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Periodic Trends: Physical Properties - I

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Periodic Trends: Physical Properties - I - Lesson Summary

A gradation in the physical and chemical properties of the elements is attributed to the arrangement of elements with respect to their electronic configurations.

Trends in physical properties such as atomic radius, ionization enthalpy, and electron gain enthalpy and electronegativity.

Atomic radius can be defined as the distance from the centre of the nucleus to the point up to which the density of electron cloud is maximum.

Depending on the nature of bonding, atomic radii are of three types. They are- covalent radius, metallic radius and van der Waals radius.

A covalent radius is one half of the distance between the nuclei of two atoms, held together by a covalent bond in a homo-diatomic molecule.

Covalent radius = Covalent diameter/2

Ex: internuclear distance between the chlorine atoms in the chlorine molecule is 198 picometers. Therefore, the radius of a chlorine atom is the half of this distance, which is, 99 picometers.

Metallic radius is half the internuclear distance between the centres of the nuclei of two adjacent metal atoms in a metallic crystal.

Ex: Distance between two adjacent copper atoms in solid copper is 256 picometers; therefore, the metallic radius of copper is 128 picometers.

Atomic radius decreases across a period and increases as we go down a group.

The decrease in atomic size across a period is due to the increasing effective nuclear charge with increase in atomic number. Nuclear charge is the net positive charge experienced by an electron of an atom.

The effective nuclear charge varies with the proximity of electrons from the nucleus because the electrons in the inner orbitals shield the electrons in the outer orbitals from experiencing the full nuclear charge.

Ex: The valence electron in Cesium is shielded from the nucleus by the inner electrons. Thus, the valence electron experiences a net positive charge that is less than the actual charge.

As moving from left to right in the periodic table, the nuclear charge increases by one unit in each succeeding element, while the principal energy level remains the same causes increase the effective nuclear charge. So the electrons in the valence orbital get more strongly attracted to the nucleus, reducing the size of the atom.

Atomic radius increases with the atomic number while moving from top to bottom, as a new energy shell is added to each succeeding element.

Noble gases, being mono-atomic, have very large non-bonded radii values. Hence the radii of noble gases should be compared with the Van der Waals radii of other elements rather than the covalent radius.

Van der Waals radius is half the distance between the centres of two atoms of different molecules which are closest to each other.

The radius of an atom changes when it loses or gains an electron to form an ion; and the resultant radius is called ionic radius. It also exhibit the same trend as the atomic radii.

When an atom loses an electron, it forms a positive ion called cation.

The radius of a cation is always smaller than its parent atom, because, while the nuclear charge remains the same, the number of electrons is decreased. This results in increase in the nuclear attraction on the valence electrons, leading to decrease in the size of the atom

Ex: The atomic radius of sodium atom (Na) is 186 picometers, whereas the ionic radius of sodium ion (Na+) is 95 picometers.

The addition of one or more electrons results in an increased magnitude of inter-electronic repulsions among the electrons and a decrease in the effective nuclear charges. Thus the size of an anion is larger than that of the parent atom.

The atomic radius of fluorine (F) is only 64 picometers, whereas the ionic radius of fluoride ion (F- ) is 136 picometers.

The variation between the atomic and ionic radius depends on the effective nuclear charge.

Iso-electronic species defined as atoms and ions which contain the same number of electrons.

Within iso-electronic species, as the nuclear charge increases, the force of attraction by the nucleus on the electrons also increases which results decrease in ionic radii.

Ex: N⁻³, O⁻², F⁻ , Na⁺, Mg⁺² and Al⁺³ ions have the same number of electrons (10-electrons. Their radii would be different because of their different nuclear charges. As cation with greater positive charge has smaller radius and anion with greater negative charge has larger radius, the order of ionic radii among the iso-electronic species increases in the order of

Al⁺³< Mg⁺² < Na⁺< F⁻< O⁻²< N⁻³

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