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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.
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 solutions for class 9 Science chapter 4 Structure of the atom |
NCERT exemplar class 9 Science chapter 4 Structure of the atom |
Topics covered in the NCERT textbook are :
4.1 Charged Particles in Matter |
4.2 The structure of an atom |
4.3 How are electrons distributed in different orbitals |
4.4 Valency |
4.5 Atomic number and Mass number |
4.6 Isotopes |
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.
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.
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)...
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.
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.
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).
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.
NCERT Class 9th Science Chapter 4 Notes |
The key concepts include:
The total of the masses of protons and neutrons in an atom's nucleus is known as its atomic mass.
Given that the mass of the helium atom is 4 u and two protons are present in its nucleus.
So the number of neutrons will be
Number of neutrons = atomic mass − number of protons
Number of neutrons = 4−2
∴Number of neutrons = 2
Therefore, the helium atom has 2 neutrons
The atomic number of carbon is 6 and the atomic number of sodium is 11. So the distribution of electrons in the carbon atom is 6 = 2,4 Distribution of electrons in the sodium atom is 11 = 2,8,1.
Understanding the structure of the atom is crucial because
Dalton's theory had limitations:
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