A BRIEF REVIEW ON PERIODIC PROPERTIES LIKE ATOMIC SIZE IONIZATION ENERGY ELECTRON AFFINITY AND ELECTRONEGATIVITY (SPECIALLY FOR STUDENTS APPEARING IN COMPETITIVE EXAMINATIONS PERIODICITY OF PROPERTIES The properties of elements, which change periodically with the change in atomic number in the periodic table is called periodicity of properties and such properties are called periodic properties. e.g. atomic size, ionization energy, electron affinity. (1) Atomic Size:- Atomic size are expressed in terms of atomic radii, covalent radii and ionic radii. (a) Atomic Radius:- As atomic has no precise boundary therefore, it is difficult to define atomic size, specially due to the wave like properties of electrons. However, we can say that, “The average distance between the valence shell and nucleus of an atom is called atomic size. ♣ The value of atomic size can be determined experimentally by spectroscopy and from X-ray study. ♣ In periodic table atomic radius decreases in going from left to right. Because the number of shells or orbits along the period remain constant, while charged particles increase. Therefore, the attractive force increases and size of atom decreases. In group atomic size increases from top to bottom, because number of shells increases element after element from top to bottom along the group. Physical properties like melting point, boiling point and density mainly depends on atomic size. (b) Covalent Radius:- It is defined as one half of the distance between the nuclei of single covalently bonded identical atoms is called covalent radius. OR ♣ It is one half of the bond length. Atomic radii (covalent radius) for H2 is 37.7pm which is the half of the bond length of H H bond. C; Ionic Radius; ♣ It is the distance b/w the nucleus of an ion and the point upto which the nucleus has influence on its electronic cloud. The size of cation is smaller while that of anion is greater than parent atom. Pauling determine the distance b/w K+ and Cl- ions in the crystal lattice and was concluded that it was the sum of the radii of two ions.It means that ionic radii is additive property.. 2; Ionization energy; ¥ The amount of energy required to remove the valence electronfrom a gaseous atom is called ionization energy. ¥ Ionization energy of Na is 496 kj/mol. ¥ Ionization energy is always endothermic. ¥ when ist electron is removed from the atom, called first ionization energy and when second electron is removed from that unipositive ion is called 2nd I.E.. Mg → Mg+1 + e- I.E = 738 kj/mol Mg+ → Mg+2 + e- I.E = 1451.0 kj/mol ¥ 2nd ionization energy is always greater than first ionization energy. FACTORS 1; Atomic size; I.E decrease with increase in size 2; Nuclear charge; I.E Increase with increase in nuclear charge. 3; shielding effect; poor shielding effect higher ionization energy 4; electronic configuration. Half filled and completely filled shell and sub shells are more stable than partially filled and sub shells. ¥ I.E decrease from top to bottom and increase from left to right. ¥ I.E of group IIA is greater than IIIA and that of VA is greater than VIA. ¥ metals have low ionization energy and non metals have high. ¥ low ionization energy mean highly reactive metal. 3; Electron affinity The amount of energy released or absorbed when an electron is added to the outermost shell of gaseous atom atom. ¥ 349 kj/mol energy is released when an electron is added to gaseous Cl atom. ¥ Electron affinity may be exothermic or endothermic. ¥ Halogen is the only group the ionization energy of which is always exothermic. ¥ Second electron affinity is always endothermic. O + e → O-1 1st E.A E.A = - 141.0 kj/mol O-1 + e → O-2 2nd E.A. E.A = + 844 kj/mol FACTORS 1; Atomic size; E.A decrease with increase in size 2; Nuclear charge; E. A Increase with increase in nuclear charge. 3; shielding effect; poor shielding effect higher E.A 4; electronic configuration. Half filled and completely filled shell and sub shells are more stable than partially filled and sub shells ¥ E.A of nonmetals is high while metals have low value of E.A. ¥ Higher electron affinity means highly reactive non metals. ¥ electron affinity increase from left to right and decrease from top to bootom. ¥ E.A of Fluorine is less than Cl due to extra ordinary low atomic size. Electronegativity The tendency of an atom to attract a shared pair of electrons towards itself in a molecule is called electro negativity. ¥ This is a relative property therefore has no unit. ¥ Pauling scale is commonly used to show electronegativity of an element. ¥ Fluorine is assigned the most electronegative element with the value of 4.0 while the least one is the Cesium (Cs). FACTORS 1; Atomic size; E.N decrease with increase in size 2; Nuclear charge; E. N Increase with increase in nuclear charge. 3; shielding effect; poor shielding effect higher E.N. 4;E.N value also depends on ionization energy and electron affinity, ¥ E.N value decrease from top to bottom and increase from left to right. ¥ electronegativity value tells us the nature of element and nature of bond between two atoms ¥ metals having E.N value lesser than 1.8 are metals.
Posted on: Tue, 23 Dec 2014 19:16:48 +0000
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