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Atom is the smallest particle of an element which take part in chemical reaction
.It is made up of three fundamental particles:
- 1.Electrons -- Negatively charged.
2. Protons -- Positively charged.
Neutrons -- No charge (Neutral) .
***J.J. Thomson in 1897 discovered electrons in an atom.
E. Goldstein in 1886 discovered canal rays or positive rays and protons in the atom

*electron*

The electron is a negatively charged particle found in the atom of all element
Relative mass of electron =1840 1 mass of hydrogen atom
Absolute mass of electron = 28 9 10− × grams
Relative charge of electron = −1

Absolute charge = 19 1.6 10− × coulomb

*proton*

The proton is positively charged particle found in the atoms ofall elements. The protons are located in the nucleus of an atom.
Relative mass = 1amu or 1840 times to that of an electron
Absolute mass = 24 1.6 10− × grams
Relative Charge = +1
Absolute Charge = 19 1.6 10− × C

neutron*

..Absolute mass = mass of proton

§ JJ Thomson’s Model Of An Atom

This model is also called Plum Pudding Model or Apple Pie Model. JJ Thomson gave the first model of the atom. In this
model electrons were uniformly distributed in the entire atom.
The mass of the atom was supposed to be uniformly distributed.
Thomson Proposed That
(i) An atom consists of a positively charged sphere and thebelectrons are embedded in it.
(ii) The negative and positive charges are equal in

magnitude. So the atom as a whole is electrically neutral.

§ The Conclusions of jj thomson

1. There is a lot of empty space in an atom as most of the alpha-particles passed straight.
2. There is a center of positive charge in atom, which repels positively charged particles and deflects their path.
3. This center of positive charge in an atom is known as nucleus.
4. The nucleus is very dense and hard which does not allow alpha-particles to pass through them that are why a few

alpha-particles completely turned back.

§ Rutherford’s Structure Of An Atom

(i) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
(ii) The electrons revolve around the nucleus in well defined orbits. (iii) The size of the nucleus is very small as compared to the size of the atom.
Drawback Of Rutherford’s Atomic Model Of An Atom It was supposed that when electrons will revolve in a high speed around the nucleus they would loose energy. When energy of electrons will decrease their speed will also be decreased. Due to this the electrons will be attracted towards nucleus more strongly. By coming more and more close to the nucleus following a spiral path ultimately they will fall into the nucleus. Such an atom is not stable.
The Bohr’s Atomic Model
According to Bohr’s atomic model the electrons revolve around the nucleus in particular orbits having fixed energy levels.
When electrons revolve in fixed orbits they do not loose energy and in turn do not fall into the nucleus and that is why they are stable.
The number of electron shells (energy levels) in which electrons revolve is also fixed.

The nucleus of an atom consists of protons and neutrons and is positively charged. An atom remains neutral due the presence of equal number of protons (positive charge) and electrons (negative charge). The Orbits, Shells, Energy Levels In An Atom
Atomic Number
The number of protons, which are equal to number of electrons, is called atomic number.
It is denoted by Z. Example Z = 1 for hydrogen => 1 electron & 1 proton Z = 6 for carbon => 6 electrons & 6 protons
Mass Number The total number of protons and neutrons present nucleus of an atom is known as mass number.

An element is represented as Symbol Number Mass Number Atomic no

§ Valence Electrons And Valency

Valence Electrons The electrons, which are present in the outermost shell of an atom, are called valence electrons.

Only the valence electrons take part in chemical reactions.

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