Match the elements given in Column I with the colour they impart to the flame given in Column II.
| Column I | Column II | ||
|---|---|---|---|
| A. | Cs | 1. | Apple green |
| B. | Na | 2. | Violet |
| C. | K | 3. | Brick reduction |
| D. | Ca | 4. | Yellow |
| E. | Sr | 5. | Crimson red |
| F. | Ba | 6. | Blue |
A. $\rightarrow$ (6)
B. $\rightarrow$ (4)
C. $\rightarrow(2)$
D. $\rightarrow$ (3)
E. $\rightarrow$ (5)
F. $\rightarrow 1$
Elements with the characteristic flame colour are as follows
A. Cs - Blue
B. $\mathrm{Na}-$ Yellow
C. K - Violet
D. Ca - Brick red
E. Sr - Crimson red
F. Ba - Apple green
Flame colours are produced from the movement of the electrons in the metal ions present in the compounds. These movement of electrons (electronic excitation-de-excitation) requires energy.
Each atom has particular energy gap between ground and excited energy level therefore each of these movements involves a specific amount of energy emitted as light energy, and each corresponds to a particular colour. As we know energy gap between ground and excited state energy level increases wavelength of decreases and complemently colouer is observed as a result.
Assertion (A) The carbonate of lithium decomposes easily on heating to form lithium oxide and $\mathrm{CO}_2$.
Reason (R) Lithium being very small in size polarises large carbonate ion leading to the formation of more stable $\mathrm{Li}_2 \mathrm{O}$ and $\mathrm{CO}_2$.
Assertion (A) Beryllium carbonate is kept in the atmosphere of carbon dioxide.
Reason (R) Beryllium carbonate is unstable and decomposes to give beryllium oxide and carbon dioxide.
The $s$-block elements are characterised by their larger atomic sizes, lower ionisation enthalpies, invariable +1 oxidation state and solubilities of their oxosalts. In the light of these features describe the nature of their oxides, halides and oxosalts.
Due to low ionisation energy and large atomic size, alkali metals form cation readily and so their compounds are ionic.
Oxides
Due to +1 oxidation state, alkali metals form normal oxides of general formula $\mathrm{M}_2 \mathrm{O}$.
Only Li forms normal oxide $\mathrm{Li}_2 \mathrm{O}$ when heated in air. Other form peroxide and superoxide.
Oxides of alkali metals are strongly basic and are soluble in water.
The basic character of oxide increases gradually from $\mathrm{Li}_2 \mathrm{O}$ to $\mathrm{Cs}_2 \mathrm{O}$ due to increased ionic character.
Halides
Except lithium halides all other alkali metal halides are ionic. Due to high polarising power of $\mathrm{Li}^{+}$. Lithium halide is covalent in nature. Due to +1 oxidation states alkali metal halides have general formula MX. Low ionisation enthalpy allows formation of ionic halides.
Oxo salts
All alkali metals form solid carbonates of general formula $\mathrm{M}_2 \mathrm{CO}_3$. Carbonates are stable except $\mathrm{Li}_2 \mathrm{CO}_3$ due to high polarising capacity of $\mathrm{Li}^{+}$ which is unstable and decomposes. All the alkali metals (except Li ) form solid bicarbonates $\mathrm{MHCO}_3$. All alkali metals form nitrates having formula $\mathrm{MNO}_3$. They are colourless, water soluble, electrovalent compounds.
Present a comparative account of the alkali and alkaline earth metals with respect to the following characteristics.
(a) Tendency to form ionic/covalent compounds
(b) Nature of oxides and their solubility in water
(c) Formation of oxosalts
(d) Solubility of oxosalts
(e) Thermal stability of oxosalts
(a) Alkaline earth metals form compounds which are predominantly ionic but less ionic than the corresponding compounds of alkali metals due to increased nuclear charge and small size.
(b) Oxides of alkaline earth metals are less basic than corresponding oxides of alkali metals. The oxides dissolve in water to form basic hydroxides and evolve a large amount of heat. The alkaline earth metal hydroxides, are however less basic and less stable than alkali metal hydroxides.
(c) Alkaline earth metals form oxoacids as alkali metals. The formation of alkali metal oxoacids is much more faster and stronger than their corresponding alkaline earth metals due to increased nuclear charge and small size.
(d) Solubility of alkaline oxoacids is more than alkali oxoacids because alkaline earth metals have small size of cation and higher hydration energy. Salts like $\mathrm{CaCO}_3$ are insoluble in water.
(e) Oxosalts of alkali metals are thermally more stable than those of alkaline earth metals. As the electropositive character increases down the group, the stability of carbonate and hydrogen carbonates of alkali metal increases.
Whereas for alkaline earth metals, carbonate decomposes on heating to give carbon dioxide and oxygen.