Anomalous Behavior of Lithium and Beryllium
Elements are arranged in a periodic table row-wise and column-wise according to similarities in their chemical and physical properties. The elements in the first column are known as Group 1 elements which have the following elements lithium, sodium, potassium, rubidium, Caesium and Francium. All these elements have similar properties except Lithium so all the elements of this group including lithium (yet it has different properties ) are known as Alkali Metals. . The elements in the second column is known as Group 2 elements which have the following elements beryllium, magnesium, calcium, strontium, barium and radium. All these elements have similar properties except Beryllium so all the elements of this group except beryllium are known as Alkaline Earth Metals.
Anomalous Properties of Lithium
|Physical Properties of Alkali Metals||Physical Properties of Lithium|
|1. These are soft metals.||1. Lithium is relatively hard than other elements of this group.|
|2. They have a lower boiling point than alkaline earth metals.||2. Boiling point of lithium is quiet high.|
|3. Alkali metals are light.||3. Lithium is the lightest metal.|
|4. All alkali metals are bigger than Lithium.||4. Ionic radii of Lithium is smallest in the group.|
|5. Ionization enthalpy of alkali metals is lower than Lithium.||5. Ionization enthalpy of Lithium is highest than alkali metals.|
Chemical Properties of Lithium
1. Alkali metals react violently with oxygen and forms oxides, peroxides and superoxides depending on the metal and its size. Lithium is the only element among alkali metals that forms oxide while the rest of the elements form peroxides and superoxides.
4Li + O2 → 2Li2O Lithium forms oxide
2. Alkali metals react with water to form hydroxides and releases H2 gas but the reaction of lithium with water is less vigorous than other alkali metals yet the value of Standard potential is most negative for lithium.
2X + 2H2O → 2M+ + 2OH– + H2 where X is an alkali metal
3. At high temperatures around 673 K, alkali metals can react with hydrogen to form corresponding hydrides but Lithium reacts with hydrogen at a very high temperature of around 1073 K.
2X + H2 → 2X+H– where X is an alkali metal
4. Alkali metals react with the halogen group which consists of fluorine, chlorine, bromine and iodine to form corresponding halides which are ionic in nature but Lithium form halides which are covalent in nature.
2M + X2 → 2MX where X is a halogen and M is alkali metal
5. Nature of alkali metals are reducing in nature while lithium is known as the strongest reducing agent.
Uses of Lithium:
1. Lithium is used in making different types of alloys one is known as white metal.
2. Lithium is also used in storing energy in electrochemical cells.
3. Lithium with aluminium is very strong and is used in the manufacturing of parts of different aircraft and fighter planes.
4. Some isotopes of Lithium is used in nuclear reactions.
5. Strong and light armour plates are manufactured by mixing lithium and magnesium.
Reasons for Anomalous properties of Lithium
There are mainly two reasons for the anomalous properties of lithium :
1. Size: Atomic size of lithium is really very small.
2. Polarising power: Polarising power can be defined as the ratio of the charge of an element to the radius of that element. Lithium has high polarising power as its size is very small so the charge divided by radius gives a big number.
Some major differences between Lithium and other Alkali Metals
1. Lithium Chloride has the ability to form hydrates while the rest of the alkali metals cannot form hydrates.
2. Alkali metals form hydrogen carbonates which are formed in the solid phase while the hydrogen carbonate of lithium cannot be formed in a solid state.
3. All alkali metals have the tendency to form ethynide when reacting with ethyne but lithium is the exception.
4. On heating, nitrates of alkali metals form nitrites while in the case of lithium nitrate it gives lithium oxide.
5. Fluorides of alkali metals are readily soluble in water but in the case of lithium fluoride, it is less soluble in water.
Anomalous Properties of Beryllium
|Physical Properties of Alkaline Earth Metals||Physical Properties of Beryllium|
|1. Appearance of alkaline earth metals is silvery white.||1. Appearance of beryllium is steel greyish.|
|2. They have higher boiling point than alkali metals.||2. Boiling point of beryllium is much higher than all others.|
|3. These are soft metals.||3. Beryllium is relatively harder than other elements of this group.|
|4. All alkaline earth metals are bigger than Beryllium.||4. Ionic radii of Beryllium is the smallest in the group.|
|5. Ionization enthalpy of alkaline earth metals is lower than Beryllium.||5. Ionization enthalpy of Beryllium is highest than alkaline earth metals.|
Chemical Properties of Beryllium
1. Alkaline earth metals react with water to form hydroxides. Among group 2 elements Beryllium is the element which does not react with water because of the protected layer which formed on the surface of this alkaline metal.
2. Alkaline earth metals also react with air to form different oxides of metals. Among group 2 elements Be is the element that does not react with air also because of the protected layer which formed on the surface of these metals.
3. When heated, alkaline earth metals can react with hydrogen to form corresponding hydrides. Hydrides of alkaline earth metals are metallic hydrides except Be which forms covalent hydrides.
X + H2 → 2XH2 where X is alkaline earth metal
4. Alkaline earth metals reacts with halogen group which consists of fluorine, chlorine, bromine and iodine to form corresponding halides which are ionic in nature but Beryllium form halides which are covalent in nature.
M + X2 → MX2 where X is a halogen and M is alkaline earth metal
5. Alkaline earth metals have a tendency to react with acids to release hydrogen gas but Beryllium does not react to give hydrogen gas.
Uses of Beryllium
1. Beryllium is used in making different types of alloys.
2. Beryllium when mixed with copper is used in the manufacturing of powerful springs which are used in shockers of automobiles.
3. Used for making mirrors of spacecrafts.
4. Some isotopes of Beryllium is used in nuclear reactions.
5. Beryllium is used in making different parts of computers, missiles and aircrafts.
Reasons for Anomalous properties of Beryllium
There are mainly two reasons for the anomalous properties of beryllium :
1. Size : Atomic size of beryllium is really very small as compared to other elements of the same group so it does not follow same trends as other alkaline earth metals do.
2. Atomic Number : As atomic number of beryllium is 4 so its coordination number cannot be greater than 4 while other members of the same group can have a coordination number 6 also.
Some major differences between beryllium and other Alkaline Earth Metals
1. Oxides and hydroxides of beryllium represents amphoteric nature while oxides and hydroxides of other alkaline earth metals are ionic in nature.
2. Beryllium does not react to with acids easily because of the protected layer which formed on the surface of Beryllium.
3. Beryllium Sulphate can be easily dissolved in water but other sulphates of alkaline earth metals are not soluble in water.
4. Carbonates of alkaline earth metals are stable but beryllium carbonate is unstable so to keep it safe it is stored in the carbon dioxide environment.
5. Beryllium nitrate is anhydrous in nature while other nitrates of alkaline earth metals are not anhydrous.
Question 1: Do Beryllium reacts with Acids? Explain.
Beryllium does not react with acids easily as it is being protected by a layer of the oxide film which forms at the surface. But Beryllium reacts with acids more easily when beryllium is in powdered form.
Question 2: Why does the solubility of alkaline earth metal hydroxides in water increase down the group?
The hydroxides of alkaline earth metals have more anion than cations so the cationic radius affects the lattice energy and as a result lattice energy become less than hydration energy as size increases so the solubility of these metal hydroxides increases in a group from top to bottom.
Question 3: Solubility of alkaline earth metal carbonates and sulphates in water decrease down the group explain why?
In alkaline earth metal carbonates and sulphates the size of anions is bigger when compared to cations so the cationic radius will not affect the lattice energy and hence lattice energy will remain constant but hydration energy decreases down the group so as a result, the solubility of these metal carbonates and sulphates decreases down the group.
Question 4: What is Diagonal Relationship between Beryllium and Aluminium?
The diagonal relationship is a term that describes the unusual behaviour of Beryllium and Aluminium. These two elements show many similarities to one another rather than their own group elements. The two common reasons for this diagonal relationship are:
- Size: The ionic size of Beryllium and Aluminium is nearly equal to 31 pm.
- Polarising power: Polarising power of Beryllium and Aluminium is also nearly the same.
Question 5: List some similarities between Beryllium and Aluminium.
Some of the similarities are:
- Both the elements follow the trend of making complexes.
- Chlorine ion bridged chloride structure is present in both chlorides of Beryllium and Aluminium.
- Beryllium and Aluminium do not react easily with acids.
- In the presence of excess alkali, hydroxides of both Beryllium and Aluminium gives beryllate and aluminate ions.
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