According to Avogadro’s law, equal volumes of all gases at the same temperature and pressure contain equal numbers of:

The correct option is Molecules.

Explanation

Avogadro’s Law is a fundamental principle in the study of the gaseous state. It describes the relationship between the volume of a gas and the amount of substance (number of moles) present, assuming constant physical conditions.

Detailed Analysis:

• Molecules (Correct): Avogadro’s Law explicitly states that equal volumes of all gases, under the same conditions of temperature and pressure, contain an equal number of molecules. Mathematically, this is expressed as \( V \propto n \) (where \( V \) is volume and \( n \) is the number of moles), at constant temperature and pressure.
• Atoms (Incorrect): While the number of molecules is the same, the number of atoms depends on the atomicity of the gas. For instance, equal volumes of Helium (monoatomic, He) and Oxygen (diatomic, O₂) will have the same number of molecules, but the Oxygen sample will contain twice as many atoms as the Helium sample.
• Protons and Neutrons (Incorrect): The number of subatomic particles (protons and neutrons) is determined by the atomic structure (atomic number and mass number) of the specific elements constituting the gas. This varies significantly between different gases even if their volumes and molecule counts are identical.

Key Takeaway:

Avogadro’s Law relates gas volume directly to the number of molecules (or moles), irrespective of the nature, size, or mass of the gas particles, provided temperature and pressure remain constant.