NCERT Class 9 Science Chapter 4 Notes - Structure Of The Atom PDF Notes

What makes up everything around us, from the air we breathe to the buildings or technology we use? The simple answer to this question is atoms. Atoms are the fundamental building blocks of everything around us from the gold or diamonds used in jewellery to the silicon used in mobile phones. Have you ever picked a grain of sand that is very small in size and that gain too is made up of millions of atoms.

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1. NCERT Notes for Class 9 Science Chapter 4: Download PDF
2. NCERT Notes for Class 9 Science Chapter 4
3. Previous Year Questions of Class 9 Science Chapter 4
4. NCERT notes for Class 9 Science Chapter-wise
5. Subject-Wise NCERT Exemplar Solutions
6. Subject-Wise NCERT Solutions

The chapter covers many important topics like atomic models, isotopes, isobars etc. that will form the basis for the concepts in the upcoming classes. The NCERT Notes for Class 9 Science chapter 4 is designed to help you memorize the concepts in the easiest way possible. These notes break down complex atomic theory in a simple and engaging way which makes learning enjoyable and effective. These NCERT notes will help students in their exams and will improve their understanding of the concepts. The formulas and diagrams are also provided for effective learning.

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• NCERT solutions for class 9 Science chapter 4 Structure of the atom
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NCERT Notes for Class 9 Science Chapter 4: Download PDF

Download the PDF to access a clear explanation of this chapter. These NCERT notes cover all the key concepts of the Structure of Atom. You can download the PDF from the button given below:

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NCERT Notes for Class 9 Science Chapter 4

These notes will work as key guide for understanding the concepts deeply. Here we will look into various experiments explaining the characteristics features of an atom. The chapter will help you build a strong command on the topics.

Atoms and molecules are the building blocks of matter. The existence of different kinds of matter around us is due to the different types of atoms and molecules present in them. For a long time it was thought that the atoms are indivisible, so they do not have an inner structure. But now we know that atoms are divisible and they do have an inner structure. Atoms have smaller particles in them which are called subatomic particles.

4.1 Charged Particles in Matter

Initially, atoms were considered indivisible, but later on, it was formulated that they are made up of subatomic particles that are negatively charged and they were called electrons. Those were discovered by J.J. Thomson.

• E. Goldstein in 1886 found out that there were some radiations present in atoms that were termed as canal rays and it was concluded later that two kinds of sub-atomical particles are there are electrons and protons.
• Protons have the same magnitude but opposite charge as compared to electrons and protons is having 2000 more mass as compared to electrons.

4.2 The Structure of an Atom

After the discovery of electrons and protons, the scientist started thinking of arranging these particles in an atom therefore different models were proposed to explain the distribution of subatomic particles in an atom.

The first simple model was proposed by JJ Thomson which is known as Thomson atomic model.

4.2.1 THOMSON’S MODEL OF AN ATOM

JJ Thomson proposed that an atom consists of a uniform sphere in which positive charge is uniformly distributed.

• The electrons are embedded into it in such a way as to give the most stable atom.
• This model was much like pudding or cake because he assumed a pudding as a positive charge and the resins inside as electrons embedded into it.
• This model was also compared with the watermelon model as a positive charge in which seeds (electrons) are embedded.
• An important feature of this model is that the mass of the atom is considered to be evenly spread over the atom.

This model explains the neutrality of the atom however it was soon discarded when other scientists experimental results inquired about some questions that were not answered by this model.

4.2.2 RUTHERFORD’S MODEL OF AN ATOM

In 1909, Rutherford and a student conducted a series of experiments to better understand how electrons and protons are arranged in an atom.

To detect alpha particles, a high-energy particle beam was aimed at a thin gold foil with a thickness of around 100 nm.

Observation

• Through the gold foil, the majority of the alpha particles remained undeflected.

• A small percentage of alpha particles were discovered to have been deflected at modest angles.

• A small percentage of particles did not travel through the foil at all, but instead suffered significant deflection or even returned after 180° deflection.

Because the majority of the alpha particles went through the gold foil undeflected, Rutherford concluded that there must be a lot of free space within the atom. Since some of the alpha particles deflected to certain angles it means that there is a heavily positively charged mass present in the atom and because just a few particles experienced large deflections, this mass must be occupying a very limited space within the atom.

Main Features of Rutherford's Model

Following were the main features of the model of the Rutherford

• The whole mass and positive charge of an atom are concentrated in a very tiny region at the core known as the nucleus; nonetheless, the volume occupied by the nucleus is negligibly small when compared to the overall volume of the atom.

• Protons are responsible for the nucleus' positive charge.

• The mass of the nucleus comes from protons and other neutral particles, each of which has a mass that is almost equivalent to that of a proton.

• The structure is electrically neutral because the nucleus is surrounded by negatively charged electrons that balance the positive charge on the nucleus.

• Electrons are not static; they spin around the nucleus at a rapid rate, similar to how planets orbit the sun.

• The electrostatic force of attraction holds electrons and the nucleus together.

This atomic model failed to explain the stability of atoms.

Drawbacks of the Rutherford Model

Electrons revolve around the positively charged nucleus, according to the Rutherford atom model. However, in the long run, it is not conceivable since atoms are stable, whereas any particle in a circular orbit will accelerate.

Charged particles would radiate energy as they accelerated. As a revolving electron loses energy and eventually falls into the nucleus, this model fails to explain atomic stability.

4.2.3 BOHR’S MODEL OF ATOM

To correct the drawbacks of Rutherford, Bohr came up with another model of an atom. Following were the postulates of Bohr’s atomic model

1. Only certain orbits known are discrete orbits are allowed inside the atom

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

These orbits or shells are known as energy levels.These are names as K(n=1), M(n=2), O(n=3), P(n=4)...

4.2.4 NEUTRONS

In 1932, J. Chadwick discovered another subatomic particle called a neutron. neutron is not having any charge and its mass is equal to that of a proton. It was suspected that the nucleus is having protons and neutrons and as neutrons are chargeless that’s why the charge of the nucleus was concluded to be positive. Neutron is represented by 'n' and the mass of an atom is given by the sum of neutrons and protons.

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

Lets find out how electrons are filled in different orbits;

The dispersion of electrons into different orbits of an atom was proposed by Bohr and Bury. They proposed a set of principles that must be observed when filling a shell with electrons. The number of electrons in different energy levels or shells is written using the following rules:

• Formula 2n² calculates the maximum number of electrons in a shell, where 'n' is the orbit number or energy level index. The values of n are 1,2,3,...

As a result, the maximum number of electrons that can be present in each shell is:

• The first orbit, often known as the K-shell, will be = 2 x 1 = 2. The second orbit, often known as the L-shell, will be = 2*4=8. The third orbit, often known as the M-shell, will be 2 * 9= 18. The fourth orbit, often known as the N-shell, will be 2*16 = 32, and so on.

• In the outermost orbit, a maximum of eight electrons can be supported.

• The inner shells must be filled before electrons may be accommodated in a given shell. That is to say, the shells are filled one at a time.

4.4 Valency

The electrons that are present in the outermost shell are known as valence electrons.

If you take the example of magnesium which has an atomic number of 12, its configuration is 2,8,2 it will be easier for magnesium to lose two electrons to gain stability.

All the elements in which electrons in the outermost shell are less than 8 are chemically reactive. They share, gain, or lose an electron to complete their octet and completing their octet provides them stability this combining capacity of the elements to complete their octet is known as their valency.

4.5 Atomic Number and Mass Number

4.5.1 ATOMIC NUMBER

The atomic number of an element is described by the total number of protons present in its nucleus. It is represented by Z, also the number of protons(p) is equal to the number of electrons(e) in an atom.

4.5.2 Mass number

The sum of the neutrons and protons is known as mass number. the total mass of the atom is equal to the mass of the nucleus and neutrons and protons are present in the nucleus, both these subatomic particles together are known as nucleons. The mass number is represented by A.

Generally, an atom is represented by its symbol for the element atomic number is written on the lower side of the symbol and the mass number is written on the upper side.

4.6 Isotopes

Atoms of the same element which have the same atomic number but different mass numbers are called isotopes.

Example- (i) carbon, ${}_6^{12}\mathrm{C}$ and ${}_6^{14}\mathrm{C}$, (ii) chlorine, ${}_{17}^{35}\mathrm{Cl}$ and ${}_{17}^{37}\mathrm{Cl}$, etc.

Similarly, chlorine has 2 isotopes having the same atomic number Z = 17 whereas their mass numbers are 35 and 37 when we take the mass number of chlorine then we take an average of these 2 numbers.

The isotopes of a given element will show the same chemical properties.

4.6.1 ISOBARS

Atoms of different elements having the same mass number but different atomic numbers are called isobars.

These atoms differ in their atomic number and therefore they have a different number of protons or electrons and also a different number of neutrons.

For example, if you take the case of argon, potassium, and calcium all of them have mass numbers 40 but their atomic numbers are 18, 19, and 20 respectively isobars are atoms of different elements and hence they have different properties.

Previous Year Questions of Class 9 Science Chapter 4

Question: Which of the following statements about the atom is correct according to Bohr's model?

1) Electrons radiate energy while revolving in orbits
2) Electrons move in random paths
3) Electrons revolve in fixed energy levels without radiating energy
4) Nucleus revolves around electrons

Answer:
According to Bohr, electrons move in specific circular orbits (energy levels) where they do not lose energy. Energy is absorbed or emitted only when an electron jumps between orbits.

Hence, the correct answer is option (3).

Question: The mass of an electron is approximately:
1) Equal to that of a proton
2) 1/1837 times the mass of a proton
3) Twice the mass of a proton
4) Zero

Answer:
An electron is much lighter than a proton. Its mass is nearly 1/1837 of a proton, which is why the mass of an atom is concentrated mostly in its nucleus.

Hence, the correct answer is option (2).

Question: Which of the following particles is not present in the nucleus of an atom?

A. Proton
B. Neutron
C. Electron
D. Both A and B

Answer:

Electrons revolve around the nucleus in shells; only protons and neutrons are present inside the nucleus.

Hence, the correct answer is option (3).

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