CBSE Class 12 Chemistry Notes: Periodic Properties – Periodicity in Atomic Properties

In a period from left to right there is a regular change in electronic configuration of elements. In a group from top to bottom the outermost shell electronic configuration is similar. The chemical properties of the elements depend upon their electronic configuration. So there is a regular change in chemical properties in a period while in a group element have similar chemical properties. In a period as well as in a group there is a regular gradation (gradual increase or decrease in a particular property) in physical and chemical properties of elements with the change in atomic number. This regular gradation in properties is called periodicity and all these properties which are directly or indirectly related to the atomic structure or electronic configuration of the elements are called atomic properties. Periodicity in some atomic properties is discussed as under

ATOMIC RADIUS

The radius of an atom may be taken as the distance between atomic nucleus and the outermost shell of electrons of the atom. The size of atom is very important because many physical and chemical properties of the atom are related to it. According to the heisenberg’s uncertainty principle the position of a moving electron can not be accurately determined. So the distance between the nucleus and the outermost electron is uncertain. Atomic radius can be determined indirectly from the inter-nuclear distance between the two atoms in a gaseous diatomic molecule. This inter-nuclear distance between the two atoms is called bond length. Different types of atomic radii are discussed below.

Covalent radius: One half of the distance between the nuclei (inter-nuclear distance) of two covalently bonded atoms in a homo-diatomic molecule is called the covalent radius of that atom. The covalent bond must be single covalent bond. The covalent radius (r_A) of atom A in a molecule A₂ may be given as

i.e. the distance between the nuclei of the two single covalently bonded atoms in a homo-diatomic molecule is equal to the sum of covalent radii of both the atoms.

d_A-A = r_A+r_A

Metallic radius: Metal atoms are assumed to be closely packed spheres in the metallic crystal. These metal atom spheres are considered to touch one another in the crystal. One half of the internuclear distance between the two closest metal atoms in the metallic crystal is called metallic radius.

Metallic radius > Covalent radius

For example – Metallic radius and covalent radius of potassium are 2.3 Å and 2.03Å respectively.

Van der Waal’s Radius or Collision radius: The molecules of non metal atoms are generally gaseous. On cooling, the gaseous state changes to liquid which is followed solid state on further cooling. In the solid state, the non metallic elements usually exist as aggregations of molecules are held together by Van-der Wall forces. One half of the distance between the nuclei of two adjacent atoms belonging to two neighbouring molecules of an element in the solid state is called Van der Waal’s radius. It may also be defined as half of the inter-nuclear distance of two non bonded neighbouring atoms of two adjacent molecules.

Vander Waal’s radius > Metallic radius> Covalent radius
The Vander Waal’s radius and covalent radius of chlorine atom are 1.80Å and 0.99Å respectively

Ionic Radius: A neutral atom changes to a cation by the loss of one or more electrons and to an anion by the gain of one or more electrons. The number of charge on cation and anion is equal to the number of electrons lost or gained respectively. The ionic radii of the ions present in an ionic crystal may be calculated from the internuclear distance between the two ions

(a) Radius of a Cation–Radius of a cation is invariably smaller than that of the corresponding neutral atom

Reasons

The effective nuclear charge increases. For example in Na atom 11 electrons are attracted by 11 protons and in Na⁺ 10 electrons are attracted by 11 protons. Thus in the formation of cation number of electrons decreases and nuclear charge remains the same.

Generally the formation of cation results in the removal of the whole outer shell due to which interelectronic repulsion decreases. The interelectronic repulsion in Na is among and in Na⁺ among

(b) Radius of an anion – Radius of an anion is invariably bigger than that of the corresponding atom

Reasons

The effective nuclear charge decreases in the formation of anion. Thus the electrostatic force of attraction between the nucleus and the outer electrons decreases and the size of the anion increases.

Interelectronic repulsion increases due to which expansion of electron cloud takes place.

Isoelectronic species:

A series of atoms, ions and molecules in which each species contains same number of electrons but different nuclear charge is called isoelectronic series

In isoelectronic series atomic radii decreases, nuclear charge increases as

(i) Number of electrons is same.

(ii) Number of protons is increasing

(iii) So the effective nuclear charge is increasing and atomic size is decreasing. In an isoelectronic series atomic size decreases with the increase of effective nuclear charge.

Some of the examples of isoelectronic series are as under

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