In 20th century, Richard Feynman one of the great physicists discovered that matter is made up of atoms.
About 2000 year ago John Dalton suggested that the smallest constituents of an element are atoms and they cannot be separated any further. Atoms of one element are identical but differ from those of other elements. A small number of atoms of each element combine to form a molecule of the compound.
As number of atom combine to form a molecule so Dalton’s atomic theory is also referred as molecular theory of matter.
Amedeo Avogadro (1776-1856) help to understand the combination of different gases in a simple manner. Avogadro Law state that equal volume of all gases at the same temperature and pressure have equal number of molecules.
When Avogadro Law combine with Dalton’s Theory to explain Gay Lussac’s Law in 19th century, Gay Lussac’s Law state that “When gas combine chemically to yield another gas, there volume are in the ratio of small integers. In other word “If the volume of the gas is taken as constant, then the pressure of a given amount of gas varies directly to its temperature.
Atomic Hypothesis
All things are made of atoms. The atoms are little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. Advancement in science and technology enable us to see the molecules with the help of the electron microscope.
So, size of atom is of the order 10⁻¹⁰ m.
One Angstrom(1A⁰) = 10⁻¹⁰ m
State Of Matter
Solid
Solid are the substances which have definite shape size and volume. In solid the interatomic spacing is least (about 2 angstrom) and inter atomic force of attraction is maximum.
Liquid
Liquid are the substances which have definite volume but no definite shape and size. In liquid, the interatomic spacing is greater in comparison to the solid but less in comparison to gas and interatomic force of attraction is relatively weaker in comparison to solid. This enables liquid to flow.
Motion Of Molecules In Gas
The average distance a molecule can travel without colliding is called the mean free path. The mean free path in gases is of order of thousands of angstrom, due to this reason atoms of gases are much free to move and can travel along distances without colliding.
Modern theories had proved that atoms are no longer individual or elementary. Every atoms consists of nucleus and electrons, the nucleus itself is made up of protons and neutrons. Protons and neutrons are again made up of quarks. It may be possible in future that quarks may be further subdivided into some short of elementary particles.