Atomic number 81

 Introduction :

• Atomic number 81 belong to P-block elements.
• Atomic number 81 is Thallium and chemical symbol is ‘Tl’ from periodic table.
• Thallium belongs to Group13 and period 6.
• General electronic configuration of p-block element is [Rare gas] nS² np^{1 to 6}
• Thallium has atomic number 81 and mass number 204.383 The data is obtained from the periodic table.
• Atomic number 81 was found in iron pyrites, crookesite, hutchinsonite, and lorandite.  It is obtained in the by-product of zinc and lead smelting.
• Electronic configuration of Thallium:
• 1S², 2S², 2P⁶, 3S², 3P⁶, 3d¹⁰, 4S², 4P⁶, 4d¹⁰, 4f¹⁴, 5s², 5p⁶, 5d¹⁰, 6S², 6P¹
• Electron per energy level: 2, 8, 18, 32, 18,3
• Number of Electrons (with no charge): 81
• Number of Neutrons (most common/stable nuclide): 123
• Number of Protons: 81
• Oxidation States: 3,1
• Crystal Structure of Atomic number 81 is Hexagonal.
• Density (293 K) of thallium is 11.85 g/cm³.
• In Greek thallos mean green twig, representation for bright green line in its spectrum.
• Thallium is a Soft gray metal that looks like lead.
• Sir William Crookes discovered Thallium in the year 1861 in England.
• Thallium belongs to metal group.

Thallium is very soft and malleable and at room temperature, it can be cut with a knife.  Thallium has a metallic luster, but by exposing to air, it quickly diminishes with a bluish-gray tinge that resembles lead. It is preserved by keeping it under oil.

Image of Thallium metal: Appearance of thallium metal is silvery white colour.
                                            


Properties of Atomic number 81

• Atomic radius and ionic radius of Group 13 elements: Atomic radius and ionic radius increases down the group from boron to thallium

Elements	Boron	Aluminum	Gallium	Indium	Thallium
Atomic radius (pm)	85	121	135	155	190
Ionic radius (pm)	41	53.5	76	94	102.5

• Ionization potential of Thallium:

      First Ionization potential: 6.1083 eV.
      Second Ionization potential: 20.428 eV.
      Third Ionization potential: 29.829 eV.

• Oxidation states of Atomic number 81: Group 13 elements exhibit oxidation state of +3.  Thallium exhibit oxidation state of +1 and +3.  It is exhibited when ns², np¹ electrons are involved in bonding.

                   Tl       : [Xe] 4f¹⁴, 5d¹⁰, 6s², 6p¹
                            Tl¹⁺     : [Xe] 4f¹⁴, 5d¹⁰, 6s², 6p⁰
                   Tl³⁺     :  [Xe] 4f¹⁴, 5d¹⁰, 6s⁰, 6p⁰

• Inert pair effect of Thallium: In Inert pair effect, the outermost s electrons to remain no ionized or unshared in compounds of post-transition metals (or p-block elements). The term inert pair effect is frequently used in relation to the increasing stability of oxidation states that are 2 less than the group valence for the elements of groups 13, 14, 15 and 16. The term "inert pair" was first proposed by Nevil Sidgwick proposed the term "inert pair" in 1927. As an example in group 13 the Tl has+1 oxidation state and it is the most stable one and Tl^III compounds are comparatively less. The stability of Group 13 elements is given in the order,

                                        Al^I < Ga^I < In^I < Tl^I.

• Melting point (M.P.) and boiling points (B.P.) of Group 13 elements: Melting point depends on the size of the atom.  Smaller the atomic size, higher is the meting point. Boiling point decreases from boron to thallium. 

Elements	Boron	Aluminum	Gallium	Indium	Thallium
M.P.  (0C)	4275	2740	2475	2350	1745
B.P.  (0C)	2300	933.25	302.9	429.75	577

• Isotopes of Thallium:  There are 25 isotopes in thallium.  Atomic masses ranges from 184 to 210. Stable isotopes are only ²⁰³Tl and ²⁰⁵Tl .  ²⁰⁴Tl is the most stable radioisotope which is having a half-life of 3.78 years.

Uses of Thallium

• Thallium sulphate is odorless and tasteless and was once widely used as rat poison and ant killer. Since 1972 it is prohibited.
• To treat ringworm, other skin infections and to reduce the night sweating of tuberculosis patients, thallium salts were used. However it is limited due to their narrow therapeutic index.

Chemical reaction of Thallium

• Chemical reaction of thallium with air: When thallium metal is heated to red hot in the presence of air, thallium (1) oxide which is poisonous is formed.

                 2Tl _(s) + O_{2 (g)} → Tl₂O _(s)

• Chemical reaction of thallium with water: When Thallium metal is exposed to moist air, it tarnishes slowly and then it dissolves in water to form thallium (1) hydroxide which is poisonous is formed.

                2Tl _(s) + 2H₂O _(l) → 2Tl (OH) _(aq) + H_{2 (g)} ↑

• Chemical reaction of thallium with halogens: Thallium metal reacts rapidly with halogen to form dihalides.  Thallium (111) fluoride, thallium (111) chloride and thallium (111) bromide are formed.  All these are poisonous.

                                        2Tl(s) + 3F₂ (g) → 2TlF₃(s)
                                        2Tl(s) + 3Cl₂ (g) → 2TlCl₃(s)
                                        2Tl(s) + 3Br₂ (l) → 2TlBr₃(s)

• Reaction of thallium with acids: Thallium reacts with sulphuric acid and hydrochloric acid slowly