Some other periodic properties
Category : JEE Main & Advanced
(1) Atomic volume : It is defined as the volume occupied by one gram atom of an element. Mathematically,
\[\text{Atomic}\,\text{volume}\,=\,\frac{\text{Gram}\,\text{atomic}\,\text{weight}}{\text{Density}\,\text{in}\,\text{solid}\,\text{state}}\]
Units of atomic volume are c.c./mole. Atomic volume signifies the volume occupied by one mole (Avogadro number) of atoms of the given element in solid state. Lower atomic volume generally leads to higher density, increased hardness and brittleness, higher melting and boiling points, less malleability and ductility.
(i) While descending a group, the atomic volume generally increases which is due to increase in the number of shells though the valence electrons in a given group remains constant. (ii) While going left to right across a period the atomic volume first decreases to a minimum and then increases. Francium has the highest atomic volume and boron has lowest atomic volume.
(2) Density : The density of the elements in solid state varies periodically with their atomic numbers. At first, the density increases gradually in a period and becomes maximum somewhere for the central members and then starts decreasing afterwards gradually.
(3) Melting and boiling points : The melting points of the elements exhibit some periodicity with rise of atomic number. It is observed that elements with low values of atomic volumes have high melting points while elements with high values of atomic volumes have low melting points. In general, melting points of elements in any periodic at first increase and become maximum somewhere in the centre and thereafter begins to decreases.
Tungsten has the maximum melting point (3410°C) amongst metals and carbon has the maximum melting point (3727°C) amongst non-metals. Helium has the minimum melting point (-270°C). The metals, \[Cs,Ga\] and Hg are known in liquid state at 30°C.
The boiling points of the elements also show similar trends, however, the regularities are not so striking as noted in the case of melting points.
(4) Oxidation state (Oxidation number, O.N.) : Oxidation number of an element in a compound is the total number of electrons it appears to have gained or lost (negative and positive oxidation states respectively) during the formation of that particular compound.
Note : For detail see chapter redox reaction.
(5) Magnetic properties : Magnetic properties of matter depend on the properties of the individual atoms. A substance (atom, ion or compound) capable of being attracted into a magnetic field is known as paramagnetic. The paramagnetic substances have a net magnetic moment which in turn is due to the presence of unpaired electron(s) in atoms, ions or molecules. Since most of the transition metal ions have unpaired d-electrons, they show paramagnetic behaviour. The exceptions are \[S{{c}^{3+}},\,T{{i}^{4+}},\,Z{{n}^{2+}},\,C{{u}^{+}},\] etc. which do not contain any unpaired electron and hence are diamagnetic.
On the other hand, a substance which is repelled by a magnetic field is known as diamagnetic. Such substances do not have any net magnetic moment because they do not have any unpaired electron. Electrons determine the magnetic properties of matter in two ways,
The observed magnetic moment is therefore the sum of the two moments: the spin moment and the orbital moment. It is expressed in units called Bohr Magnetons (BM). In terms of n (number of unpaired electron), magnetic moment is given by the formula, \[\mu =\sqrt{n\,(n+2)}\]
Greater the number of unpaired electrons in a substance, the greater is the magnetic moment of the substance. The value of magnetic moment has been used to calculate the number of unpaired electrons in an ion. In some cases, even the structure of the molecule or complex is indicated by its magnetic moment.
Paramagnetism is generally measured by a simple device known as Guoy's balance which involves weighing the species in presence of a magnetic field.
Ferromagnetism is a special property observed in some substances in the solid state. Such substances are strongly attracted to magnetic field and may retain the magnetic properties for some time even after the removal of the field. The most common example is of Fe followed by Co and Ni.
(6) Hydration and hydration energy
(i) Hydration energy is the enthalpy change that accompanies the dissolving of 1 mol of gaseous ions in water.
(ii) Size of ions and its charge determines extent of hydration. Greater the charge smaller the size of the ion, greater the attraction for the lone pair of O of \[{{H}_{2}}O\], hence greater the extent of hydration energy.
(a) Size of the hydration ion increases.
(b) Ionic mobility decreases i.e. heavier (hydrated) ions moves slower.
(7) Acid-base-character of oxides
(i) On moving across a period, the basic character of the oxides gradually changes first into amphoteric and finally into acidic character.
(ii) On moving down a group, reverse behaviour is observed i.e., from more acidic to more basic.
(iii) Stability of oxides decreases across a period.
(8) Hydrides
(i) Hydrogen combines with a number of other elements including metals and non-metals to form compounds called hydrides.
(ii) Covalent nature of hydrides increases across a period and decreases down the group.
(iii) Ionic hydride are better reducing agents than covalent hydride and reducing nature of hydride decreases across a period and increases down the group.
(iv) Covalent and ionic hydrides are classified as follows,
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