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Atomic Radius

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Define Atomic Radius and Atomic Size

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An atom is made of three tiny subatomic particles, which are protons, neutrons, and electrons. Protons and neutrons constitute the atom's centre and are together known as the nucleus. The electrons fly around and above the nucleus in a smaller cloud. Electrons carry a negative charge, protons have a positive charge, and neutrons are neutral. If an atom is neutral, it has an equal number of electrons and protons; but not mandatorily the equal number of neutrons.


Define Atomic Size 

Atomic size refers to the distance between the nucleus's centre in an atom and its outermost shell. In introductory chemistry, one can define atomic radius as the shortest distance between the atom's nucleus and its outermost shell.


Radius and Atomic Radius  

The radius of a circle is the distance from the centre point to the circle's edge. This distance stays the same anywhere on the circle due to its radial symmetry. In an atom, the radius of the nucleus is almost negligible due to its smaller size.

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Atomic radius is half of the distance between the adjacent atoms of the same element in any molecule. It is difficult to measure the atomic radius of chemical elements as the atomic size is of the order 1.2 x 10⁻¹⁰ m. The electron cloud acts as the atom's shell and does not have a fixed shape, making it tough to determine the atomic size.


For precisely measuring the radius of an atom, one can use Heisenberg Uncertainty Principle. According to it, the atom's radius is calculated based on the distance between the nuclei of two of its bonded atoms. Thus, the bonds formed by an atom help in determining its radii. The radii of the atoms vary based on their bond formations; thus, there is no fixed radius of an atom. There are different types of atomic radii, which are atomic radius, Bohr radius, covalent radius, metallic radius, Van Der Waals radius. 


Common Types of Radius with Respect to Bonds


1. Van Der Waals Radius: 

Van Der Waals radius is the measurement of an atom's size that does not bind chemically, ionically, or covalently. It is defined as half of the closest distance of the two equal and non bonded atoms.


2. Ionic Radius: 

Ionic radius definition refers to the radius of an atom that forms an ionic bond or an ion. Ionic radii are measured in Armstrong or Picometers, and this radius ranges from 30 to 200 pm. In these, the electrons and nucleus are restricted by atomic bonds, so the atoms or ions do not have any specific shape. The ionic radii are not static, and they differ according to the spin state of the electrons, their coordination number, and other parameters. As the coordination number increases, the ionic size also rises. And, the ions having a higher spin state of the electron have a larger ionic size.


3. Metallic Radius: 

It is the atoms' radii that are joined by a metallic bond. The metallic radius is half of the total distance between the two adjacent atoms' nuclei, which belong to a metallic cluster.


4. Covalent Radius: 

It is defined as the atom's radius under a covalent bond with another atom of a similar element. The covalent radius of an atom is determined by measuring the bond lengths between the two covalently bonded atoms. In this case, if the two atoms are on the same side, then the covalent radius is half of its bond length.


5. Bohr Radius: 

This is a physical constant equal to the most probable distance between the electron and nucleus of the hydrogen atom in its ground state. The Van der Waals radius of a hydrogen atom is 120 pm, and the atomic radius of a hydrogen atom is 53 pm.

FAQ (Frequently Asked Questions)

Q1: How to Calculate the Atomic Size? Describe the Trends of Atomic Size in a Periodic Table.

Ans: While moving downwards in a group or across a column or row in the periodic table, there are various trends in all elements' properties that one can observe. While combining the two atoms, their atomic size is calculated by checking the distance between the atoms. The other method for measuring the atomic size of non-metallic elements is by forming a single covalent bond between the two atoms and then calculating the distance between them.


An atom's atomic radius is measured by spectroscopy or x-rays, and the atomic radii of elements vary in a periodic table in a fixed pattern. The atomic radius decreases while moving from left to right in the periodic table, increasing while moving down in a group. This is because the valence electrons are in the same outermost shells in the periods.


Atomic number increases in the same period while moving from left to right, and this raises the effective nuclear charge, increasing the attractive forces to reduce the atomic radius.

Q2: Differentiate Between Atomic Radius and Ionic Radius.

Ans: Atomic radius is the distance between the atomic nucleus and the neutral atom's outermost stable electron. It is calculated by measuring the diameter of the atom and then dividing it in half. Atomic radius helps in describing the size of an atom.


In contrast, the ionic radius is half of the distance between two gaseous atoms that just touch each other. It is the same as the atomic radii in the neutral atom but not in the charged ions. For the cations, the ionic radius is smaller than the atomic radius, and it is larger than the atomic radii in the case of anions.


However, while moving in the modern periodic table, both atomic radii and ionic radii follow the same trend. Generally, the radii decrease while moving from left to right in the periods, and it increases while moving from top to bottom in the groups.