Characteristics of the Compounds of Alkali Metals (original) (raw)

Last Updated : 2 Dec, 2021

There are a large number of elements around us having different properties and different uses based on those properties. For using these elements, properties are important so there should be some table to group these elements. This table is known as a periodic table which is created by using the work of different scientists. Elements of the first group are called Alkali metals. This group contains the following elements lithium, sodium, potassium, rubidium, Caesium and Francium. Apart from lithium, all other elements follow a similar trend in chemical and physical properties.

Group 1 Elements

Characteristics of Alkali Metals

• Atomic Radii: Atomic radii of alkali metals increases down the group as the atomic number increases due to which new energy shells are added as the atomic number increases. Due to the addition of energy shells the size of atoms gets increases moving from beryllium to radium.

Be< Mg< Ca< Sr< Ba< Ra

• Boiling Point: The boiling point of alkali metals is quite lower than alkaline earth metals and it decreases down the group, it follows the trend opposite to the atomic size as electrons from smaller atoms are hard to excite by heat energy than electrons in the larger atoms.

Be> Mg> Ca> Sr> Ba> Ra

• Electronic Configuration: The electronic configuration of alkali metals can be represented by a noble gas and one valence electron is placed in an s orbital. So the representation is like [Noble Gas] ns1.
• Standard Potential: Standard potential of alkaline earth metals follows the same trend as that of atomic size means the negative standard potential of ions increases down the group as atomic size increases.

Be> Mg> Ca> Sr> Ba> Ra

Physical Properties of Alkali Metals

1. Alkali metals can be easily ionised by heat energy.
2. These metals show the lustrous property.
3. They can be beaten into thin sheets as these elements are malleable.
4. The thermal conductivity of alkali metals is quite high.
5. These elements can be cut easily as they are soft in nature.
6. Alkali metals float on water due to their low density.
7. These elements have the tendency to show exothermic reactions.

Chemical Properties of Alkali Metals

• Alkali metals react with water to form hydroxides and releases H2 gas.

2X + 2H2O → 2X+ + 2OH- + H2

where X is an alkali metal

• These also react with air to form different oxides of metals.
• These elements react with the halogen group which consists of fluorine, chlorine, bromine and iodine to form corresponding halides.

2M + X2 → 2MX

where X is a halogen and M is alkali metal

• At high temperatures, alkali metals can react with hydrogen to form corresponding hydrides.

2X + H2 → 2X+H-

where X is an alkali metal

• These elements react with air and water so to avoid any reaction these are kept in kerosene oil.
• The nature of alkali metals is reducing in nature.

Uses of Alkali Metals

1. Lithium is also used in the medical field by heart pacemakers.
2. Lithium is mainly used in batteries both rechargeable and non-rechargeable.
3. Caesium is used as caesium sulphide in thermoelectric power generation.
4. Potassium has many uses in soaps, cleaners, detergents and gold mining also.
5. Sodium is used in the purification process of many metals.

Two or more elements join to form compounds so alkali metals form ionic compounds as alkali metals are of group 1 and have an oxidation state of +1.

Characteristics of Compound formed by Alkali Metals

Oxides

Alkali metals can form corresponding oxides and some peroxide also when react with oxygen. These oxides are colourless in pure form. The structure of these oxides is of rock salt type. These oxides are dissolvable in water.

X + O2 → X2O

where X is an alkali metal

Hydroxides

The oxides of alkali metals when reacting with water form hydroxides. These hydroxides are basic in nature and they are thermally stable as well. These hydroxides can be dissolved in water.

X2O + H2O → 2X+ + 2(OH)-

where X is an alkali metal

Apart from hydroxides, peroxides can also be formed.

2XO2 + 2H2O → 2X+ + 2OH- + H2O2 + O2

Halides

Compounds of alkali metals react with the halogen group which consists of fluorine, chlorine, bromine and iodine to form corresponding halides. These halides have high melting points and are colourless.

Salts of Oxo-Acids

The alkali metals have the ability to form salts with the oxo-acid group, so let's understand what is an oxoacid. Oxo-acids can be defined as those which has acidic proton attached to the hydroxyl group along with the oxo group. Examples are sulphuric acid and carbonic acid. Following are the properties of Oxo-acids:

• They are readily hydrolysed by water.
• They are stable to heat or temperature change.
• They are electropositive in nature which increases in the top to bottom manner.
• Except for Lithium carbonate, all other carbonates of oxoacids are stable to heat.
• The oxo-acid group is attached to the same atom on which the hydroxyl group is attached.

Sample Questions

Question 1: Explain the reaction of alkali metals with oxygen?

Answer:

Alkali metals react violently with oxygen and forms oxides, peroxides and superoxides depending on the metal and its size. As superoxides are generally unstable because of small cation but with large cation it will be stable.

Lithium forms oxide: 4Li + O2 → 2Li2O

Sodium forms peroxide: 2Na + O2 → Na2O2

Potassium forms superoxide: K + O2 → KO2

Question 2: Why lithium carbonate is not stable to heat?

Answer:

Lithium carbonate is not so stable to heat as the atomic size of lithium is very small due to which it polarises a large carbonate ion which results in the formation of Lithium oxide and carbon dioxide that's why lithium carbonate is not stable to heat.

Question 3: How alkali metals can be tested?

Answer:

Alkali metals can be tested by dissolving them in liquid ammonia as in liquid ammonia they give deep blue colour because of ammoniated electrons. These electrons make this solution conductive in nature. This solution is paramagnetic in nature.

Question 4: Why caesium and potassium are used in photoelectric cells?

Answer:

Caesium and Potassium have a special ability to lose electrons by absorbing energy from sunlight when light rays fall on these elements and this results in a photoelectric effect due to which caesium and potassium are used as electrodes in photoelectric cells.

Question 5: What is the oxidation state of Potassium in Potassium oxide?

Answer:

Potassium reacts with oxygen to give potassium oxide as:

K + O2 → KO2

Potassium oxide is superoxide and the oxygen ion is represented as O-2 and hence the oxidation state of +1, this compound is neutral in nature.