Atom, Evidence of atoms (FSC Punjab Text book, notes)

 Atom

Atom: Greek philosophers thought that matter is made up of individual particles which cannot be further subdivided.

Democritus (460-370) called these indivisible particles an atom (the Greek word atom means indivisible.)

In 1880, an English school teacher John Dalton gave atomic theory. Its main points are given below.

1   All matter is composed of atoms.
2. The atoms of the same elements have similar properties.
3. The atoms of different elements have different properties.

The modern definition of an atom is given below.

The smallest particle of an element that may or may exist independently is called an atom. For example, He, Ne, H, C, Na, Cu, S, etc.

Modern research shows that an atom is further composed of electrons, protons, neutrons, positrons, hyperons, neutrinos, anti-neutrino, etc. they are called sub-atomic particles. More than 100 sub-atomic particles have been discovered. But electrons, protons, and neutrons are the fundamental particles of the atom.

Berzelius (1779-1848) was a Swedish chemist. He determined the atomic masses of elements. Moreover, he introduced the symbols for elements.

Easy notes: Determination of atomic masses by Mass Spectrometry

Evidence of atoms:

A clear image of an atom can not be obtained because it is smaller than the wavelength of visible light.

An ordinary microscope can measure up to or above 500nm. An atom can be observed in an electron microscope. An electron microscope uses a beam of electrons instead of visible light. the X-ray work also gives evidence of atoms. 

The diameter of the atoms is of the order  2 x 10^-10m or 0.2nm (1nm= 10^-9m). 

The mass of an atom ranges from 10^-27 kg to 10^-25kg. We may say that an atom is an extremely small particle.

A student can imagine that a full stop can contain two million atoms in it. A photograph of graphite obtained by electron microscope is shown in the figure, This photograph has been magnified 15 million times.