Atomic structure part 2

Sir Humphry Davy

* By passing electricity Many elements isolated from their compound.

eg: potassium, sodium Calcium etc

* He conducted experiments which involve the passage of electricity through 

liquids.

* He established there are two types of charges (+ve &-ve), 

these charges that enable a substance to react with another substance.

 

Michael Faraday

* Michael Faraday is known as the father of electricity.

* He proposed the laws in connection with electricity (Law of electrolysis).

 

J J Thomson

* He proved that the rays orginating from the discharge tube (Cathode) are made 

up of negatively charged particles.

* These particles possess mass and energy.

* These particles are common to all substances, they are much smaller than the 

atom and are part of atoms.

*  The negatively charged particle in the atom is called Electron.

Ernest Rutherford

* He conducted experiments by passing positively charged alpha particles through the gold foil, allowed to fall on a circular photographic film.

* He analysed the major portion of an atom as empty and there is a small part 

inside the atom where all the positive charge is concentrated.

* The central region of an atom is called the Nucleus.

* He recognised the presence of a positively charged centre in an atom.

* This particles that were responsible for the positive charge are called Protons.

Its charge was found to be equal and opposite to that of an electrons.

* Mass of proton is equal to the mass of a hydrogen atom.

James Chadwick

* He proved scientifically, inside the nucleus of an atom, there are neutral particle having mass equal to that of protons. This chargeless particle is the Neutron.

Features of Electron, Proton and Neutron

Atoms are electrically neutral. why?

*  An atom contains an equal number of negatively charged electrons and positively charged protons. Therefore atoms are said to be neutral

Rutherford's planetary Model of Atom

Demerits of Rutherford model atom

 According to this model electrons should eventually collapse into the nucleus as they loss their energy while revolving within the field of attraction of the nucleus. But this does not happen in atoms. He failed to give an explanation to this doubt.so this model was eventually ddiscarded.

Bohr's Model of Atom

 Niels Bohr proposed a new atomic model by giving a better explanation to the Rutherford model. This model is known as Bohr's model.

1. Main ideas of the Bohr Model

* Electrons revolve around the nucleus of an atom infixed paths called orbits or shells.

* Electrons in each shell have a definite energy. Hence shells are also called Energy levels.

* As long as an electron revolves in a particular orbit,its energy remains constant.

* The energy of the shells increases as the distance from the nucleus increases.

* The shells around the nucleus can be numbered from near the nucleus as 1,2,3,4…...or represented by 

the letters K,L,M,N……

Mass Number and Atomic Number

* The total number of protons and neutrons in an atom is called the Mass Number. This is represented 

by the letter A.symbol eg:

 

* The total number of protons in an atom is called its Atomic Number (Z).

* In an atom Number of protons and the number of electrons are equal.

* Mass number on the top left side of the symbol and atomic number on the bottom left side of the symbol

 eg: