Chemistry

Periodicity of Compounds in periodic Table

The properties of the elements affect the properties of their compounds. We find a number of correlations between the properties of compounds and the positions of their constituent elements in the periodic table.

Such correlations are especially clear in halides, hydrides, oxides and Sulphides.

(i) Halides. With the exception of few elements, all other elements combine directly with halogens to form halides. Chlorides are compounds with chlorine in the -1 oxidation state.

Going from left to right in a period, there is a general trend in the properties of halides. Elements on the left side of the periodic table, such as groups.

Groups in Periodic Table

IA and IIA metals, tend to form stable ionic halides with high melting point Elements on the right side of the periodic table tend to form unstable nonionic halides that are gases or low boiling liquids. However, as we move along a period with a decrease in ionic radius, i.e., with an increase in the charge radius ratio, the covalent character of the halides increases.periodic Table

Thus amongst chlorides of 2nd-period elements e.g. LiCl, BeC12, BC13, CC14 and NC13, LiCl is an ionic compound, BeC1 has considerable covalent character, BC13 is partially ionic and CC14 is essentially covalent and volatile liquid. NC13 is covalent and typical gas.

On moving down the group, the electropositive nature of the element increases and thus the ionic nature of the halides increases.

Some of the metals form halides in several oxidation states. In such cases, the halides formed in the lower oxidation states are ionic while those in the higher oxidation states tend to be covalent. Thus while PbC12 is largely ionic, PbC14  is largely covalent.this is because Pb4+ has high polarizing power as compared to that of  Pb 2+.  Pb4+ polarizes chloride ions and induces covalent character according to Fajan’s rules.

Size of the Halide

The size of the halide ion is another important factor in determining the character of the halides of the elements. The covalent character of the halide containing the same action but different anions belonging to the same  group increases from top to bottom in a group.

For example,  AlF3 has essentially ionic character, A1Cl3 has intermediate character while A1Br3 and AlI 3 have essentially a covalent character.

Most of the ionic halides are soluble in water. There are, however, a fewer exception as for example PbC12 , AgCl. The covalent halides do not show any trends in their solubility.

Polymeric halides in Periodic Tablepolymerric halides

In between ionic and covalent halide,s there is another class of halides in which the halogen atom acts as a bridge between the two atoms of the other elements; sue halides are called “Polymeric halides”. Less electropositive elements like Be, G and Al form polymeric halides having partly ionic bonding with layer or chai lattice.

Hydrides

Hydrides are binary compounds of hydrogen.  The elements of group IA and the heavier elements of· group IIA form ionic hydrides which contain H (hydride) ion. These hydrides are crystalline solid compounds, with high melting and boiling points and they conduct electricity in the molten state.

These hydrides react with water to liberate H2 gas and form hydroxide ion producing alkaline solution Ionic hydrides are useful reducing agents and usually, their reducing power decreases across a period and increase down a group. The hydrides of Be and  Mg are in between the ionic and covalent hydrides.

They have polymeric structures and covalent nature. Almost all the elements of p-block react with. hydrogen to form covalent hydrides which are volatile. There· is, however, some ionic character in them depending upon the electronegativity of the element.

As electronegativity increases along the period, the covalent hydrides become more and more acidic.

Thus  the acidity of hydrides  of 2nd-period elements  CH 4 ,  NH 3  ,  H2O and  HF  increases  with the electronegativity of the non-metal present from  C  to  F.  The  acidity  can  also  be related to increasing the stability of their anions as shown below

CH    <  NH;   < OH-  < F

Increasing stability in Periodic Table

The elements of group IIIA form characteristic covalent hydrides·. These hydrides are electron deficient molecules and form addition compounds molecules with donor The groups VIA and VIIA hydrides (H2O, H2S, H2Se and HF, HCl, HBr, HI) show an increase in acidity on going down the group which is against normal expectation based op electronegativity concept.

Here the major factor is the heat of dissociation which shows a fall down a group in the hydrides of these groups, thus causing an increase in the acid strength of the hydrides. The hydrides of the heavier elements in group IIIA through VIA are not stable and often are virtually impossible to prepare.

The hydrides of transition metals usually are hard compounds with high melting points. They tend to have infinite compositions with vary number of hydrogen atoms fitted into spaces between metal atoms in the solid. Periodic Table

Oxides

With the exception of some of the noble gases, every element forms at least one binary compound with oxygen called oxide. Some elements form a number of different oxides.

Vanadium, a transition metal, forms VO, V2O3 ,. VO2 , V2O5 . · Chlorine, a non-metal, form Cl2O, Cl2O3, Cl0 2, Cl2O6 and Cl2O7 . The nature of the oxide is determined primarily by the position of that element in the periodic table.

Oxides of elements on the left side of the periodic table, such as those of the alkali and alkaline earth metals (except for BeO, which is covalent) are ionic solids. These oxides are basic; they react with water to form hydroxides.  For  sodium  oxide the reaction is

Na 2O     + H2O     →       2Na+ + 2OH-

Many ionic oxides are very refractory, that is, they can be heated to the high temperature without melting or decomposition: For example, calcium oxide, CaO “quicklime” has a melting point of above2005°C.

Oxides of the metallic elements toward the middle· of the table and of the semimetals are solids but often are not ionic Oxides of non-metals are discrete, separate molecules,  generally, exist as liquids or gases at room temperature.

These properties of the elements affect the properties of their compounds. We find a number of correlations between the properties of compounds and the positions of their constituent elements in the periodic table. Such correlations are especially clear in halides, hydrides, oxides and Sulphides.

Halides

With the exception of few elements, all other elements combine directly with halogens to form halides. Chlorides are compounds with chlorine in the -1 oxidation state.

Going from left to right in a period, there is a general trend in the properties of halides. Elements on the left side of the periodic table, such as groups IA and IIA metals, tend to form stable ionic halides with high melting point. Elements on the right side of the periodic table tend to form unstable nonionic halides that are gases or low boiling liquids.

Ionic Radius in Periodic Table

However, as we move along a period with a decrease in ionic radius, i.e., with an increase in the charge radius ratio, the covalent character of the halides increases.Ionic Radius in Periodic Table

Thus amongst chlorides of 2nd-period elements e.g. LiCl, BeC12, BC13, CC14 and NC13, LiCl is an ionic compound, BeC1 has considerable covalent character, BC13 is partially ionic and CC14 is essentially covalent and volatile liquid.

NC13 is covalent and typical gas. On moving down the group, the electropositive nature of the element increases and thus the ionic nature of the halides increases.

Some of the metals form halides in several oxidation states. In such cases, the halides formed in the lower oxidation states are ionic while those in the higher oxidation states tend to be covalent. Thus while PbC12 is largely ionic, PbC14  is largely covalent.this is because Pb4+ has high polarizing power as compared to that of  Pb 2+.  Pb4+ polarizes chloride ions and induces covalent character according to Fajan’s rules.

Size of Halide Ion

The size of the halide ion is  another important factor in determining the character of the halides of the elements. The covalent character of the halide containing  the  same  cation   but  different  an’ ions  belonging  to  the  same  group increases  from top to bottom in  a group.

For example,  AlF3 has essentially ionic character, A1Cl3 has intermediate character while A1Br3 and AlI 3 have essentially a covalent character. Most of the ionic halides are soluble in water. There are, however, a fewer exception as for example PbC12 , AgCl. The covalent halides do not show any trends in their solubility.

In between ionic and covalent halides, there is another class of halides in which the halogen atom acts as a bridge between the two atoms of the other elements; such halides are called “Polymeric halides”. Less electropositive elements like Be, G and Al form polymeric halides having partly ionic bonding with layer or chai lattice.

Hydrides

Hydrides are binary compounds of hydrogen.  The elements of group IA and the heavier elements of· group IIA form ionic hydrides which contain H (hydride) ion. These hydrides are crystalline solid compounds with high melting and boiling points, and they conduct electricity in the molten state.

These hydrides react with water to liberate H2 gas and form hydroxide ion producing alkaline solution Ionic hydrides are useful reducing agents and usually, their reducing power decreases across a period and increase down a group. The hydrides of Be and  Mg are in between the ionic and covalent hydrides.

They have polymeric structures and covalent nature. Almost all the elements of p-block react with hydrogen to form covalent hydrides which are volatile. There· is, however, some ionic character in them depending upon the electronegativity of the element.

As electronegativity increases along the period, the covalent hydrides become more and more acidic.  Thus  the acidity of hydrides  of 2nd-period elements  CH4,  NH3,  H2O and  HF  increases  with the electronegativity of the non-metal present from  C  to  F.  The  acidity  can  also  be related to increase the stability of their anions as shown below

CH    <  NH;   < OH-  < F

Increasing Stability

The elements of group IIIA form characteristic covalent hydrides. These hydrides are electron deficient molecules and form addition compounds molecules with donor.

The groups VIA and VIIA hydrides (H2O, H2S, H2Se and HF, HCl, HBr, HI) show an increase in acidity on going down the group which is against normal expectation based op electronegativity concept.

Here the major factor is the heat of dissociation which shows a fall down a group in the hydrides of these groups, thus causing an increase in the acid strength of the hydrides. The hydrides of the heavier elements in group IIIA through VIA are not stable and often are virtually impossible to prepare.

The hydrides of transition metals usually are hard compounds with high melting points. They tend to have infinite compositions with vary number of hydrogen atoms fitted into spaces between metal  atoms in the solid.                                                                                                 –

Oxides

With the exception of some of the noble gases, every element forms at least one binary compound with oxygen called oxide. Some elements form a number of different oxides. Vanadium, a transition metal, forms VO, V2O3 ,. VO2 , V2O5 . · Chlorine, a non-metal, form Cl2O, Cl2O3 , Cl0 2, Cl2O6 and Cl2O7 . The nature of the oxide is determined primarily by the position of that element in the periodic table.

Oxides of elements on the left side of the periodic table, such as those  of  the alkali and alkaline earth metals (except for BeO, which is covalent) are ionic solids. These oxides are basic; they react with water to form hydroxides.  For  sodium  oxide the reaction is:

Na 2O     + H2O —-, 2Na+ + 2OH-

Many ionic oxides are very refractory, that is, they can be heated to high temperature without melting or decomposition: For example, calcium oxide, CaO “quicklime” has a melting point above 2005°C.

Oxides of the metallic elements toward the middle· of the table and of the semimetals are solids, but often are not ionic. Oxides .of non-metals are discrete, separate molecules. Generally exist as  liquids or gases  at  room temperature.

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