Periodicity

Variation of periodic properties across the periodic table

1. Shri shankaracharya mahavidyalaya junwani , bhilai Presented by Dr. Neelam Department of chemistry Periodicity & their properties

2. PERIODICITY Repetition of properties of elements after a certain interval when the elements all arranged in increasing order of atomic number. PERIODIC PROPERTIES OF ELEMENTS ATOMIC RADIUS It is distance between outermost electron and nucleus. Atomic radius depends on the type of chemical bond between atoms in a molecule. These are : 1. Covalent radius 2. Ionic radius 3. Metallic radius 4. Vander waal’s radius

3. PERIODIC TRENDS ATOMIC SIZE/ATOMIC RADIUS atomic size/radius increases from top to bottom (due to increasing number of shells thus decreasing effective nuclear charge, Z eff ) atomic size/radius decreases with increasing atomic number, number of shells remain same but number of electrons & protons increases, thus increasing effective nuclear charge, Z eff ) Along the period Top to bottom

4. Factors affecting atomic size Effective nuclear charge Atomic radius ∝ ( ii) Number of shells Atomic radius ∝ No. of shells (iii) screening effect Atomic radius ∝ Screening effect (iv) Magnitude of + ve charge Atomic radius ∝ (v) Magnitude of - ve charge Atomic radius ∝ Magnitude of - ve charge (vi ) Bond order Atomic radius ∝ Effective nuclear charge ( Z eff ) 1 1 Magnitude of + ve charge 1 Bond order

5. IE increases with atomic number increase due to the decrease in size & increase in effective nuclear charge ( Z eff ) along the period IE decrease with increase in size & number of shells increases So effective nuclear charge ( Z eff ) decreases along the group IONISATION ENERGY Ionisation energy (IE) is defined as the amount of energy required to remove the most loosely bound electron from an isolated gaseous atom to form a cation. M g + M g ++ + e - IE2 M g ++ M g +++ + e - IE3 M g M g + + e - IE1 (IE)1 < (IE)2 < (IE)3 Along the period Top to bottom Perioddic trends

6. Factors Influencing Ionisation energy Ionisation energy ∝ (ii) Ionisation energy ∝ Effective nuclear charge ( Z eff ) (iii) Ionisation energy ∝ (iv) Electronic Configuration: If an atom has exactly half-filled or completely filled orbitals , then such an arrangement has extra stability. 1 Atomic size 1 Screening effect

7. ELECTRON AFFINITY Electron affinity is conventionally defined as the energy released when an electron is added to the valence shell of an isolated gaseous atom. M g + e - M g - Exothermic reaction EA1 M g - + e - M g - - Endothermic reaction EA2 EA increases atomic number increase decrease in size effective nuclear charge ( Z eff ) increases EA decrease size increase number of shells increases effective nuclear charge ( Z eff ) decreases Along the period Top to bottom

8. Electron affinity ∝ (ii) Electron affinity ∝ Effective nuclear charge ( z effs ) (iii) Electron affinity ∝ (iv) Stability of half filled and completely filled orbitals of a subshell is comparatively more and the addition of an extra electron to such an system is difficult and hence the electron affinity value decreases. 1 Atomic size 1 Screening effect Factors Influencing Electron affinity

9. Variation of electronegativity in a group Variation of electronegativity in a period On moving down the groups, Z increases but Z eff almost remains constant, number of shells (n) increases, r n (atomic radius) increases. Therefore, electronegativity decreases moving down the groups. While moving across a period left to right, Z, Z eff increases & r n decreases. Therefore, electronegativity increases along a period. Electronegativity: Electronegativity is a measure of the tendency of an element to attract shared electrons towards itself in a covalently bonded molecules.

10. Method to measure the value of electronegativity Pauling’s scale : Linus Pauling developed a method for calculating relative electronegativities of most elements. According to Pauling (b) Mulliken’s scale : Electronegativity can be regarded as the average of the ionisation energy (IE) and the electron affinity (EA) of an atom (both expressed in electron volts).

11. 11 Thank you