Introduction: Nature of matter was indicated quite early inform of frictional electricity.

      It was produced by rubbing two articles together as a ebonite rod or glass rod with silk or fur.

Discovery of electron: In (1897) J.J Thomson performed an experiment in a discharge tube which is made up of glass, long cylindrical tube at both ends. Negative terminal electrode is known as cathode and positive terminal is known as anode.

       A current (10000 volt) was passed through the tube. It was found that the fluorescent is due to bombardment of the tube by the rays emitted from cathode. These rays are known as cathode rays.

    Characteristics of cathode rays:

1.Rays are always travel in a straight path. 

2.It consists of negative charged particles i.e electron. 

3.These are deflected by electrical and magnetic field etc.

Discovery of proton: Eugen Goldstein repeated J.J Thomson experiment by modifying the discharging tube.He used a perforated tube that is cathode is attach not at the end, but in the centre of tube.

       He surprise to observe that another types of rays emitted from tube move away from the side of anode.

The rays were named anode rays, they are positive rays. They are initially  also called canal rays.

     Characteristics of anode rays:

1. It is also travel in the straight path.

2. It consists of positively charged particles i.e proton.

3. It is deflected by electric and magnetic field in a direction opposite to the cathode rays etc.

Structure of atom:  We are briefly discussed in the contribution made by different scientist in this direction.

Thomson's model of an atom: In (1904), he was the first to predict the model of an atom known as Thomson model of an atom.

   According to him:

1. Atom may be predicted as positively charged sphere in which protons are present.

2. Negatively charged particles are embedded in the square.

3. Both the opposite charges balance each other, it has no net charge.

4. Atom can be visualised as plum pudding or cake in the same way as seeds in the water melon.

Limitation:

1.  He could not verify his atomic model from an experimental support.

2. He could not explain the result of the experiments that carried by Rutherford and other scientists at later stage.

Rutherford's model of an atom: He perform the famous Alpha particle scattering experiment in the year (1911).

           These alpha particles (positively charged helium nuclei)(He²+) bombarded on the thin gold foil/ thick about 1000 atoms with the speed of Alpha particles from a radioactive element radium enclosed in a lead box.

             These particle strike against the gold foil. These particle got scattered produce bright flashes on a circular screen with a substance called zinc sulphide.

Observation from the experiment:

Most of the Alpha particles were able to pass through the gold foil undeflected.

Some were defeated by small angles.

 Very few (one out of every 12000) retrace back in the same direction.

Conclusion: As alpha particle are positively charged that is regarded very small mass massive and same charge obstruction in an atom as nucleus (means centre).

     Rutherford's predicted that all the proton (known at that time) present in the small space is called nucleus.

Major part in atom is  unoccupied that's why most of Alpha particles pass undeflected.

Features: Atoms are consists of two parts that is nucleus and extra nuclear portion.

Electron present in extra nuclear portion are not stationary or fixed. They are revolving around the nucleus at high speed around the circular path.

The nucleus consists of positively charged.

In this reaction nucleus is discovered.

Limitation: He could not explain the stability of atom.

    How electron are distributed in the extra nuclear portion in an atom.

      Electron will lose energy at every turn and move closer to the nucleus it will follow a spiral path and fall ultimately  into the nucleus i.e. atom should collapse. This never happens, therefore he could not explain stability of atom.

Bohr's model of an atom (1912): Neils Bohr explain the structure of atom based on new concept.

Postulate: In atom, a central nucleus surrounded by electron.

Electrons revolve around the nucleus in a in a certain fixed circular path,these paths are called orbits or shells.

Each cells has fixed energy these are known as energy level or energy cells.

As long as an electron remains in a particular orbit it does not lose or gain energy.

Energy change in an electron it gives energy during it jumps from lower to higher  and vice versa.

Limitation:

No idea about shapes of molecule from the combination of elements (atoms).

According to Bohr's theory, an electron follow well define circular path called shell.

       However later studies revealed that the path is not exactly followed.

             It is not possible to explain the concept at the present level of the students.

Discovery of neutron: Neutron was discovered by James chadwick (1931).

      He  bombarded Alpha particle to thin foil of beryllium metal.

        Each particle found that they have equal mass of proton (nearly same).

       However it did not any charge i.e it was neutral in nature. Hence it is as neutrons and is donated by symbol 'n'.

Atomic number (Z): Number of positive charge present in the nucleus of an atom is known as atomic number of that element.

     It is denoted by Z 

In isolated neutral atom:-

Atomic number= number of protons= number of electrons.

Mass number (A): Total number of protons and neutrons present in the nucleus of an atom.

         It is denoted by A.

Both protons and neutrons are together known as nucleons.

    Mass number= number of protons + number of neutrons.

Distribution of electrons in different orbits/shells:

According to Bohr's bury  scheme electrons are distributed in different cells orbits.

  Electrons maximum are present in different cells orbits by this formula 2n².

Maximum number of electrons that can be accommodated in the outermost shell is 8.

Electrons do not occupy a new cell unless the inner cells are completely filled i.e shells?orbits are arranged  in a stepwise manner.(n=1,2,3,4.....or K,L,M,N......).

Valency: Combining capacity of an atom with another atom in order to acquire 8 electrons.(2 in some exception cases).

Valence shell: The outermost shell of an atom is called its valence shell.

Valence electrons: The number of electrons present in valence shell are called valence electrons.

Relation between valence electrons and chemical properties of elements:

Elements with same valency have similar chemical properties.

 Elements having 1,2&3 valence electrons tend to lose 3,2&1. These are generally metal.

 Elements having 5,6&7 valence electron tend to gain 3,2&-1. These are generally non metals.

Elements with 4 valence electron they possess both character (metal and non-metal). Carbon is non metal but Silicon is semi metal or metalloid.

Formation of ions: When charges appear in an atom then ions are formed.

There are two types of iron

1. Positive ion (cation): When an atom donate one or more electrons to other atom,positive ion gets formed.

2. Negative ion (anion): When an atom except one or more atom from another atom,negative ion gets formed.

Isotopes: Different atom of same element having same atomic number but different mass number.

e.g. c₆¹²,c₆¹³,c₆¹⁴.

Radioactive isotopes: Isotopes of some element are of radioactive nature. These are known as radioactive isotopes i.e they emit alpha, beta and gamma radiation.

Application of isotopes (radioactive isotopes):

1. Radioactive Cobalt (Co-60 isotope) used for cancer treatment.

2. Radioactive Phosphorus (P-32 isotope) used for agriculture field.

3. Radioactive uranium (U-235 isotope) used for nuclear reactors.

Isobars: They belong to different element with same mass number but different atomic number.

       They have different physical and chemical properties.