law: The atomic radius can be estimated using Coulomb’s law, which describes the electrostatic interaction between charged particles. For example, the distance between the nuclei of two bonded atoms in a molecule can provide an estimate of the atomic radius.
Atomic Radii for Elements
Here is a tabular description of approximate atomic radii for selected elements. The values are given in picometers (pm), where 1 picometer is equal to 1×10-12 meters.
|
Element
|
Atomic Number
|
Atomic Radius
|
|
Hydrogen
|
1
|
53
|
|
Helium
|
2
|
31
|
|
Carbon
|
6
|
67
|
|
Nitrogen
|
7
|
56
|
|
Oxygen
|
8
|
48
|
|
Fluorine
|
9
|
42
|
|
Neon
|
10
|
38
|
|
Sodium
|
11
|
90
|
|
Magnesium
|
12
|
145
|
|
Aluminum
|
13
|
118
|
|
Silicon
|
14
|
11
|
|
Phosphorus
|
15
|
98
|
|
Xenon
|
54
|
108
|
|
Bromine
|
35
|
94
|
|
Zinc
|
30
|
142
|
Atomic Radii of Carbon
- Atomic Number: 6
- Atomic Radius: Approximately 67 picometers (pm)
Atomic Radii of Hydrogen
- Atomic Number: 1
- Atomic Radius: Approximately 53 picometers (pm)
Atomic Radii of Oxygen
- Atomic Number: 8
- Atomic Radius: Approximately 48 picometers (pm)
Atomic Radii of Sodium
- Atomic Number: 11
- Atomic Radius: Approximately 190 picometers (pm)
Atomic Radii Trends in Periodic Table
As you move from left to right across a period (horizontal row), atomic radii generally decrease. This is because the number of protons in the nucleus increases, which results in a greater nuclear charge, pulling the electrons closer to the nucleus.
In contrast, when you move down a group (vertical column) in the periodic table, atomic radii tend to increase. This is because new electron shells are added as you move down the group, increasing the distance between the outermost electrons and the nucleus.
Conclusively,
- The radius of atoms increases as you go down a certain group.
- The size of an atom will decrease as you move from left to the right of a period.
Limitations of Atomic Radii
Some of the key limitations of atomic radii are:
- Atomic radii are based on electron density and the concept of the electron cloud, which does not have precise boundaries.
- Atomic radii are typically discussed in the context of isolated atoms. In molecules, atoms can have different effective sizes due to the influence of neighboring atoms and the nature of chemical bonds.
- Atomic radii are typically discussed in one dimension (e.g., as a radius or distance from the nucleus to the outermost electron). They do not provide information about the overall three-dimensional shape or electron distribution within an atom.
- Atomic radii are estimated based on various measurement methods and theoretical models. Different methods may yield slightly different values, and the choice of method can impact the reported radii.
Read More,
Frequently Asked Questions on Atomic Radii
What is the Atomic Radius of an Atom?
Atomic radius is the size of an atom, specifically the distance from the nucleus to the outermost electron shell. It is a fundamental concept in chemistry used to compare the sizes of atoms.
What are the Atomic Radii trends across the Periodic Table?
Generally, atomic radii decrease from left to right across a period (horizontal row) of the periodic table because of increasing nuclear charge. Atomic radii increase as you move down a group (vertical column) due to the addition of electron shells.
Why does Atomic Radii increase down a Group?
Atomic radii increase down a group due to additional energy levels, resulting in larger electron cloud distribution and weaker attraction between the nucleus and electrons.
Why do Atomic Radii decrease from Left to Right?
Atomic radii decrease from left to right across a period due to increased effective nuclear charge, pulling electrons closer, and fewer energy levels.
Are Atomic Radii precise Measurements?
Atomic radii are not precise measurements in the strictest sense because atoms do not have well-defined boundaries. They are estimates that can vary depending on the method and context.
Share your thoughts in the comments
Please Login to comment...