Mg and Mn are highly reactive metals, they are able to displace hydrogen from the nitric acid, however, less reactive metals do not displace hydrogen gas but get oxidized due to strong oxidising nature of nitric acid, due to high electronegativity of mn, it can't be dissolved in hydrogen,but it reacts with nitric acid (HNO3) to produce hydrogen .Got it ?

Introduction:
When metals react with dilute nitric acid (HNO3), they undergo a redox reaction, where the metal is oxidized and the nitric acid is reduced. However, not all metals react with dilute HNO3 to evolve hydrogen gas. In the case of metals other than magnesium and manganese, the absence of hydrogen gas evolution can be explained by considering the reaction mechanism and the reaction products.

Explanation:
1. Oxidation of metals:
When metals react with dilute HNO3, they undergo oxidation. The nitric acid acts as an oxidizing agent, providing oxygen for the oxidation reaction. The oxidation state of the metal increases during the reaction.

2. Nitric acid as an oxidizing agent:
Dilute nitric acid (HNO3) is a strong oxidizing agent. It can easily oxidize most metals, except for those that are highly resistant to oxidation, such as gold and platinum. In the reaction with metals, nitric acid is reduced to nitrogen dioxide (NO2) gas.

3. Formation of metal nitrate:
During the reaction, the metal reacts with nitric acid to form a metal nitrate. The metal cation combines with the nitrate anion to form a salt. For example, when copper reacts with dilute HNO3, copper nitrate (Cu(NO3)2) is formed.

4. Absence of hydrogen gas evolution:
The absence of hydrogen gas evolution can be attributed to the formation of nitrogen dioxide (NO2) gas as one of the products of the reaction. Nitrogen dioxide is a brown gas that is evolved during the reaction and can be seen as brown fumes.

5. Reaction mechanism:
The reaction between a metal and dilute HNO3 involves the following steps:
- Oxidation of metal: The metal is oxidized by nitric acid, resulting in the formation of metal ions.
- Reduction of nitric acid: Nitric acid is reduced to nitrogen dioxide (NO2) gas.
- Formation of metal nitrate: The metal ions combine with nitrate ions to form metal nitrates.

6. Role of hydrogen ions:
In the absence of magnesium and manganese, the reaction does not produce enough hydrogen ions (H+) to evolve hydrogen gas. This is because magnesium and manganese have a higher reactivity and are capable of displacing hydrogen from the acid, resulting in the evolution of hydrogen gas.

Conclusion:
In conclusion, when metals other than magnesium and manganese react with dilute HNO3, they do not evolve hydrogen gas due to the formation of nitrogen dioxide gas and the absence of sufficient hydrogen ions. The reaction involves the oxidation of the metal and the reduction of nitric acid, resulting in the formation of metal nitrates.