Class 9 Science Chapter 4 Structure of the Atom Notes pdf

Class 9 Science chapter 4 Structure of the atom is very important chapter of chemistry. The chapter contains different atomic models and their structure proposed by various scientists. Class 9 Science chapter 4 Structure of the atom chapter is vey important from examination point of view also. Many questions from this chapter come in exams.

Here is pdf notes for Class 9 Science chapter 4 Structure of the atom. The notes contain all the topics from Class 9 NCERT and enough examples for better understanding of the student. The students can revise from these notes for their examinations as it contains all topics in brief and short.

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Class 09 Science Chapter-04 Structure of the Atom Notes

Dalton proposed that atoms are indivisible.

Studying static electricity and the conditions under which electricity is conducted by different substance indicated that atoms are not indivisible.

When two objects rubbed together they get charged (static electricity). It indicates that atoms contains charged particle.

Sub-atomic particles in atom:

Electrons: Electrons are negatively charged particle.

Protons: Protons are positively charged particle.

Neutrons: Neutrons are neutral particle.

Particle	Charge	Mass	Discovered by
electron	-1.6 x 10^-19	9.108 x 10^-31 kg	J.J Thomson
proton	+1.6 x 10^-19	1.6726 x 10^-27 kg	Ernest Rutherford
neutron	0	1.675 x 10^-27 kg	James Chadwick

Only electrons get transferred from one body to another and the objects get charged in this way. (Protons does not transferred).

Consider the rubbing of two objects A and B:

static electricity between two objects

The object from which electrons are transferred get positively charged and the object on which electrons are transferred get negatively charged.

The Structure of the atom:

To know how electrons and protons are arranged within an atoms, various atomic model were proposed by different scientists.

Thomson's Model of an Atom:

Thomson's model of atom was similar to a Christmas pudding. He proposed that atom is a sphere of positive charge and electrons are embedded in it.

Thomson's atomic model can be compared to a watermelon- the positive charge in the atom is spread all over like red edible part in watermelon and the electrons are studded in the the positively charged sphere, like the seeds in the in the watermelon.

Thomson's Model of an Atom

Thomson proposed that:

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

The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

Rutherford's Model of an Atom:

Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. So, he performed an experiment called α-particle scattering experiment.

α-particle scattering experiment:

In this experiment α-particles (doubly charged helium ion - He²⁺) are directed to a surface of thin gold foil (thickness ~ 10^-7m).

Rutherford's α-particle scattering experiment

Observations made in the α-particle scattering experiment:

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

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

Surprisingly one out of every 12000 particles appeared to rebound.

Conclusions drawn from α-particle scattering experiment:

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

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

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.

From the data he also calculated that the radius of the nucleus is about 10⁵ times less than the radius of the atom.

Features of Rutherford's Nuclear Model of an Atom:

There is a positively charged center in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.

The electrons revolve around the nucleus in circular paths.

The size of the nucleus is very small as compared to the size of the atom.

Drawbacks of Rutherford’s model of an atom:

Rutherford proposed that the electrons revolve around the nucleus in circular orbits. When an object is moving in an orbit, it undergoes acceleration even if it is moving at constant speed because of change in direction. During acceleration charge particles would radiate energy and its orbit will continue to shrink and finally fall into the nucleus. But this does not happen.

Thus, Rutherford's model cannot explain the stability of an atom.

Bohr's Model of an Atom:

Postulates about Bohr's atomic model:

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

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

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

Bohr's model of an atom

Distribution of electrons 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:

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-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.

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

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.

Valency:

The electrons present in the outermost shell of an atom are known as the valence electrons.

The atoms of elements, completely filled with 8 electrons in the outermost shell show little chemical activity (least reactive). Their valency is zero.

The combining capacity of the atoms of elements is the attempt to attain a fully-filled outermost shell.

An outermost-shell, which had eight electrons was said to possess an octet. Atoms would thus react, so as to achieve an octet in the outermost shell.

This was done by sharing, gaining or losing electrons. The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives us directly the combining capacity of the element (valency of the element).

Valency is determined by the number of electrons gained/lost/shared in order to complete the outer most shell (octet).

If the number of electrons in the outer most orbit of an element are less than or equal to 4, it will most likely lose its electrons to complete its octet.

e.g. Consider Magnesium and Aluminium:

Magnesium:

Magnesium has 2, 8, 2 electrons in K, L, M shell respectively.
Number of electrons in the outermost orbit = 2 (less than 4)
it will lose its 2 electrons to complete its octet (now it has 2, 8 electrons in K, L shell)
Now it has 8 electrons in the outermost orbit (octet).
Valency of Magnesium = 2

Aluminium:

Aluminium has 2, 8, 3 electrons in K, L, M shell respectively.
Number of electrons in the outermost orbit = 3 (less than 4)
it will lose its 3 electrons to complete its octet (now it has 2, 8 electrons in K, L shell)
Now it has 8 electrons in the outermost orbit (octet).
Valency of Aluminium = 3
If the number of electrons in the outer most orbit of an element are more than 4, it will most likely gain electrons to complete its octet.

e.g. Consider Oxygen and Chlorine:

Oxygen:

Chlorine:

Neutrons:

J. Chadwick discovered neutrons in 1932.

Neutrons are neutral (contains no charge) and the mass is nearly equal to that of a proton.

Neutrons are present in the nucleus of all atoms, except hydrogen.

The mass of an atom is given by the sum of the masses of protons and neutrons present in the nucleus.

Atomic Number and Mass Number:

Atomic Number:

The atomic number is defined as the total number of protons present in the nucleus of an atom.
It is denoted by 'Z'. All atoms of an element have the same atomic number.
For Hydrogen, Z = 1, because in hydrogen atom, only one proton is present in the nucleus.
For Carbon, Z = 6, because in Carbon atom, 6 protons are present in the nucleus.
For Oxygen, Z = 8, because in Oxygen atom, 8 protons are present in the nucleus.

Mass Number:

The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom.
It is denoted by 'A'.
Protons and neutrons are also called nucleons (present in the nucleus of an atom).
For example, mass of carbon is 12 u because it has 6 protons and 6 neutrons, 6 u + 6 u = 12 u.
The mass of aluminium is 27 u (13 protons + 14 neutrons).

In the notation for an atom, the atomic number, mass number and symbol of the element are to be written as:

Isotopes:

The elements which have same same atomic number but different mass numbers are called isotopes.
For example, Hydrogen exists in three isotopic forms: ¹₁H (protium), ²₁H (deuterium), ³₁H ( tritium).
More examples of isotopes are:

Carbon: ¹²₆C, ¹⁴₆C

Chlorine: ³⁵₁₇Cl, ³⁷₁₇Cl

Isobars:

Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.

Let us consider two elements — calcium, atomic number 20, and argon, atomic number 18. The number of protons in these atoms is different, but the mass number of both these elements is 40. That is, the total number of nucleons is the same in the atoms of this pair of elements.