Class 9 Structure of an atom

CHAPTER – 4

STRUCTURE OF ATOM

1. Atom is indivisible as proposed by Dalton. It is made up of smaller particles. There are charged particles in the atom, positively charged and negatively charged.

ACTIVITY

To show that atom consists of charged particles.

Material required: comb, pieces of paper, glass rod, silk cloth, inflated balloon.

Procedure:

1. Comb dry hair.
2. Bring the comb near the pieces of paper.
3. Now rub the glass rod with a silken cloth.
4. Bring the rod near the balloon.

Observation:

We will observe that the pieces of paper are attracted towards the comb. Similarly the inflated balloon is attracted towards the glass rod.

Conclusions:

From the above activity we conclude that the atoms consist of charged particle.

DISCOVERY OF SUB – ATOMIC PARTICLE

S.NO.	SUB –ATOMIC PARTICLE	DISCOVERED BY
1.	Electron	J.J. Thomson
2.	Proton	E. Goldstein
3.	Neutron	J. Chadwick

PROPERTIES OF ELECTON, PROTON, NEUTRON

S.NO.	PROPRTY	ELECTRON	PROTON	NEUTRON
1.	Symbol	e	p	n
2.	Nature of charge	-	+	neutral
3.	Absolute charge	-1.6 * 10 c	-1.6 * 10c	0
4.	Absolute mass
5.	Relative mass	1/1840u	1u	1u
6.	Relative charge	-1	+1	0
7.	Location in the atom	Outside the nucleus	Inside the nucleus	Inside the nucleus

THOMSON MODEL OF AN ATOM

Thomson proposed a model of an atom which looked like a Christmas pudding in which the currants were the electrons embedded into it. We can also think of a watermelon in which the seeds were the electrons and the hard outer covering was its positively charged sphere.

Thomson proposed that:

1. An atom consists of a positively charged sphere and the electrons are embedded in it.
2. The negative and positive charges are equal in magnitude. So the atom as a whole is electrically neutral.

But Thomson could not explain the results of various experiments.

RUTHERFORDS MODEL OF AN ATOM

Rutherford performed an experiment in which fast moving alpha particles were made to fall on a thin gold foil. Alpha particles are helium nuclei with a mass of 4u and a charge of +2. He chose the gold foil because gold is the most meliable metal and cab be beaten into thin sheets.

OBSERVATIONS

1. Most of the fast moving alpha particles passed straight through the gold foil.
2. Some of the particles were deflected by small angles.
3. Surprisingly, very few particles appeared to be rebound.

CONCLUSIONS

1. There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
2. The electrons revolve around the nucleus in well-defined orbits.
3. The size of the nucleus is very small as compared to the size of the atom.
4. Most of the space inside the atom is empty.

DRAWBACKS OF RUTHERFORDS MODEL

1. Any charged particle which is moving in a circular orbit/path would lose energy and finally will fall into the nucleus. This would lead to destruction of atom.

But we know that this does not happen. Atom is quite stable. Rutherford model does not explain this anomaly.

2. He did not tell anything about the distributions of electrons in various orbits.

BOHR’S MODEL OF AN ATOM

POSTULATES OF BOHR’S MODEL

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

2. While revolving in discrete orbits the electrons do not radiate energy.

These orbits or shells are called energy levels.

BOHR AND BURY SCHEME

1. The maximum number of electrons present in a shell is given by the formula 2n² where ‘n’ is the orbit number or energy level index, 1,2,3,…. Hence the maximum number of electrons in different shells are as follows:

First orbit or k is 2, second orbit L is 8 and so on.

2. The maximum number of electrons that can be accommodated in the outermost shell is 8.

3. Electrons are not accommodated in a given shell, unless the inner shells are filled in a step-wise manner.

VALENCY

The number of electrons that an atom needs to gain, loose or share to complete its octet is known as its valency.

ATOMIC NUMBER

Atomic number is equal to the number of electrons or protons in an atom. It is denoted by ‘Z’.

MASS NUMBER

Mass no. is the sum of the number of protons and neutrons present in the nucleus of an atom. It is denoted by ‘A’.

ISOTOPES

Isotopes are the atoms of the same element having different mass number.

EXAMPLE: C₆¹², C₆¹⁴;Cl₁₇³⁵,Cl₁₇³⁷

ISOBARS

Isobars are the atoms having the same mass number but

different atomic numbers.

EXAMPLE: Ca-20, Ar-18

^{Mass number}

Symbol

Atomic number

CANAL RAYS

Canal rays are positively charged radiations which were discovered by E. Goldstein in 1886. These rays travel in a direction opposite to cathode rays. These rays are also called anode rays or positive rays. Canal rays led to the discovery of positively charged subatomic particle known as proton.

APPLICATIONS OF ISOTOPES

1. An isotope of uranium is used as a fuel in nuclear reactors.

2. An isotope of cobalt is used in the treatment of cancer.

3. An isotope of iodine is used in the treatment of goitre.

AVERAGE ATOMIC MASSS

(Mass of first isotope x mass % + mass of second isotope x mass %)

100

For example: - CHLORINE

35 x 75 + 37 x 25 = 35.5u

100