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 block 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. The size of atoms is very small and we cannot see them with the naked eye but they are very powerful as they are the building blocks of everything around us. Science class 9 Structure of Atoms is a very important chapter from the point of view of CBSE exams and it forms the foundations of very crucial topics in the class 11 and class 12 syllabus.

This Story also Contains

1. NCERT Class 9 Chapter 4 Class Notes
2. Structure Of The Atom
3. Charged Particles in Matter
4. The Structure of an Atom
5. Thomson’s Model of an Atom
6. Rutherford's Model of an Atom
7. Main Features of Rutherford's Model
8. Bohr's Model of Atom
9. How are Electrons Distributed in Different Orbits (Shells)?
10. Valency
11. Atomic Number and Mass Number
12. Isotopes
13. Isobars
14. Most Important Questions of Class 9 Chapter 4 Structure of the Atom
15. Approach To Solve Class 9 Science Chapter 4 Questions
16. Chapter-Wise NCERT Class 9 Notes Science
17. Subject-Wise NCERT Exemplar Solutions
18. Subject-wise NCERT Solutions

Sometimes it becomes very difficult to read chapters again and again from textbooks and memorize concepts. So to solve this problem, NCERT Class 9 Science Chapter 4 notes are prepared by our subject experts in a very comprehensive and easy-to-understand way. These notes break down complex atomic theory in a simple and engaging way which makes learning enjoyable and effective. NCERT Notes will help students in their boards and competitive exams. Each concept, including formulae and examples of the structure of atoms, is provided as easily and effectively as possible in these notes.

Also, students can refer to

NCERT Class 9 Chapter 4 Class Notes

Topics covered in the NCERT textbook are :

Structure Of The Atom

• Every matter is made up of atoms and molecules.
• Structure of the atom class 9 notes tells that there was a fact which was earlier raised that atoms are indivisible but now scientists were able to prove that some sub-atomical particles are present in atoms.
• A Series of experiments have been performed to find the arrangement of these sub-atomical particles.
• Here we will look into various experiments explaining the characteristics features of an atom.
• We will also look into the merits and demerits of different structures of atoms.

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.

The Structure of an Atom

• The theory that the atom is indivisible was put forward by Dalton and later on, it was proved wrong.
• 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.

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.

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.
• Rutherford's scattering experiments were the name of his experiment.
• A bit of radioactive material, such as radium, was inserted in the lead block in this experiment.
• The block is designed with slits so that only a limited beam of an alpha particle can escape.
• To detect alpha particles, a high-energy particle beam was aimed at a thin gold foil with a thickness of around 100 nm.
• After scattering, a moveable circular screen covered with zinc sulfide was placed around the gold foil.

When an alpha particle hits the zinc sulfide screen, it causes flashes of light that may be seen. The proportions of alpha particles deflected through various angles could be determined, and the following observations could be made as a result of this experiment

• 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. Alpha particles are positively charged and have a lot of mass. Due to the huge force of repulsion, they can only be deflected if they approach close to a heavy positively charged mass. 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.

• The direct impact of the massive positively charged mass of the atom was described as the cause of the strong deflections or even bouncing back of alpha particles from the foil.

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. For various atoms, the size of the positive charge on the nucleus, i.e. the number of protons, varies.

• 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 nucleus of an atom represents the sun while circulating electrons represent the planets.

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

• The majority of the space between the nucleus and the spinning electron is vacant.

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.

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)...

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.

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.

Valency

• We have now already learned how the electrons are filled in shells.
• The electrons that are present in the outermost shell are known as valence electrons.
• In some elements, the outermost shell is complete which is they are having a total of eight electrons and are not capable of chemically reacting, these elements are known as inert elements. for eg Helium, neon, argon, etc.
• We can take the example of fluorine and oxygen as the atomic number of fluorine is nine that is the total number of protons that is equal to several electrons is equal to 9, and its electronic configuration is 2,7.
• It can either lose seven electrons or gain one electron to an octet Gaining one electron will require less energy as compared to losing seven electrons fluorine will gain one electron.
• 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.

Atomic Number and Mass Number

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.

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.

Isotopes

• Atoms of the same element which have the same atomic number but different mass numbers are called isotopes.
• Since the atomic number of different isotopes of the same element is the same it means that they have the same number of electrons and protons, the difference in their mass numbers is due to a different number of neutrons present in their nuclei.
• Hydrogen is the common example which has 3 isotopes these are commonly known as hydrogen, deuterium, and tritium.
• These 3 isotopes have the same atomic number one but different mass numbers 1,2 and 3 respectively.
• The term hydrogen is used only for the first isotope all these isotopes have only one electron.
• 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.
• It may be noted that the chemical properties of atoms are mainly controlled by electrons which are determined by the number of protons in the nucleus therefore all the isotopes that have given element will show the same chemical properties.

Isobars

• Atoms of different elements having the same mass number but different atomic numbers are called isobars since Isobars have the same mass number therefore the sum of protons and neutrons in the nucleus of each atom is the same.
• 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.

Most Important Questions of Class 9 Chapter 4 Structure of the Atom

Q 1. 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).

Q 2. 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).

Approach To Solve Class 9 Science Chapter 4 Questions

The approach is crucial while attempting any questions, as it can affect your accuracy and speed. The following are some tips that can help you build a good approach.

1. Understand Atomic models
First try to learn features and limitations of models by Dalton, Thomson, Rutherford and Bohr. Focus on how each model improved on the previous one.

2. Know about subatomic particles
Try to memorize the charge, mass and location of electrons, protons, and neutrons as sometimes they are directly asked in exam.

3. Practice electron configuration
Use the 2n² rule to distribute electrons in shells (K, L, M...). Also, solve examples with atomic numbers to write configurations. This can help you a lot while answering the questions related to it.

4. Learn important terms

Atomic number (Z) = number of protons = number of electrons
Mass number (A) = protons + neutrons
Number of neutrons = Mass number – Atomic number

Distinguish between Isotopes & Isobars
Isotopes- Same atomic number, different mass numbers.
Isobars- Same mass number, different atomic numbers.

5. Solve NCERT & Exemplar questions
Practice is the key. Solve more and more questions for better understanding and you can also refer to the NCERT solved examples to get the correct pattern to answer the questions.

Chapter-Wise NCERT Class 9 Notes Science

Subject-Wise NCERT Exemplar Solutions

Subject-wise NCERT Solutions