Advertisements
Advertisements
प्रश्न
What are the characteristics of the transition elements and why are they called transition elements?
Advertisements
उत्तर
- Electronic configuration: General electronic configuration is (n − 1) d1−10ns0−2. This configuration reflects the filling of d-orbitals in addition to the s-orbitals of the outermost shell.
- Metallic character: Transition elements, except for Zn, Cd and Hg, exhibit metallic structures and typical metallic properties like malleability, ductility and conductivity.
- Atomic and ionic size: Across a transition series, the atomic and ionic radii decrease due to the increasing nuclear charge, which pulls the d-electrons closer to the nucleus (a phenomenon called the lanthanide contraction).
- Oxidation state: They exhibit variable oxidation states due to the participation of both d and s-electrons in bonding. Common oxidation states range from +2 to +7, depending on the element.
- Paramagnetism: Ions with unpaired d-electrons exhibit paramagnetism. The greater the number of unpaired electrons, the stronger the paramagnetic behaviour.
- Ionisation enthalpy: Ionisation enthalpy increases gradually across the series as the nuclear charge increases, making it harder to remove electrons.
- Formation of coloured ions: Transition metal ions are often colored because of electronic transitions within the d-orbitals (specifically d-d transitions) when they absorb visible light.
- Formation of complex compounds: Transition metals form complexes due to their small size, high charge density, and availability of vacant d-orbitals to accept electron pairs from ligands.
- They possess catalytic properties: They act as catalysts because they can exist in multiple oxidation states, which facilitates various chemical reactions.
- Formation of interstitial compounds: Transition metals can form interstitial compounds by trapping small non-metal atoms (such as H, C, or N) within their crystal lattice.
- Alloy formation: They readily form alloys due to similar atomic sizes, which allows different metal atoms to substitute for one another in the crystal structure.
They are positioned between s and p-block elements and are known as transition elements because of their incompletely filled d-orbitals in the ground state or any stable oxidation state.
संबंधित प्रश्न
Which of the following cations are coloured in aqueous solutions and why ?
Sc3+, V3+, Ti4+, Mn2+ (At. Nos. Sc = 21, V = 23, Ti = 22, Mn = 25)
Complete the following chemical equations:
`(i) Cr_2O_7^(2-)+6Fe^(2+)+14H^+ ->`
`(ii) 2CrO_4^(2-)+2H^+ ->`
`(iii) 2MnO_4^-+5C_2O_4^(2-)+16H^+ ->`
To what extent do the electronic configurations decide the stability of oxidation states in the first series of the transition elements? Illustrate your answer with examples.
NF3 is possible, but NF5 is not. Why?
Explain why transition elements form alloys.
Give reason for the following:
The transition metals generally form coloured compounds.
Read the passage given below and answer the following question:
The transition metals when exposed to oxygen at low and intermediate temperatures form thin, protective oxide films of up to some thousands of Angstroms in thickness. Transition metal oxides lie between the extremes of ionic and covalent binary compounds formed by elements from the left or right side of the periodic table. They range from metallic to semiconducting and deviate by both large and small degrees from stoichiometry. Since electron bonding levels are involved, the cations exist in various valence states and hence give rise to a large number of oxides. The crystal structures are often classified by considering a cubic or hexagonal close-packed lattice of one set of ions with the other set of ions filling the octahedral or tetrahedral interstices. The actual oxide structures, however, generally show departures from such regular arrays due in part to distortions caused by packing of ions of different size and to ligand field effects. These distortions depend not only on the number of d-electrons but also on the valence and the position of the transition metal in a period or group.
In the following questions, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices on the basis of the above passage.
Assertion: Cations of transition elements occur in various valence states.
Reason: Large number of oxides of transition elements are possible.
Interstitial compounds are formed when small atoms are trapped inside the crystal lattice of metals. Which of the following is not the characteristic property of interstitial compounds?
When a chromite ore (A) is fused with sodium carbonate in free excess of air and the product is dissolved in water, a yellow solution of compound (B) is obtained. After treatment of this yellow solution with sulphuric acid, compound (C) can be crystallised from the solution. When compound (C) is treated with KCl, orange crystals of compound (D) crystallise out. Identify A to D and also explain the reactions.
Read the passage given below and answer the following question.
|
Are there nuclear reactions going on in our bodies? There are nuclear reactions constantly occurring in our bodies, but there are very few of them compared to the chemical reactions, and they do not affect our bodies much. All of the physical processes that take place to keep a human body running are chemical processes. Nuclear reactions can lead to chemical damage, which the body may notice and try to fix. The nuclear reaction occurring in our bodies is radioactive decay. This is the change of a less stable nucleus to a more stable nucleus. Every atom has either a stable nucleus or an unstable nucleus, depending on how big it is and on the ratio of protons to neutrons. The ratio of neutrons to protons in a stable nucleus is thus around 1 : 1 for small nuclei (Z < 20). Nuclei with too many neutrons, too few neutrons, or that are simply too big are unstable. They eventually transform to a stable form through radioactive decay. Wherever there are atoms with unstable nuclei (radioactive atoms), there are nuclear reactions occurring naturally. The interesting thing is that there are small amounts of radioactive atoms everywhere: in your chair, in the ground, in the food you eat, and yes, in your body. The most common natural radioactive isotopes in humans are carbon-14 and potassium-40. Chemically, these isotopes behave exactly like stable carbon and potassium. For this reason, the body uses carbon-14 and potassium-40 just like it does normal carbon and potassium; building them into the different parts of the cells, without knowing that they are radioactive. In time, carbon-14 atoms decay to stable nitrogen atoms and potassium-40 atoms decay to stable calcium atoms. Chemicals in the body that relied on having a carbon-14 atom or potassium-40 atom in a certain spot will suddenly have a nitrogen or calcium atom. Such a change damages the chemical. Normally, such changes are so rare, that the body can repair the damage or filter away the damaged chemicals. The natural occurrence of carbon-14 decay in the body is the core principle behind carbon dating. As long as a person is alive and still eating, every carbon-14 atom that decays into a nitrogen atom is replaced on average with a new carbon-14 atom. But once a person dies, he stops replacing the decaying carbon-14 atoms. Slowly the carbon-14 atoms decay to nitrogen without being replaced, so that there is less and less carbon-14 in a dead body. The rate at which carbon-14 decays is constant and follows first order kinetics. It has a half-life of nearly 6000 years, so by measuring the relative amount of carbon-14 in a bone, archeologists can calculate when the person died. All living organisms consume carbon, so carbon dating can be used to date any living organism, and any object made from a living organism. Bones, wood, leather, and even paper can be accurately dated, as long as they first existed within the last 60,000 years. This is all because of the fact that nuclear reactions naturally occur in living organisms. |
Researchers have uncovered the youngest known dinosaur bone, dating around 65 million years ago. How was the age of this fossil estimated?
The standard electrode potentials of four elements A, B, C and D are – 3.05, – 1.66, – 0.40 and + 0.80. The highest chemical reactivity will be exhibited by
On strong heating AgNO3, the gases evolved are:-
Mercury is the only metal liquid at room temperature due to its:-
Passing H2S gas into a mixture of Mn2+ and Ni2+, Cu2+, ions in an acidified aqueous solution precipitates.
Among the following pairs of ions, the lower oxidation state in aqueous solution is more stable than the other in:-
Agcl is soluble in NH4OH. The solubility is due to the information of:-
On adding NaOH, solution to the aqueous solution of K2CrO7 the colour of the solution changes from
Assertion (A): Transition metals show their highest oxidation state with oxygen.
Reason (R): The ability of oxygen to form multiple bonds to metals.
Give a reason for the following:
Zinc, cadmium and mercury are considered as d-block elements but not regarded as transition elements.
Decide which of the following atomic numbers are the atomic numbers of the inner transition elements:
29, 59, 74, 95, 102, 104
