Periodic Relationships Among the Elements Chapter 8

8.2 PERIODIC CLASSIFICATION OF THE ELEMENTS 8.3 PERIODIC VARIATION IN PHYSICAL PROPERTIES 8.4 IONIZATION ENERGY 8.5 ELECTRON AFFINITY

Classification of the Elements Elements are classified into (5) types 1.Representative elements( Group 1A to 7A) 2. Transition elements (Group 3B to 8B, 1B and 2B) 3. Noble gases(or) Inert gases (Group 8A) 4. Lanthanides (Atomic No. 58 to 71) 5. Actinides (Atomic No. 90 to 103)

Electron Configurations of Cations and Anions Na [Ne]3s 1 Na + [Ne] Ca [Ar]4s 2 Ca 2+ [Ar] Al [Ne]3s 2 3p 1 Al 3+ [Ne] Atoms lose electrons so that cation has a noble-gas outer electron configuration. H 1s 1 H - 1s 2 or [He] F 1s 2 2s 2 2p 5 F - 1s 2 2s 2 2p 6 or [Ne] O 1s 2 2s 2 2p 4 O 2- 1s 2 2s 2 2p 6 or [Ne] N 1s 2 2s 2 2p 3 N 3- 1s 2 2s 2 2p 6 or [Ne] Atoms gain electrons so that anion has a noble-gas outer electron configuration. Of Representative Elements

Na + : [Ne]Al 3+ : [Ne]F - : 1s 2 2s 2 2p 6 or [Ne] O 2- : 1s 2 2s 2 2p 6 or [Ne]N 3- : 1s 2 2s 2 2p 6 or [Ne] Na + (10e), Al 3+ (10e), F - (10e), O 2- (10e), and N 3- (10e) are all isoelectronic with Ne (10 e) Isoelectronic means atoms or ions having same numbers of electrons. What neutral atom is isoelectronic with H - ? H - : 1s 2 same electron configuration as He

Electron Configurations of Cations of Transition Metals When a cation is formed from an atom of a transition metal, electrons are always removed first from the ns orbital and then from the (n – 1)d orbitals. Fe: [Ar]4s 2 3d 6 Fe 2+ : [Ar]4s 0 3d 6 or [Ar]3d 6 Fe 3+ : [Ar]4s 0 3d 5 or [Ar]3d 5 Mn: [Ar]4s 2 3d 5 Mn 2+ : [Ar]4s 0 3d 5 or [Ar]3d 5

Effective nuclear charge (Z eff ) is the “ positive charge ” felt by an electron. Na Mg Al Si Z eff Core Z (At.No) Radius Z eff = Z -  0 <  < Z (  = shielding constant) Z eff  Z – number of inner or core electrons

(a) One half the distance between the nuclei in two neighboring atoms. One-half the distance between the nuclei of the two atoms in a particular molecule.

Cation is always smaller than atom from which it is formed. Anion is always larger than atom from which it is formed.

Ionization energy is the minimum energy (kJ/mol) required to remove an electron from a gaseous atom in its ground state. I 1 + X (g) X + (g) + e - I 2 + X (g) X 2 + (g) + e - I 3 + X (g) X 3 + (g) + e - I 1 first ionization energy I 2 second ionization energy I 3 third ionization energy I 1 < I 2 < I 3 When an electron is removed from an atom, repulsive force among remaining electron decreases.More energy is required to remove another electron from the positively charged ion.Therefore …

Electron affinity (EA) is the negative of the energy change that occurs when an electron is accepted by an atom in the gaseous state to form an anion. X (g) + e - X - (g) 8.5 F (g) + e - X - (g) O (g) + e - O - (g)  H = -328 kJ/mol EA = +328 kJ/mol  H = -141 kJ/mol EA = +141 kJ/mol