An Italian physicist, Amedeo Avogadro in 1811 suggested that the smallest particle in gases (except noble gases) were not atoms but molecules.

These molecules could contain two, three or more atoms.

For example, the number of atoms in one molecule of certain gases are,

 Helium (He)       Oxygen (O₂)        Nitrogen (N₂)     Hydrogen (H₂)

        1                      2                      2                      2

 Chlorine (Cl₂)     Ozone (O₃)         Sulphur dioxide (SO₂)

 2                     3                      3

Avogadro’s law

Avogadro proposed his famous hypothesis, known as Avogadro’s hypothesis.

The hypothesis (now known as Avogadro’s law) states that, “under similar conditions of temperature and pressure equal volumes of all gases contain the

same number of molecules.”

For example, if equal volumes of different gases are held in separate containers under similar conditions of temperature and pressure, then each container will contain the same number of molecules.

The total mass of each gas in the container may differ from each other.

To compare the behaviour of gases, it is necessary to choose certain fixed values of temperature and pressure.

Such fixed values of temperature and pressure are called normal temperature and pressure (NTP), and standard temperature and pressure (STP).

• Normal temperature and pressure

Normal temperature = 0°C or 273 K

Normal pressure = 1 atm = 76 cm Hg = 760 mm Hg = 760 torr

• Standard temperature and pressure

Standard temperature = 0°C = 273 K

Standard pressure = 1 bar = 105 Pa (1 atm = 1.01325 bar)

The NTP and STP conditions are not the same.

This difference; however, is small.

Therefore, in common use both the terms may be used as if the same.