Notes On Periodic Trends: Chemical Properties - CBSE Class 11 Chemistry
The periodicity in the fundamental properties like atomic or ionic radii, ionization enthalpy and electron gain enthalpy is related to the chemical reactivity of elements. As moving from left to right in the period, the atomic or ionic radii decrease this result increase ionization enthalpies and the negative electron gain enthalpies. Ex: Alkali metals of Group 1 have the lowest ionization enthalpies as compare to halogens of Group 17 have the most negative electron gain enthalpies. Low ionization enthalpy, as in the case of alkali metals, means that the elements are willing to give away their most loosely bound electrons. In other words, alkali metals are good reducing agents and are highly reactive in nature. The most negative electron gain enthalpy being attributed to halogens indicates that halogens behave as good oxidizing agents and exhibit high chemical reactivity. This is in contrast to the noble gases that have completely filled outer electronic shells and thus exhibit a positive electron gain enthalpy and the least reactivity. Chemical reactivity is highest for both for alkali metals and halogens. The maximum chemical reactivity of the alkali metals is due to the lowest ionization enthalpy which enables it to lose an electron that leads to the formation of a cation. The maximum chemical reactivity of the halogens is due to the highest electron gain enthalpy which facilitates an atom to gain an electron leading to the formation of an anion. This property of forming cations and anions can be related to the oxidizing and reducing behavior of the elements. The alkali metals and halogens show a higher tendency to combine with oxygen as compared to the elements in the center. The oxides formed by the elements of group-1 are basic, while those formed by the elements of group-17 are acidic in nature. Ex: Sodium, a group 1 alkali metal, forms sodium oxide with oxygen which on reaction with water forms a base called sodium hydroxide (NaOH). In contrast, Chlorine, a Group 17 halogen, on reaction with oxygen, forms a highly acidic oxide called chlorine heptaoxide which on reaction with water forms perchloric acid. Oxides formed by elements like aluminium are amphoteric in nature. Amphoteric oxides can behave both as acids and as bases depending upon whether they react with a strong base or a strong acid. 4 Al+ 3O₂ → 2Al₂O₃ (amphoteric oxide) Al₂O₃ (acts as base) + 3H₂SO₄ (acid) → Al₂(SO₄)₃ + 3 H₂O Al₂O₃ (acts as acid) + 2NaOH (base) + 3H₂O → 2 NaAl(OH) ₄ Trend of oxides of the 3rd period is observed as: from strongly basic to weakly basic to amphoteric to weakly acidic to strongly acidic. The atomic and ionic radii increase down a group in representative elements causes decrease in the ionization energies and electron gain enthalpies become less negative down a group. Metallic character increases down the group while the non-metallic character decreases down the group. Thus chemical reactivity increases down a group for alkali and alkaline earth elements while it decreases for halogens down the group. In the transition metals in the 3d series of the periodic table, the change in atomic radii is much smaller as compared to those of representative elements across the period.. Similarly, the inner-transition series,4f and 5f elements show even smaller variation in atomic radii. The ionization enthalpies of the transition and the inner transition elements are intermediate between those of s- and p-blocks. As a result they are less electropositive than group 1 and 2 metals.

#### Summary

The periodicity in the fundamental properties like atomic or ionic radii, ionization enthalpy and electron gain enthalpy is related to the chemical reactivity of elements. As moving from left to right in the period, the atomic or ionic radii decrease this result increase ionization enthalpies and the negative electron gain enthalpies. Ex: Alkali metals of Group 1 have the lowest ionization enthalpies as compare to halogens of Group 17 have the most negative electron gain enthalpies. Low ionization enthalpy, as in the case of alkali metals, means that the elements are willing to give away their most loosely bound electrons. In other words, alkali metals are good reducing agents and are highly reactive in nature. The most negative electron gain enthalpy being attributed to halogens indicates that halogens behave as good oxidizing agents and exhibit high chemical reactivity. This is in contrast to the noble gases that have completely filled outer electronic shells and thus exhibit a positive electron gain enthalpy and the least reactivity. Chemical reactivity is highest for both for alkali metals and halogens. The maximum chemical reactivity of the alkali metals is due to the lowest ionization enthalpy which enables it to lose an electron that leads to the formation of a cation. The maximum chemical reactivity of the halogens is due to the highest electron gain enthalpy which facilitates an atom to gain an electron leading to the formation of an anion. This property of forming cations and anions can be related to the oxidizing and reducing behavior of the elements. The alkali metals and halogens show a higher tendency to combine with oxygen as compared to the elements in the center. The oxides formed by the elements of group-1 are basic, while those formed by the elements of group-17 are acidic in nature. Ex: Sodium, a group 1 alkali metal, forms sodium oxide with oxygen which on reaction with water forms a base called sodium hydroxide (NaOH). In contrast, Chlorine, a Group 17 halogen, on reaction with oxygen, forms a highly acidic oxide called chlorine heptaoxide which on reaction with water forms perchloric acid. Oxides formed by elements like aluminium are amphoteric in nature. Amphoteric oxides can behave both as acids and as bases depending upon whether they react with a strong base or a strong acid. 4 Al+ 3O₂ → 2Al₂O₃ (amphoteric oxide) Al₂O₃ (acts as base) + 3H₂SO₄ (acid) → Al₂(SO₄)₃ + 3 H₂O Al₂O₃ (acts as acid) + 2NaOH (base) + 3H₂O → 2 NaAl(OH) ₄ Trend of oxides of the 3rd period is observed as: from strongly basic to weakly basic to amphoteric to weakly acidic to strongly acidic. The atomic and ionic radii increase down a group in representative elements causes decrease in the ionization energies and electron gain enthalpies become less negative down a group. Metallic character increases down the group while the non-metallic character decreases down the group. Thus chemical reactivity increases down a group for alkali and alkaline earth elements while it decreases for halogens down the group. In the transition metals in the 3d series of the periodic table, the change in atomic radii is much smaller as compared to those of representative elements across the period.. Similarly, the inner-transition series,4f and 5f elements show even smaller variation in atomic radii. The ionization enthalpies of the transition and the inner transition elements are intermediate between those of s- and p-blocks. As a result they are less electropositive than group 1 and 2 metals.

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