Group Study of Physical and Chemical Characteristics of Transition Elements

Scandium Group

Transition elements have been put in ten vertical columns or groups. The groups are III_B, IV_B, V_B, VI_B, VII_B, VIII (comprising of three vertical columns) and I_B and II_B. These elements are placed between alkaline earth metals and p-block elements in the long form table.

In the long form table, they start from group 3 to 12. Group III includes elements like Scandium, Yttrium, Lanthanum and Actinium and are called as scandium group elements (four elements). The trends in their properties arc quite regular and similar to trends in group I and II elements. They cater to important industrial uses. These elements were earlier called rare earths, but it is a misnomer as many of them are quite abundant on the earth’s surface. Due to similarity in their properties it is difficult to separate the individual elements. They are found together with lanthanides in the form of MⁿⁱC0₃F (bastnaesite) and monazite MⁿⁱP0₄. The extraction of these metals from their compounds is difficult. They are electropositive and react with water.

Physical Characteristics

Occurrence

These elements occur together with lanthanides. They are found mixed with monazite MⁿⁱP0₄ and other minerals. Their oxides are very stable therefore, they cannot be separated by thermite reaction. So, the metals are obtained by reduction of the chlorides and fluorides with calcium at 1000°C under inert atmosphere of argon.

Yttrium is used in red phosphor for TV tubes and to make synthetic garnets used in radar and as gemstones. Lanthanum is produced as Mischmetal (unseparated metal). It is used to improve steel and form alloys with magnesuim. La^O, is used in optical glass to protect from UV light. Actinium is obtained from thorium and uranium in the form of decay product. Ac is radioactive and has half life from 6 hours to 21.8 years. It has ²^Ac and ²²²Ac as radio isotopes.

Atomic Size

The covalent and ionic radii of these elements increase gradually down the group. The second and third elements have almost same size due to lanthanide contraction, but it is observed only after lanthanum.

Chemical Characteristics

These metals are reactive and reactivity increases with increased atomic size down the group. These metals react with water to form basic oxides (MO • OH) and liberate H, gas. The reaction is fast in hot water.

(//) Oxides

Similarly, the basic oxide of ScO.OH is obtained. Sc dissolves in NaOH liberating H₂ gas.

The basic character of hydroxides is in the following order.

The oxides and hydroxides being basic form salt with acids.

Electronic Configuration

The valence shell configuration of these elements are (w-1 )d¹ns². Their most common oxidations state is +3.

Oxidation States

All the four elements show +3 oxidation state in solution in which both ns and one electron from (ji - l)d are removed. The M³⁺ salts have all paired electrons and hence arc colourless and diamagnetic.

La(OH)₃ is a strong base and can liberate NH₃ from ammonium salts. Their hydroxides are decomposed to oxides on heating (similar to Group 2 elements)

Halides

They react with halogens to form MX₃ trihalides which resemble Ca halides. The fluorides are insoluble like CaF₂. The chlorides are deliquescent and crystallise as hydrates.

Anhydrous Sc€l₃ cannot be used in FCR reaction in place of anhydrous A1C1₃.

The elements react with hydrogen on heating at 300°C forming MH₃. The hydrides react with H-,0 liberating hydrogen. The hydrides are salt like containing H“. When heated with carbon, Sc forms ScC, which in turn reacts with H-,0 to form ethyne.

Complexes

The elements in the group do not have a strong tendency to form complexes in +3 state. Sc³⁺ ion is the smallest in the group, hence it readily forms complexes e.g., [Sc(OH)₆]^3_ and [ScF₆]^3_ both are octahedral in shape.

Y and La have larger atomic size, they form complex with coordination number of 7 or 8. The common complexes with complexing ligands oxalic acid, citric acid, acetylacctone and edta e.g. [Y(acac)₃(H,0)] 7-coordinate with monocapped trigonal prism and {La(ac ac)₃(H,0)₂} 8 coordinate with square antiprism structure, {La edta (11,0)4} is 10 coordinate and {La (S0₄)₃ 9(H,0)} is 12 coordinate.

Similarity in La³⁺ and Ca²⁺ compounds is observed because of similar size [La = 1.032A and Ca = 1.00А]. In biological role, La³⁺ appears to replace Ca²⁺ in nerve impulses.