Atomic radii is a useful property for determining many aspects of chemistry of elements such as physical and chemical properties. The periodic table greatly assists in finding a number of trends. The transition metals belong to group 3 - 12 and are generally characterised by partially filled d-orbitals in the free elements or their cations. The atomic radii of transition metals show a small decrease from left to right across a period, then remain nearly constant, and finally, slight increase towards the end of the series. The trend is a result of interplay between the increasing nuclear charge and the increasing shielding effect from the filling of d-orbitals. Initially the nuclear charge dominates, leading to decrease in size, but as more d-electrons are added, their shielding becomes more effective, balancing the increased nuclear charge and causing the radii to become constant.
The increase in size between 3d and 4d metals is much greater than between the 4d and 5d metals. The atomic radii of 2nd and 3rd series are almost the same due to lanthanoid contraction. Increase in density is also observed on moving from left to right in a period.
Why do transition metals have high \( \Delta_{a}H^{\circ} \) ?
Atomic radii is a useful property for determining many aspects of chemistry of elements such as physical and chemical properties. The periodic table greatly assists in finding a number of trends. The transition metals belong to group 3 - 12 and are generally characterised by partially filled d-orbitals in the free elements or their cations. The atomic radii of transition metals show a small decrease from left to right across a period, then remain nearly constant, and finally, slight increase towards the end of the series. The trend is a result of interplay between the increasing nuclear charge and the increasing shielding effect from the filling of d-orbitals. Initially the nuclear charge dominates, leading to decrease in size, but as more d-electrons are added, their shielding becomes more effective, balancing the increased nuclear charge and causing the radii to become constant.
The increase in size between 3d and 4d metals is much greater than between the 4d and 5d metals. The atomic radii of 2nd and 3rd series are almost the same due to lanthanoid contraction. Increase in density is also observed on moving from left to right in a period.
Why do transition metals have high \( \Delta_{a}H^{\circ} \) ?
Show Hint
Solution and Explanation
\( \Delta_{a}H^{\circ} \) is the enthalpy of atomisation. It is the energy needed to convert one mole of the solid metal into gaseous atoms.
Step 2: Link it to the bonding in the metal:
Transition metals have many unpaired electrons in the (n-1)d subshell, besides the ns electrons.
All of these electrons take part in metallic bonding between the atoms.
More electrons in bonding means stronger interatomic attraction, so the metallic bond is very strong.
A lot of energy is needed to break these bonds and free the atoms.
Hence the values of \( \Delta_{a}H^{\circ} \) are high. The metals also have high melting and boiling points for the same reason.
Final Answer:
Many unpaired (n-1)d electrons add to the ns electrons in metallic bonding, so the bonds are strong and atomisation needs a lot of energy.
Top CBSE CLASS XII Chemistry Questions
- ${AgNO_3}$ does not decompose where :
- Azeotropic mixture of water and HCl boils at 381.5 K. By distilling the mixture it is possible to obtain
Write IUPAC names of the following compounds and classify them into primary, secondary and tertiary amines.
(i) (CH3 )2CHNH2 (ii) CH3 (CH2 )2NH2 (iii) CH3NHCH(CH3 )2
(iv) (CH3 )3CNH2 (v) C6H5NHCH3 (vi) (CH3CH2 )2NCH3 (vii) m–BrC6H4NH2
Give one chemical test to distinguish between the following pairs of compounds.
(i) Methylamine and dimethylamine
(ii) Secondary and tertiary amines
(iii) Ethylamine and aniline
(iv) Aniline and benzylamine
(v) Aniline and N-methylanilineAccount for the following:
(i) pKb of aniline is more than that of methylamine.
(ii) Ethylamine is soluble in water whereas aniline is not.
(iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide.
(iv) Although amino group is o– and p– directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m-nitroaniline.
(v) Aniline does not undergo Friedel-Crafts reaction.
(vi) Diazonium salts of aromatic amines are more stable than those of aliphatic amines. (vii) Gabriel phthalimide synthesis is preferred for synthesising primary amines.
Top CBSE CLASS XII General Properties of the Transition Elements (d-Block) Questions
- From the given data of standard electrode potential E°M2+/M values (in volts): Cr = -1.18, Mn = -1.18, Fe = -0.44, Co = -0.28, Ni = -0.25, Cu = +0.34, answer the following: (I) Why does E°M2+/M show an irregular trend in the above values? (II) Why is E°Cu2+/Cu value exceptionally positive? (III) Why is E°Mn2+/Mn value highly negative?
- Why do transition metals act as good catalysts?
- Why does Cr have a higher melting point than Mn?
- From the given data of E° values, answer the following questions:
E°M2+/M (V): Cr = –1.18, Mn = –1.18, Fe = –0.44, Co = –0.28, Ni = –0.25, Cu = +0.34
(I) Why does E°M2+/M show an irregular trend in the above values?
(II) Why is the E°Cu2+/Cu value exceptionally positive?
(III) Why is the E°Mn2+/Mn value highly negative? - In aqueous acidified solution, CrO$_4^{2-}$ ion converts to which of the following ?
Top CBSE CLASS XII Questions
- 2, b, c are in A.P. and the range of determinant $\begin{vmatrix}1&1&1\\ 2&b&c\\ 4&b^{2}&c^{2}\end{vmatrix}$ is [2, 16]. Then find range of c is
- A boy of mass 50 kg is standing at one end of a, boat of length 9 m and mass 400 kg. He runs to the other, end. The distance through which the centre of mass of the boat boy system moves is
- A capillary tube of radius r is dipped inside a large vessel of water. The mass of water raised above water level is M. If the radius of capillary is doubled, the mass of water inside capillary will be
- A circular disc is rotating about its own axis. An external opposing torque 0.02 Nm is applied on the disc by which it comes rest in 5 seconds. The initial angular momentum of disc is
- A circular disc is rotating about its own axis at uniform angular velocity \(\omega.\) The disc is subjected to uniform angular retardation by which its angular velocity is decreased to \(\frac {\omega}{2}\) during 120 rotations. The number of rotations further made by it before coming to rest is