+2 UNIT 9 PAGE- 4
PHOTOGRAPHY
The process of producing an exact impression of an object on an art paper by using light is called photography. It is based on the chemical behaviour of silver halides, which undego decomposition in light and turn black due to liberation of free silver. The various steps involved are :
(i) Preparation of a sensitive plate or film : A photographic plate or film is essentially a layer of an emulsion of a silver halide (most frequently silver bromide) in gelatine and water applied to a glass or celluloid sheet. The emulsion is prepared by mixing a solution of silver nitrate, gelatine and potassium bromide in hot water (a little potassium iodide is usually present). The silver bromide with little iodide is precipitated and the warm mixture is allowed to stand for some time. The process is called ripening and makes the emulsion more sensitive to light. The emulsion is allowed to cool and set and then washed with water to remove soluble material. It is then melted and applied to glass or celluloid sheet and dried.
(ii) Exposure : The sensitive plate or film is mounted in a camera and exposed for a few seconds to the image of a properly focussed object. Depending up on the colour , shade and intensity of light reflected from different parts of the object, an invisible change occurs in parts of emulsion on which light falls. The light reduces silver bromide to extremely small particles of silver. Inverted images of the object which is formed on the plate is not visible and is therefore called latent image.
2 AgBr 2 Ag + Br2
(iii) Developing : The exposed film or plate is immersed in a solution of developer. A developer is a weak reducing agent such as potassium ferrous oxalate or alkaline solution of organic reducing agents like quinol or pyrogallol. During developing, the parts affected by the light are reduced more so that silver bromide gets reduced to give more black silver.
2 AgBr + C6H4(OH)2 2 Ag + 2 HBr + C6H4O2
quinol (developer) quinone
The developer does not be affected by light. As result, the image becomes visible by developing but its shade is negative in relationship with that of the object i.e., it has exactly opposite shades as compared to those of the object. The plate is therefore , called a negative plate.
(iv) Fixing the image : After the developing, the sensitive emulsion of silver halide is still present on the plate in the parts unaffected by light. Therefore it is necessary to remove it in order to get the permanent image. The negative plate after washing is dipped in a fixing solution of sodium thiosulphate. It dissolves unaffected silver bromide but leaves metallic silver unchanged.
2 Na2S2O3 + AgBr Na3[Ag(S2O3)2] + NaBr
soluble
The plate is throughly washed after the treatment with hypo solution and is subsequently dried. The negative plate can now be taken into light.
(v) Printing : In this process , the negative is placed over a printing paper that has been coated with photographic emulsion(silver bromide mixed with gelatin). The paper is then exposed to light for a fraction of a second. The printing paper is then subjected to developing and fixing as usual, when the shades of the negative are reversed on a printing paper. We get positive print which has shades exactly similar to that of the object.
(vi) Toning : The print obtained is black and white. It can be given a variety of colour shades by various chemical reactions. This process is called toning. The positive print after developing is put into a solution of a desired toning agent. The dark silver particles are replaced by another metal or other coloured salt. For example, when photographic plate is dipped in gold chloride solution or potassium hexachloroplatinate (K2PtCl6) solution, then the silver gains are replaced by gold or platinum and the grey shades become golden or steel grey.
AuCl3 + 3 Ag 3 AgCl + Au (golden)
K2PtCl6 PtCl4 + 2 KCl
PtCl4 + 4 Ag 4 AgCl + Pt (steel grey)
The different stages during photography process are shown below .
LANTHANOIDS AND ACTINOIDS
The elements in which the last electron enters the f-orbital of their atoms are called f-block elements. In these elements , the last electron is added to the third outer most (called antepenultimate) energy level i.e., (n2)f . These elements are called inner transition elements. They consist of two series of elements placed at the bottom of the periodic table. These two series are generated by the filling of characteristic electrons in the 4f and 5f-orbitals.
LANTHANOIDS
The series involving the filling of 4f-orbitals following lanthanum La(Z=57) is called lanthanide series . The elements present in the series are called lanthanides. There are fourteen elements in this series starting from Cerium(Ce) and ending with lutetium, Lu(Z = 71).
ACTINOIDS
The series involving the filling of 5f-orbital is called actinide series . It follows actinium Ac (Z = 89) and the elements present in this series are called actinides. They include the elements from thorium Th ( Z =90) to lawrencium Lw (Z = 103). As in the case of lanthanides, the name of the element is after the preceding element actinium with which these elements closely resemble. This series also consists of 14 elements.
General characteristics
The important general characteristics of lanthanides and actinides are discussed below :
1. Electronic configuration
The lanthanides involve the gradual filling of 4f-orbitals. It may be noted that the energies of d-orbitals (5d) and the next inner shell f-orbitals(4f) are closely similar and therefore, the order of filling the f-orbitals in the atoms shows quite irregularities. The commonly accepted electronic configurations of lanthanides are given in the TABLE.
Element Symbol Atomic number Outer Electronic
configuration
Lanthanum La 57 [Xe] 5d16s2
Cerium Ce 58 [Xe] 4f15d16s2
Praseodymium Pr 59 [Xe] 4f35d06s2
Neodymium Nd 60 [Xe] 4f45d06s2
Promethium Pm 61 [Xe] 4f55d06s2
Samarium Sm 62 [Xe] 4f65d06s2
Europium Eu 63 [Xe] 4f75d06s2
Gadolinium Gd 64 [Xe] 4f75d16s2
Terbium Tb 65 [Xe] 4f95d06s2
Dysprosium Dy 66 [Xe] 4f105d06s2
Holmium Ho 67 [Xe] 4f115d06s2
Erbium Er 68 [Xe] 4f125d06s2
Thulium Tm 69 [Xe] 4f135d06s2
Ytterbium Yb 70 [Xe] 4f145d06s2
Lutetium Lu 71 [Xe] 4f145d16s2
Lanthanum has the electronic configuration [Xe]5d16s2. In the succeeding 14 elements, 14 electrons successively added to 4f-subshell. The single 5d-electrons shift to 4f-subshell in all cases except gadolinium(Z=64), where such a shift gives a symmetry of half-filled 4f-subshell and in Lu (Z = 71), where 4f-subshell has already been completely filled.
The actinides involve the filling of 5f-subbshell. Actinium has the electronic configuration [Rn]6d17s2.From Thorium Th (Z=90) onwards, the 5f-orbitals gets progressively filled. Because of almost equal energy of 5f and 6d-subshells, there are uncertainties regarding the filling of 5f and 6d-subshells. The commonly accepted electronic configurations of these elements are given in the following TABLE.
Element Symbol Atomic number Outer Electronic
configuration
Actinium Ac 89 [Rn] 6d17s2
Thorium Th 90 [Rn] 5f16d17s2
Protactinium Pa 91 [Rn] 5f26d17s2
Uranium U 92 [Rn] 5f36d17s2
Neptunium Np 93 [Rn] 5f46d17s2
Plutonium Pu 94 [Rn] 5f56d17s2
Americium Am 95 [Rn] 5f66d17s2
Curium Cm 96 [Rn] 5f76d17s2
Berkelium Bk 97 [Rn] 5f86d17s2
Californium Cf 98 [Rn] 5f106d07s2
Einsteinium Es 99 [Rn] 5f116d07s2
Fermium Fm 100 [Rn] 5f126d07s2
Mendelevium Md 101 [Rn] 5f136d07s2
Nobelium No 102 [Rn] 5 f146d07s2
Lawrencium Lr 103 [Rn] 5 f146d17s2
2. Oxidation states
All lanthanides exhibit a common oxidation state of +3. In addition, some lanthanides show +2 and +4 oxidation states also. These are shown by those elements which by doing so attain the stable f0, f7 and f14 configurations. For example,
i) Ce and Tb exhibit +4 oxidation states . Cerium and terbium attain f0and f7configurations respectively, when they get +4 oxidation state as shown below :
Ce4+ : [Xe] 4f0
Tb4+ : [Xe] 4 f7
ii) Eu and Yb exhibit +2 oxidation states . Europium and ytterbium get f7and f14 configurations in +2 oxidation state as shown below.
Eu2+ : [Xe] 4 f 7
Yb2+ : [Xe] 4 f14
iii) La, Gd and Lu exhibit +3 oxidation state only because by loosing three electrons, they acquire stable configurations of empty, half-filled and completely filled 4f-subshells.
Exceptional examples : Some other elements show +2 and +3 oxidation states even though they have electronic configurations other than f0, f7 or f14. For example, Sm2+(4f6), Tm2+(4f13), Pr4+(4f1), Dy4+(4f8), Nd4+(4f2), etc.
Actinides show oxidation states of +2, +3, +4, +5 and +6. However, +3 oxidation state is most common among actinides.
3. Physical characteristics
All the lanthanides are soft, malleable and ductile with low tensile strengths. They are not good conductors of heat and electricity.
The actinides are silvery white metals which are highly reactive. They get tarnished when exposed to the attack of alkalies and are less reactive towards acids. All the actinides except thorium and americium have high densities.
4. Colour
Some of the lanthanides and actinides are coloured in the solid state as well as in solution. The colour depends upon the number of f-electrons. The ions containing zero or seven f-electrons are colourless.
5. Magnetic properties
Among the lanthanides, La3+ and Lu3+ which have 4f0 and 4f14 electronic configurations are diamagnetic and all other trivalent lanthanide ions are paramagnetic because they have unpaired electrons. Many of the acinide ions are paramagnetic.
6. Reactivity
All the lanthanides are highly electropositive metals and have almost similar chemical reactivity. This is due to the fact that the lanthanides differ only in the number of 4f-electrons. Since these electrons are very effectively shielded from the interaction by the overlying 5s, 5p and 6s-electrons, they show very little difference in their chemical reactivity. Because of their similar chemical reactivities, their separation from one another is very difficult. The lanthanides are , in general, more reactive than d-block elements. They tarnish readily on exposure to air. In finely divided state, all burn in air to form sesquioxide of the formula Ln2O3 (where Ln stands for lanthanide) except cerium which gives CeO2. Ytterbium, however, resists the action of air even at 1000C due to the formation of a protective coating of its oxide.
7. Basic character of hydroxides
All the lanthanides form hydroxides of formula Ln(OH)3. These are ionic and basic in character. They are stronger bases than Al(OH)3, but weaker than Ca(OH)2. Since ionic sizes decreases from La3+ to Lu3+, the basicity of the hydroxides decreases in the same order. Thus La(OH)3 is the strongest base while Lu(OH)3 is the weakest base.
8. Atomic / Ionic radii and Lanthanide Contraction
In the lanthanide series , with increase in atomic number, the atomic and ionic radii decrease from one element to another but the decrease is very small.
Oxidation states and atomic/ionic radii of Lanthanides
Element Oxidation states Atomic radius (pm) Ionic radius (M3+)
(pm)
La +3 169 106
Ce +3, +4 165 103
Pr +3, +4 165 101
Nd +2, +3, +4 164 101
Pm +3 - 98
Sm +2, +3 166 96
Eu +2, +3 185 95
Gd +3 161 94
Tb +3, +4 159 92
Dy +3, +4 158 91
Ho +3 158 89
Er +3 157 88
Tm +2, +3 156 87
Yb +2, +3 170 86
Lu +3 156 85
For example, on moving from Ce to Lu, the atomic radii decrease from 165 pm to 156 pm and the decrease is only 9 pm. Similarly the ionic radii decrease from 103 pm to 85 pm on moving from Ce3+ to Lu3+ ions and the decrease is only 18 pm. Thus for an increase of atomic number 14, the decrease in atomic radii or ionic radii is very small ; only 9 pm and 18 pm respectively. This is very small decrease in comparison to elements of other groups and periods. The steady state decrease in atomic and ionic sizes of lanthanide elements with increasing atomic number is called lanthanide contraction.
Cause of Lanthanide Contraction
In the lanthanide series, the 4f-electrons are being added one at each step as we move from one element to another. The 4f-electrons shield each other from the nuclear charge quite poorly because of the very diffused shapes of f-orbitals. The nuclear charge, however, increases by one at each step. Hence with increasing atomic number and nuclear charge, the effective nuclear charge experienced by each 4f-electron increases. As a result, the whole of 4f-electron shell contracts on passing across the lanthanides. The sum of the of the successive reductions gives the total lanthanide contraction.
Consequences of Lanthanide Contraction
Some significant consequences of Lanthanide contraction are :
1. Similarity of second and third transition series
The atomic radii of second row of transition elements are almost similar to those of third row of transition elements. For example, among the elements of Group 3, there is normal increase in size from Sc to Y to La. But after lanthanides the atomic radii from second to third transition series do not increase as shown below for groups 4 and 5.
3 4 5
21 Sc
144pm 22 Ti
132 pm 23 V
122 pm
39Y
180 pm 40Zr
160 pm 41Nb
146 pm
57La Lanthanides
187 pm 58 - 71 72Hf
159 pm 73Ta
146 pm
Here the usual increase in size on moving down the group from second to the third transition elements is cancelled by the decrease in size due to lanthanide contraction. Also as a result of lanthanide contraction the second and third rows of transition elements resemble each other more closely than do the first and second rows.
2. Separation of Lanthanides
Separation of lanthanides is also possible due to lanthanide contraction. All the lanthanides have quite similar properties and due to this reason they are difficult to be separated. However, because of lanthanide contraction their properties (such as ability to form complexes ) vary slightly. This slight variation in properties is utilised in the separation of lanthanides by ion exchange methods.
3. Variation in basic strength of hydroxides
Due to lanthanide contraction, the size of the lanthanide ions decreases regularly with increase in atomic number. As a result of decrease in size, their covalent character between lanthanide ion and OH ions increases from La3+ to Lu3+. Therefore the basic strength of hydroxides decreases with increase in atomic number. Thus La(OH)3is most basic and Lu(OH)3 is the least basic.
Comparison of Lanthanides and Actinides
Lanthanides Actinides
1. Lanthanides show mainly +3 oxidation state except in a few cases where it is +2 and +4. 1. In addition to +3 oxidation
state, actinides show higher
oxidation states such as
+4, +5, +6 and +7.
2. The tendency to form
complex is less. 2. They have a greater
tendency to form
complexes.
3. Lanthanide compounds are less basic. 3. Actinide compounds are
more basic.
4. They do not form oxo
ions. 4. They form oxo ions such as
UO2+, NpO+, PuO2+
5. Except promethium, these
are non-radioactive. 5. All the actinides are
radioactive
Uses of Lanthanides
The pure metals have no specific uses and therefore, these metals are extracted as mixtures or alloys. These are called misch metals. The common uses of lanthanides and their salts are given below:
(i) Cerium constitutes 30 - 50% of the alloys of lanthanides. They are used for scavenging oxygen and sulphur from other metals.
(ii) Steel mixed with La, Ce, Pr and Nd is used in the manufacture of flame throwing tanks.
(iii) Addition of 3% misch metal to magnesium increases its strength and it is used in making jet engine parts.
(iv) Lanthanide oxides are used for polishing glass. Neodymium and praseodymium oxides are used for making coloured glasses for goggles. These are particularly useful for glass blowers as they absorb the bright yellow light.
(v) Cerium salts are used in dyeing cotton. They are also used as catalysts.
(vi) Lanthanide compounds are used as catalysts for hydrogenation, dehydrogenation, oxidation and petroleum cracking. They are used in magnetic and electronic devices for their paramagnetic and ferromagnetic properties.
Uses of Actinides
(i) Thorium is used in the form of oxide, ThO2 for making incandescent gas mantles. The mantle is made from silk fibres, is dipped into a mixed solution of thorium and cerium nitrates in the ratio of 99% and 1% respectively. When fixed in the lamp and ignited, the silk fibres burn away leaving a net work of thoria(ThO2) and ceria(CeO2). The small amount of cerium oxide is essential because , ThO2 itself gives only a poor light.
(ii) Thorium salts are also used in medicines in the treatment of cancer.
(iii) Thorium is used in the manufacture of fine rods for atomic reactors.
(iv) Uranium salts impart green colour to glass.
(v) Uranium salts are used in textile industry, ceramic industry as well as in medicines.
(vi) Uranium used in the production of nuclear energy by the process of nuclear fission.
(vii) Plutonium is fissionable material and is used for fuelling atomic reactors. It is used as an ingredient of atomic explosive weapons. It can also be used to make atom bombs.
QUESTIONS
1. Write the electronic configurations of : (I) Cr3+ (ii) Cu+ (iii) Co2+ (iv) Mn2+ (v) Pm3+ (vi) Ce4+ (vii) Lu2+ (viii) Th4+
2. Why are Mn2+ compounds are more stable than Fe2+ towards oxidation to + 3 state ?
3. Explain briefly how +2 state becomes more and more stable in the first half of the first row transition elements with increasing atomic number ?
4. To what extent do the electronic configurations decide the stability of oxidation states in the first series of transition elements ? Illustrate your answer with example.
5. What may be the stable oxidation state of the transition element with the following d-electron configurations in the ground state of their atoms : 3d3, 3d5, 3d8 and 3d4 ?
6. Name the oxometal anions of first series of the transition metals in which the metal exhibits the oxidation state equal to its group number.
7. What is Lanthanide contraction?
8. What the consequences of lanthanoid contraction ?
9. What the characteristics of transition elements and why they are called transition elements ?
10. Which of the d-block elements may not be regarded as transition elements ?
11. What are the different oxidation states exhibited by lanthanoids ?
12. Explain giving reason :
(i) Transition metals and many of their compounds show paramagnetic behaviour.
(ii) The enthalpies of atomisation of the transition metals are high.
(iii) Transition metals generally form coloured compounds.
(iv) Transition metals and theircompounds act as good catalyst.
13. What are interstitial compounds ? Why are such compounds well-known for transition metals ?
14. How is variability in oxidation states of transition metals differ from that of the non-transition metals ? Illustrate with examples.
15. Describe the preparation of potassium dichromate from chromite ore.
16. What is the effect of increasing pH on a solution of potassium dichromate ?
17. Describe the oxidising action on potassium dichromate and write the ionic equations for its reaction with : (a) iodide (b) iron(II) solution (c) H2S
18. Describe the preparation of potassium permanganate.
19. How does the acidified potassium permanganate eacts with (a) ferrous ion (b) SO2 (c) oxalic acid ? Write the ionic requations for the reactions.
20. Predict which of the following will be coloured in aqueous solution?
Ti3+, V3+, Cu+, Sc3+, Mn2+, Fe3+, Co2+ and MnO4
21. Compare the stability of +2 oxidation state for elements of first transition series.
22. Name the chief ore of iron. How is the pig iron convereted into steel ?
23. Describe a method of steel manufacture.
24. Name the chief ore of copper and zinc. Describe the principle of extraction of these metals from the respective ore.
25. Describe the chemistry of the three stages of photography, i.e, exposure, developing and fixing.
26. Compare the chemistry of actinoids with that of lanthanoids with special reference to : (a) electronic configuration (b) oxidation state (c) atomic and ionic sizes (d) chemical reactivity.
27. How would you account for the following :
(a) Of the d4 species, Cr2+ is strongly reducing while manganese(III) is strongly oxidising.
(b) Cobalat (II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised.
(c) The d1 configuration is very unstable in ions.
28. What is meant by disproportionation ? Give examples of disproportionation reaction in aqueous solution.
29. Which metal in the first series of transition metals exhibits +1 oxidation state most frequently and why ?
30. Calculate the number of unpaired electrons in the following gaseous ions : Mn3+, Cr3+, V3+, and Ti3+. Which one of these is the most stable in aqueous solution ?
31. Give example and suggest reasons for the following features of transition metal chemistry :
(a) The lowest oxide of the transition metal is basic, the highest is acidic.
(b) A transition metal exhibits higher oxidation states in oxides and fluorides.
(c) The highest oxidation state is exhibited in oxoanions of a metal.
32. Indicate the steps involved in the preparation of :
(a) K2Cr2O7 from chromite ore.
(b) KMnO4 from pyrolusite ore.
(c) Copper sulphate from metallic copper.
(d) Calomel from corrosive sublimate.
33. What happens when aqueous ammonia reacts with :
(a) siver chloride
(b) Mercury(I)chloride (c) Mercury(II) chloride
34. Write the electronic configurations of elements with atomic number 61, 91, 101 and 109.
35. Compare the chemistry of the actinoids with that of lanthanoids with reference to :
(a) Electronic configuration
(b) Oxidation states
(c) Chemical reactivity