Write Lewis structure of $\mathrm{O}_2{ }^{-}$ion and find out oxidation state of each oxygen atom? What is the average oxidation state of oxygen in this ion?
The Lewis structure of $\mathrm{O}_2{ }^{-}$is
Oxygen atom carrying no charge has six electrons, so its oxidation number is zero. But oxygen atom carrying -1 charge has 7 electrons, so its oxidation number is -1.
Average oxidation number of each oxygen atom $=\frac{1}{2}$
$$\begin{aligned} \mathrm{O}_2^{-} & =2 x=-1 \\ x & =-\frac{1}{2} \end{aligned}$$
Why do beryllium and magnesium not impart colour to the flame in the flame test?
All alkaline earth metals (except Be and Mg ) impart a characteristic colour to the Bunsen flame. The different colours arise due to different energies required for electronic excitation and de-excitation.
Be and Mg atoms due to their small size, bind their electrons more strongly (because of higher effective nuclear charge). Hence, require high excitation energy and are not excited by the energy of the flame with the result that no flame colour is shown by them.
What is the structure of $\mathrm{BeCl}_2$ molecule in gaseous and solid state?
Beryllium chloride has different structures in solid and vapour state. In solid state, it exists in the form of polymeric chain structure in which each Be-atom is surrounded by four chlorine atoms having two of the chlorine atoms covalently bonded while the other two by coordinate bonds. The resulting bridge structure contains infinite chains.
In vapour state, above 1200 K , it exists as a monomer having linear structure and zero dipole moment. But below 1200 K , it exists as dimer structure even in vapour state.
Match the elements given in Column I with the properties mentioned in Column II.
| Column I | Column II | ||
|---|---|---|---|
| A. | Li | 1. | Insoluble sulphate |
| B. | Na | 2. | Strongest monoacidic base |
| C. | Ca | 3. | Most negative $E^{\ominus}$ value among alkali metals |
| D. | Ba | 4. | Insoluble oxalate |
| 5. | 6s$$^2$$ outer electronic configuration |
A. $\rightarrow(3)$
B. $\rightarrow(2)$
C. $\rightarrow(4)$
D. $\rightarrow(5)$
A. Li-Most negative $E^{-}$among alkali metals
[Due to very high hydration energy the resulting $E^{\ominus}$ is most negative].
B. Na-Strongest monoacidic base
[Alkalies are more acidic than alkaline earth metals. LiOH has covalent character].
C. Ca-insoluble oxalate
[Calciuim oxalate is insoluble in water]
D. Ba-Insoluble sulphate
[Hydration energy decreases as size of cation increases].
$6 s^2$ outer electronic configuration
$$\left.{ }_{56} \mathrm{Ba}=1 s^2, 2 s^2, 2 p^6, 3 s^2, 3 p^6, 3 d^{10}, 4 s^2, 4 p^6, 4 d^{10}, 5 s^2, 5 p^6, 6 s^2\right]$$
Match the compounds given in Column I with their uses mentioned in Column II.
| Column I | Column II | ||
|---|---|---|---|
| A. | CaCO$$_3$$ | 1. | Dentistry, ornamental work |
| B. | Ca(OH)$$_2$$ | 2. | Manufacture of sodium carbonate from caustic soda |
| C. | CaO | 3. | Manufacture of high quality paper |
| D. | CaSO$$_4$$ | 4. | Used in white washing |
A. $\rightarrow$ (3)
B. $\rightarrow$ (4)
C. $\rightarrow(2)$
D. $\rightarrow 1$
A. $\mathrm{CaCO}_3-$ Manufacture of high quality paper
B. $\mathrm{Ca}(\mathrm{OH})_2-$ Used in white washing
C. $\mathrm{CaO}-$ Manufacture of sodium carbonate from caustic soda
D. $\mathrm{CaSO}_4-$ Dentistry, ornamental work