Most metals, except for magnesium and manganese, do not react with nitric acid to form hydrogen gas. This is because nitric acid is a strong oxidizing agent and it oxidizes the metals to form metal nitrates and water instead of hydrogen gas.

The reaction between a metal and nitric acid can be represented by the following general equation:

$$Metal + HNO_3 → Metal nitrate + Water + Nitrogen oxides$$

In this reaction, the metal atoms are oxidized by the nitrate ions (NO3-) in nitric acid to form metal ions (M+). The nitrate ions are reduced to nitrogen oxides, such as nitric oxide (NO) and nitrogen dioxide (NO2). The hydrogen atoms in nitric acid are not released as hydrogen gas, but instead they combine with oxygen atoms to form water molecules.

Magnesium and manganese are two exceptions to this general rule. They react with nitric acid to form hydrogen gas because they are more easily oxidized than most other metals. The reactions between magnesium and manganese with nitric acid can be represented by the following equations:

$$Mg + 2HNO_3 → Mg(NO_3)_2 + H_2$$

$$Mn + 2HNO_3 → Mn(NO_3)_2 + H_2$$

In these reactions, the magnesium and manganese atoms are oxidized by the nitrate ions to form magnesium and manganese ions (Mg2+ and Mn2+), respectively. The hydrogen atoms in nitric acid are released as hydrogen gas.