how is atomic radius expressed in different types of radius
Answers
Answer:
The atomic size is expressed in terms of Van der Waals radius since they do not form covalent bonds. The atomic radii of elements increases with increase in atomic number as we move from top to bottom in a group. ... As a result the attraction of the nucleus for the electron decreases and hence the atomic radius increases.
Question :
Explain the different types of atomic radius
Class : 11
Chapter : Periodic Classification of Elements
Topic : Atomic Radius
Answer :
The distance between the nucleus and the electron present in the outermost shell of an atom is called atomic radius. Atomic radius is commonly referred to atomic size. But atomic size is truly regarded as the diameter of the atom. Practically it is not convenient to isolate the individual atom and determine its radius.
More over atomic radius is influenced by many factors like :
- nuclear charge
- principal quantum number
- bond character
- multiplicity of bond
- oxidation state of atom
- Coordination number of the atom etc..
The following radii are experimentally significant :
(a) Covalent Radius
(b) Metallic Radius
(c) Van der Waals Radius
Covalent Radius : Covalent radius is one-half of the internuclear distance between atoms of a homonuclear covalent molecule. It is defined for atoms that are bonded with a similar atom. It is calculated by electron diffraction or x-ray diffraction or other spectroscopic techniques. It is measured in Angstrom units, A⁰.
1A⁰ = 1 x 10⁻⁸cm = 1 x 10⁻¹⁰m =100pm
Generally for non-metals, covalent radius is taken as atomic radius. If a molecule contains atoms of two different elements, the sum of the covalent radii is expected to be equal to the internuclear distance. The covalent radius of an atom in a heteronuclear molecule is defined as the distance between the nucleus of an atom and the mean position of one shared pair of electron between the bounded atoms.
If the bonded atoms have more than one bond present between them, the attraction between the atoms will be more. Due to the increased attraction between the bonded atoms, the internuclear distance decreases and the covalent radius decreases.
Metallic Radius : All metals are crystalline and hence metallic radius is also called crystalline radius. Metallic radius is defined as one-half of the internuclear distance between adjacent atoms of a metal. Generally for metals, metallic radius is taken as atomic radius. For metallic atoms, atomic radius is larger than covalent radius. For non-metallic atoms, atomic radius is equal to covalent radius.
Van der Waals Radius : When atoms or molecules of an element are closely packed in the solid state, they are held together by very weak attractive forces. These forces are called van der Waals forces. Due to these forces one atom can approach another without forming a covalent bond. van der Waals radius is defined as one-half of the internuclear distance between atoms of two adjacent molecules of an element facing each other, in solid state.
Know More :
Comparison of radii : Compared to the theoretical atomic radius, covalent radius of an atom is about 20% shorter. The formation of covalent bond involves overlapping of atomic orbitals. As a result of this, the internuclear distance between the bonded atoms is decreased. Compared to the radius of an isolated metal atom, the metallic radius is about 10% larger. For example, the covalent and metallic radius of potassium are 2.03A⁰ and 2.31A⁰ respectively.
The van der Waals radius of an atom is usually 40% larger than the atomic radius. This is because the non-bonded atoms of the molecules are held together by weak attractive forces. Usually inert gas elements do not combine themselves or other elements and are monoatomic. For elements which exist as monoatomic molecules, the van der Waals radius is taken as atomic radius.
Periodic Trends : In a group from top to bottom, the atomic radius gradually increases. On moving down a down a group, the number of shells increases. The nuclear charge also increases. The effect of increase in the number of shells is more pronounced than the effect of increase in the nuclear charge. Consequently, the distance between the nucleus and the outermost electron increases.
In a period from left to right, the atomic radius gradually decreases. On moving across the period, the atomic number increases gradually. The nuclear charge increases while the added electron enters the same shell. Consequently the electrons are pulled strongly towards the nucleus and the radius decreases.
Atomic radius is least for hydrogen among all the elements and is highest for caesium among the available elements.