$LiCH _3$ have metal carbon bond so considered organometic compound and other do not have. 

$MgCO _3\xrightarrow{\Delta} MgO+CO _2$

Only $LiNO _3$ gives $NO _2$ on heating. All other nitrates give oxygen.
$4LiNO _3 \, \xrightarrow{\Delta} \, 2Li _2O \, + \, 4NO _2 \, + \, O _2.$
$2NaNO _3 \, \xrightarrow{\Delta} \, 2NaNO _2  \, + \, O _2$

Option $(C)$ is correct.

The diagonally opposite elements possess almost similar electronegativities and there fore almost similar electropositive character.

$ { Li }^{ + } $ ion being small in size is surrounded by large no. of water molecules on account of which it has very hydration energy. Hence, it is heavily hydrated and have high degree of hydration.

Option $(D)$ is correct.

This is because of almost same size and electronegativity values of magnesium and lithium, the show similar properties.

Lithium is the only alkali metal which combines directly with nitrigen to form lithium nitride.

Only $Rb$ does not combine with $N _2$ at high temperature. Because heavier metals like $Rb$ are seen to be chemically inactive towards $N _2$ upon heating.

$N{a _2}C{O _3} \to state\,\,than\,\,L{i _2}C{O _3}$

Similarities of  lithium with magnesium and dissimilarities with that of the other members of the group

Lithium is a very strong element and is equally strong as magnesium.

Li forms $Li _3N$ with nitrogen.

It has high melting and boiling points. The boiling point of lithium is $1330$ degree centigrade that is comparable to that of magnesium which is $1100$ degrees.

Due to small size, the lithium atom has high ionization energy. Hence, it remains inactive to dry air, slowly reactivity with water, liquid bromine and forms a highly stable hydride unlike other alkali metals.

The reaction is ,

Loss in weight due to following reactions:
$Li _{2}CO _{3} \rightarrow Li _{2}O+CO _{2} \uparrow$
$Na _{2}CO _{3}\cdot10H _{2}O \rightarrow Na _{2}CO _{3} + 10H _{2}O \uparrow$
As sodium carbonate is thermally stable so it will not dissociates on heating.

Reaction on heating:
$2LiNO _{3} \rightarrow Li _{2}O+2NO _{2}+ \frac{1}{2} O _{2}$
$Mg(NO _{3}) _{2} \rightarrow MgO+2NO _{2}+ \frac{1}{2} O _{2}$
$2NaNO _3\rightarrow 2NaNO _2+O _2$

Answer(A):- $Li _2Se$

Li has chemical properties most similar to Mg due to diagonal relationship.
Both exhibits same properties like
(i).

Li resembles to element X which is not a member of alkali metals. The group of X must be II group, because certain elements of 2nd period show similarity with their diagonal elements in the 3rd period as shown below : Thus, Li resembles Mg, Be resembles Al and B resembles Si. This is called diagonal relationship and is due to the reason that these pairs of element have almost identical ionic radii and polarizing power (i.e. charge/size ratio). Element of second period are known as bridge elements.

$Li$ and $Mg$ show diagonal relationship due to the similarity in size (radius), electronegativity, ionisation enthalpy and charge/radius ratio.

Li resembles to element X which is not the member of alkali metals. The element X is Mg. This is due to diagonal relationship. Certain elements of 2nd period show similarity with their diagonal elements in the 3rd period as shown below : Thus, Li resembles Mg, Be resembles Al and B resembles Si. 

Lithium shows similarities with magnesium in its chemical behaviour because they have similar size and charge/size ratio. They have similar electronegativity.

In certain matters, lithium differs from other alkali metals(anamalous property); the main reason for this is small size of lithium atom and $Li^+$ and high polarizing power.

The chemistry of lithium shows several differences from that of the rest of the group as the small lithium cation polarises anions and gives its compounds as the more covalent character.

A diagonal relationship is said to exist between certain pairs of diagonally adjacent elements in the second and third periods of the periodic table. Thus, $Li$ and $Mg$ are diagonally related.

Option $(C)$ is correct.

$LiF$ is the correct formula for lithium fluoride. $Li$ has a +1 charge and $F$ has a -1 charge so, when they come together, to form $LiF$ having an ionic bond. 

Fluorides and phosphates of $Li$ and $Mg$ are sparingly soluble in water whereas those of other alkali metals are soluble in water.

Due to its small size, lithium differs from other alkali metals but resembles with Mg as its size and other periodic properties are closer to Mg. Its resemblance with Mg is known as diagonal relationship.

Size of Mg is less than that of Li because of the $Z _{eff}$ of Mg. As the protons are added, electrons are attracted with a greater strength towards the core, making the radius of Mg smaller. 

 Fluorides and phosphates of $Li$ and $Mg$ are sparingly soluble in water whereas those of other alkali metals are soluble in water.

Carbonates of $Li$, $Ca$ $Mg$ decompose on heating and liberate $CO _2$.Carbonates of other alkali metals are stable towards heat and decomposed only by fusion. 

Isomorphic compounds have same crystal structure at given temperature and conditions. While $LiSO _4$ and $Na _2SO _4$ are not isomorphous due to the exceptionally small size of Lithium (Li).

• The properties of lithium are similar to those of $Mg$ because they are having diagonal relationship.
  
• The elements having diagonal relationship show similar properties and the reason is because the both of them will have similar electronegativity and polarising power i.e. charge per size ratio of both the elements are nearly the same.
  

A Diagonal Relationship is said to exist between certain pairs of diagonally adjacent elements in the second and third periods of the periodic table. These pairs (Li & Mg, Be & Al, B & Si etc.) exhibit similar properties.

• LiH is produced by treating lithium metal with hydrogen gas.
  
• It is used for filling meteorological balloons which carries instruments aloft to send back information on atmospheric pressure, temperature, humidity and wind speed by means of a small, expendable measuring device called a radiosonde
  

Lowest solubility of $LiF$ is due to high lattice enthalpy which is higher than hydration enthalpy. Hence, the dissociation of $Li-F$ bond becomes difficult.

(A). Lithium carbonate, being less stable, decomposes into its oxide on heating, whereas other group IA carbonates are stable and do not decompose.

(A) $Li^{+}$ and $Mg^{2+}$ have higher polarising power and thus attract more number of water molecules.
(B) $Li$ is least reactive because of its highest ionisation energy on account of smaller size.
(C) It is one of the uses of liquid sodium as it has low melting point.
(D) $LiI$ is the most covalent due to smaller cation and bigger anion (according to Fajan's rule).

Balanced chemical equation is: