Electrical Conductance

One of the most familiar properties of metals is their ability to conduct electricity. This property is mainly due to the presence of relatively loose electrons in the outermost shell of the element and ease of their movement in the solid lattice. The electrical conductance of metals in groups IA and IIA, generally increases from top to bottom. However, the trend is not free from the individual variation in different atoms. Metals of group IB, which are known as coinage metals, have extraordinary high values of electrical conductance. Non-metals, on the other hand, especially of groups VIA and VIIA, show such low electrical conductance that they can be considered as bad conductors or insulators.

In the series of transition metals, the values of electrical conductance vary so abruptly That no general trend can be assigned to them. Carbon, in the form of diamond is non-conductor Because all of its valence electrons are tetrahedrally bound and unable to move freely, while in the Form of graphite, carbon is fairly good conductor because one of its four valence electrons is Relatively free to move. The lower elements of group IVA, tin and lead, are fairly good conductors And their values of electrical conductivity are comparable with those of their counterparts in Group IA.

Hydration Energy

The hydration energy is the heat absorbed or evolved when one Mole of gaseous ions dissolve in water to give an infinitely dilute solution. For example, when one mole of gaseous hydrogen ions are dissolved in Water resulting an infinitely dilute solution, a large amount of heat is Liberated. Hydration energies of a few negative and positive ions are shown In the Table 1.2.

It is evident from the table that hydration energies highly depend Upon charge to size ratio of the ions. For a given set of ions, for example of Group IA, charge to size ratio decreases from top to bottom in a group, the Hydration energy also decreases in the same fashion. On the contrary, the Hydration energy increases significantly on moving from left to right in a Period as the charge to size ratio increases, as found in the metal ions of Third period.

PERIODIC RELATIONSHIP IN COMPOUNDS

Halides:

Halides are the binary compounds which halogens form with other elements. The Physical properties of halides are largely determined by the nature of bonding present in them. On this basis, halides can be classified into two general classes: ionic and covalent. In between The two, there is another class of halides in which the halogen atom acts as a bridge between the Two atoms of the other element, such halides are termed as “Polymeric” halides. Strongly Electropositive elements, having greater electronegativity difference with halogen atom, form Ionic halides. The halides of group IA are considered purely ionic compounds, which are high Melting point solids.

 Such halides have three-dimensional lattices consisting of discrete ions. Among the pure ionic compounds, the fluorides have the highest lattice energies due to the small Size of fluoride ion. Thus for ionic halides, the fluorides have the highest melting and boiling Points which decrease in the order: fluoride >chloride > bromide > iodide. Less electropositive elements, such as Be, Ga and Al form polymeric halides having Partly ionic bonding with layer or chain Lattices. The lattice of SiCl, consists of discrete Molecules, which are highly polar.

The bonds In PCI,, and S,Cl, are less polar than those of SiCl, On moving across the periodic table from Left to right, the electronegativity difference Decreases and the trend shifts towards covalent Halides. The gradual change in bond type and Melting points of the chlorides on moving Across period 3 of the periodic table is shown in Table. 1.3.

As the intermolecular forces in covalent halide molecules are weak van der Waal’s forces So they are often gases, liquids or low melting point solids. Physical properties of covalent Halides are influenced by the size and polarizability of the halogen atom. Iodides, as being the Largest and more polarizable ions, possess the strongest van der Waal’s forces and therefore have Higher melting and boiling points than those of other covalent halides. The variation in bonding character is also present in descending from top to bottom in the Halogen group.

 In general, for a metal the order of decreasing ionic character of the halides is: Fluoride > chloride > bromide > iodide. For example, AIF, is purely ionic compound having Melting point 1290°C and fairly a good conductor, whereas, All, is predominantly covalent with Melting point 198°C and electrically a non-conductor. In case of an element forming more than

One halides, the metal halide in its lower oxidation state tends to be ionic, while that in the higher Oxidation state is covalent. For example, PbCl, is mainly ionic and PbCl, is fairly covalent. This Can again be explained by the high polarizing power of Pb” as compared to that of Pb².

Hydrides

The binary compounds of hydrogen with other elements are called hydrides. According To the nature of bonding, hydrides may be broadly classified into three classes: ionic, covalent And intermediate. The elements of group IA and the heavier members of group IIA form ionic Hydrides, which contain H (Hydride) ion. These hydrides are crystalline solid compounds, with High melting and boiling points and which conduct electricity in molten state. The tendency Towards covalent character increases on moving from left to right in the Periodic Table. Hydrides Of beryllium and magnesium represent the class of intermediate hydrides. Their properties are in Between the ionic and covalent hydrides. They have polymeric structures and covalent nature, Table 1.4.

The covalent hydrides are usually gases or volatile liquids. They are non-conductors and Dissolve in organic solvents. Their bond energies depend on the size and the electronegativity of The element. Stability of covalent hydrides increases from left to right in a period and decreases From top to bottom in a group. Fluorine forms the most stable hydride and the least stable are Those of thallium, lead and bismuth. These hydrides are formed by elements with Electronegativity values greater than 1.8 (Pauling Scale).

 Since the electronegativity of hydrogen is 2.1, most of these hydrides have polar Covalent bonds in which hydrogen is Carrying a slight positive charge. On Moving from left to right across a period The electronegativity of the other Element increases and the hydrogen- Element bond becomes more polar. Due To high polarity the hydrides like H₂O And HF are capable of forming hydrogen Bonds between their molecules. The Boiling points of covalent hydrides Generally increase on descending a Group as shown in Table 1.5, except the Hydrides like H,O, HF and NH, which,