HNO3, also known as Nitric Acid, is a highly corrosive and reactive compound that is commonly used in various industries and laboratory applications. In its gaseous state, the HNO3 molecule adopts a planar shape.

Explanation:
1. Molecular Formula:
- The molecular formula of Nitric Acid is HNO3.
- This means that the molecule consists of one hydrogen atom (H), one nitrogen atom (N), and three oxygen atoms (O).

2. Lewis Structure:
- To determine the shape of the molecule, we first need to draw the Lewis structure.
- The Lewis structure of HNO3 shows that the central nitrogen atom is bonded to three oxygen atoms.
- The nitrogen atom has a lone pair of electrons, and each oxygen atom has two lone pairs of electrons.
- The Lewis structure can be represented as follows:

O
//
H - N - O
\\
O

3. Electron Pair Arrangement:
- The electron pair arrangement for HNO3 is trigonal planar.
- This means that the three oxygen atoms and the lone pair of electrons on the nitrogen atom are arranged in a flat plane around the central nitrogen atom.

4. Molecular Geometry:
- The molecular geometry is determined by the arrangement of the bonded atoms.
- In HNO3, the three oxygen atoms are bonded to the nitrogen atom, resulting in a trigonal planar molecular geometry.
- The lone pair of electrons on the nitrogen atom also contributes to the overall shape of the molecule.

5. Conclusion:
- Based on the Lewis structure, electron pair arrangement, and molecular geometry, the HNO3 molecule adopts a planar shape in its gaseous state.
- This means that the three oxygen atoms and the lone pair of electrons on the nitrogen atom are arranged in a flat plane around the central nitrogen atom.