Metals
Any substance which is typical shiny solid, good conductor of electricity and heat, has high to moderate melting point and tends to lose electrons in chemical reactions is called metal.
Electropositive character or Metal character
The tendency of an element to lose electron to form cation is called electropositivity or metal character.
M ---------- > M+n + ne-
Where M stands for any metal.
Metals have large atomic size and low ionization energies. Ionization energy decreases down the group. Hence e electropositive character also increases.
Question: Why is sodium is more metallic than Li?
The property of an element that makes it a metal is its ability to lose the electron.As atomic size of Na is greater than Li, Therefore, Na has greater ability to lose the electrons so it is more metallic (electropositive) than Li.
Question: Why is Mg is more metallic than Be?
The property of an element that makes it metal is its ability to lose the electron.. As atomic size of Mg is greater than Be, Therefore, Mg has greater ability to lose the electrons so it is more metallic (electropositive) than Be..
Question: Why Li and Be are metals, B is metalloid, whereas C, N,O, F and Ne are non metals.
Answer.
As we move from left to right in the period ability to lose the electron decreases so electropositive character or Metallic character also decreases. This is because of increase in their nuclear charge and decrease atomic sizes.
As atomic sizes of Li and Be are relatively greater so they are metal. While atomic radio of C, N, O, F and Ne are smaller, so they are non metals. Atomic size of B is greater than non metals and lesser than metal so it is a metalloid.
Trend In electropositivity and reactivity
Trend in electropositivity is reflected in chemical reactivity of metals.
Electropositivity @ chemical reactivity
In group
As we move from top to bottom in the group, the reactivity of metals with oxygen increases. This indicates the increase in electropositive character or Metallic character.
For example, reactivity of alkali metal with oxygen (air) increases from top to bottom in group. For instance, Lithium form normal oxide.
Li + O2 ------------- > Li2O
Sodium forms peroxide.
Na + O2 ------------- > Na2O2
Potassium rubidium and Cs farms peroxide.
K + O2 ------------- > KO2
Along period
As we move from left to right, the reactivity of metals with oxygen decreases from left to right in a period. This indicates the decrease in electropositive character or metallic character.
For example, In third period, sodium readily reacts with oxygen to form sodium peroxide, at room temperature. Mg reacts with oxygen only on ignition. Al reacts with oxygen on heating to form Al2O3 which prevent further oxidation.
Trend In electropositivity and nature of oxides
Trend in electropositivity is reflected in nature of metals oxides
Metal oxides are basic in character because they produce bases in water. Non metals oxides are acidic in character because they produce acid in water.
In group
The basic character of metal oxide increases from top to bottom in the group. This indicates increase in metallic character. For example, basic character of metal oxide of group IIA increases in the following order.
BeO<MgO<CaO<SrOBaO
Along the period
Basic character of oxides decreases as we move from left to right in the period. This indicates decrease in metallic character. For example oxides of 3rd period elements such as Na2O, MgO, Al2O3, SiO2,P4O10, SO2, Cl2O. These oxides change from strongly basic through weakly basic, amphoteric, weakly acidic to strongly acidic.
Na2O (strongly basic), Mgo (basic), Al2O3 (amphoteric, both acidicas wellasbasic), SiO2 (weakly acidic), P4O10(acidi), SO2(strongly acidic), Cl2O7( very strongly acidic .
Comparison of reactivity of alkali and alkaline earth metals
In order to make comparison of reactivity of alkali and alkaline earth metals, we need to know their position in the periodic table and their tendency to loose the electrons.
The group IA elements except hydrogen are known as alkali metals. These elements have a general electronic configuration ns1 in their valence shell.
The group IIA elements are known as aalkaline earth metals. These elements have a general electronic configuration ns2 in their valence shell.
As alkali metals have large atomic radii as compared alkaline earth metals in their respective periods. Alkali metals have relatively higher ionization energies, lesser electropositive or metallic character and hence lesser ìreactivity. Therefore, they are placed in kerosene oil and these elements never found in free state. The reactivity of alkali metals increases down the group. This is because, increase in atomic size, decrease in ionization energy, increase electropositive character. Therefore, Na is more reactive than Li and more reactive than K. T
On the other hand, alkaline earth metals have relatively smaller atomic radii as compared alkali metals in their respective periods, Alkaline earth metals have low ionization energies, more electropositive or metallic character and hence more reactivity. Therefore, they are placed in kerosene oil and these elements never found in free state. The reactivity of alkali metals increases down the group. This is because, increase in atomic size, decrease in ionization energy, increase electropositive character. Therefore, Na is more reactive than Li and more reactive than K. T
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