Cbse guide: Structure of atom Lesson 4 class 9th Chemistry

Chapter 4 class 9th notes

Introduction :

Till now we have learnt that matter is made up of atom and molecules
Atom of same element are identical
Atom of different elements are different .
But the ques arises in what sense these are different?
THis was a major challenge to scientists to give answer .
This mystery was solved towards the end of 19th century by scientists like J.J Thomson , Goldstein, Rutherford, Chadwick , Bohr and many more.
With their researches , it was established that atom is not as simple as was predicted by Dalton .
It is made up of a large number of particles out of which the most important are Electrons , protons and Neutrons

Charged particles in matter

The electric nature of matter was indicated quite early in the form of fictional electricity. It was produced by rubbing two articles together such as glass rod with silk or fur . As a result of rubbing , they got electrically charged . As matter is constituted by atoms , this suggested the presence of charge in an atom.

Discovery of electrons (e-) and cathode rays :

First identified by JJ Thomson.

He performed this experiment in discharge tube . It is a long cylindrical tube sealed at both the ends and is fitted with two electrodes. Two electrodes are connected to a source of high voltage called a battery.

The electrodes connected to the negative terminal of the battery is called cathode .

The electrodes connected to the positive terminal of the battery is called anode .

The tube is connected to a vacuum pump so as to reduce the pressure inside the tube to an extent.

Fluorescent material is used on opposite side of cathode .

Observations :

(i) A current of very high voltage was passed through discharge tube under normal pressure of one atmosphere.

No current passed as air is a bad conductor of electricity.

(ii) The pressure inside the discharge tube was slowly reduced to about 1 mm by working the vacuum pump and then current is passed .

Electricity start flowing and and light was emitted inside the tube.

(iii) The emossion of light stopped when the pressure inside the tube was further reduced to about 0.01 mm of mercury.

The wall of tube opposite to cathode glow with greenish light called fluorescence.

Conclusion :

Florescence is due to bombardment of the walls of tube by the rays emitted from the cathode . these were known as cathode rays .

Thomson repeat the same experiment by replacing gases . He got same results.

Since these rays of particles moves towardspositivecharges electrode , so it itself carry negative charge. These negatrive charged particles are called electrons .

Discovery of protons(p+) and anode rays :

First identified by E. Goldstein.

He performed this experiment in modified discharge tube.It is a long cylindrical tube sealed at both the ends and is fitted with two electrodes. Two electrodes are connected to a source of high voltage called a battery.

The electrodes connected to the negative terminal of the battery is called cathode .

He used perforated electrode as cathode and placed it in centre and not at end.

The electrodes connected to the positive terminal of the battery is called anode .

The tube is connected to a vacuum pump so as to reduce the pressure inside the tube to an extent.

Fluorescent material is used on opposite side of anode

He used hydrogen gas first for his experiment.

Observation :

Red colour glow on the side opposite to anode .

Conclusion :

Florescence is due to bombardment of the walls of tube by the rays emitted from the anode. these were known as anode rays .

These rays are also called canal rays as these passed through canal of cathode.

Thomson repeat the same experiment by replacing gases . He got different results.

Since these rays of particles moves towards negative charges electrode , so it itself carry positive charge. These positive charged particles are called protons .

Discovery of Neutrons (n) :

Rutherford found that except for hydrogen atom , the atomic mass of no other element could be explained by electrons and protons only. . So Rutherford predicted the presence of some neutral particles present in the nucleus of an atom.

These neutral particles were discovered by Chadwick in 1931 , when bombarded a thin foil of beryllium metal with the fast moving alpha particles .

These particles have mass similar to proton but carry no charge.

How does the atom look like?

Thomson` Model of an atom :

According to him

(i)An atom consists of a positively charged sphere and the electrons are embedded in it.

(ii) The negative and positive charges are equal in magnitude.

So, the atom as a whole is electrically neutral

Limitations:

He could not offer any experimental support to his atomic model.
He could not explain the results of the experiments carried by Rutherfod and other scientists at a later stage.

Rutherford model of atom:

In this experiment, fast moving alpha (α)-particles were made to fall on a thin gold foil.

• He selected a gold foil of about 1000 atoms thick.

• the fast-moving α-particles was bombarded on this gold foil.

Observations

(i) Most of the fast moving α-particles passed straight through the gold foil.

(ii) Some of the α-particles were deflected by the foil by small angles.

(iii) Surprisingly one out of every 12000 particles appeared to rebound

Conclusion:

(i) Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.

(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.

(iii) A very small fraction of α-particles were deflected by 1800,indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom

Rutherford nuclear model of an atom(features):

(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 circular paths.

(iii) The size of the nucleus is very small as compared to the size of the atom

Drawbacks of Rutherford’s model of the atom:

Maxwell Questioned that: Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know.

So he could not explain the stability of atom .

BOHR’S MODEL OF ATOM:

(i)Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(ii) While revolving in discrete orbits the electrons do not radiate energy

These orbits or shells are called energy levels. Energy levels in an atom are shown.

These orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….

How are Electrons Distributed in Different Orbits (Shells)? in Different Orbits (Shells)?

The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury. The following rules are followed for writing the number of electrons in different energy levels or shells:

(i) The maximum number of electrons present in a shell is given by the formula 2n2, where ‘n’ is the orbit number or energy level Hence the maximum number of electrons in different shells are as follows:

first orbit or K-shell will be = 2 × 12 = 2,

second orbit or L-shell will be = 2 × 22 = 8,

third orbit or M-shell will be = 2 × 32 = 18,

fourth orbit or N-shell will be = 2 × 42 = 32, and so on.

(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8. (iii) Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.

Atomic structure of the first eighteen elements is shown schematically: