What is the mass of an electron approximately compared to a proton?

About 1/2000 times that of a proton

About 2000 times more than a proton

About 1/2000 times that of a proton

About 1/100 times that of a proton

In the given diagram of Rutherford's experiment, what does the path labeled 'A' represent?

Alpha particles passing straight through

Alpha particles deflected at small angles

Alpha particles rebounding back

Which part of the given Bohr's model represents the 'L' shell?

The second circle from the center

The third circle from the center

Assertion (A): Rutherford's model of the atom could not explain the stability of the atom. Reason (R): According to classical electromagnetic theory, an orbiting electron would continuously radiate energy and eventually spiral into the nucleus.

A and R both correct, R explains A

Assertion (A): Isotopes of an element have the same chemical properties. Reason (R): Chemical properties are determined by the number of valence electrons, and isotopes have the same number of electrons.

A and R both correct, R explains A

Assertion (A): Neutrons are present in the nucleus of all atoms. Reason (R): Neutrons contribute to the mass of the atom but not to its charge.

A correct, R correct, R does not explain A

A student is analyzing a sample of an unknown element. They determine that the sample contains two types of atoms. Atom X has 17 protons and 18 neutrons. Atom Y has 17 protons and 20 neutrons. Based on this information, what can be concluded about atoms X and Y?

X and Y are isotopes of the same element.

X and Y are atoms of different elements.

X and Y have different atomic numbers.

Rutherford's alpha-particle scattering experiment provided evidence for which of the following conclusions?

The atom has a small, dense, positively charged nucleus.

Electrons are embedded in a sphere of positive charge.

The atom has a small, dense, positively charged nucleus.

Electrons revolve in discrete orbits without radiating energy.

Atoms are indivisible and indestructible.

The diagram shows a simplified model of an atom. If the red spheres represent protons and the blue spheres represent neutrons, what does this diagram depict?

A Helium atom with 2 protons and 2 neutrons.

A Hydrogen atom with 1 proton and 1 neutron.

A Lithium atom with 3 protons and 3 neutrons.

A Carbon atom with 6 protons and 6 neutrons.

Which of the following statements about the distribution of electrons in an atom are correct? (Select all that apply)

The outermost shell can accommodate a maximum of 8 electrons.

Electrons are filled in shells in a stepwise manner, from inner to outer shells.

The maximum number of electrons in a shell is given by the formula 2n, where n is the orbit number.

The K shell is always filled before the L shell.

Consider two atomic species: A (18 protons, 22 neutrons, 18 electrons) and B (20 protons, 20 neutrons, 20 electrons). Which of the following statements are true about A and B?

Species A is Argon (Ar) and Species B is Calcium (Ca).

Species A has a mass number of 38.

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Home › CBSE › Class 9 › Science › STRUCTURE OF THE ATOM

CBSE · Class 9 · 🔬 Science · Chapter 4

STRUCTURE OF THE ATOM

Subatomic ParticlesThomson's Model of AtomRutherford's Model of AtomBohr's Model of AtomNeutronsElectron Distribution

Chapter 4, 'Structure of the Atom,' introduces students to the fundamental building blocks of matter. It delves into the discovery of subatomic particles like electrons, protons, and neutrons, and explores various atomic models proposed by scientists like J.J. Thomson, Ernest Rutherford, and Neils Bohr. Understanding this chapter is crucial for grasping the basic principles of chemistry, including electron distribution, valency, atomic number, mass number, and the concepts of isotopes and isobars. This knowledge forms the bedrock for advanced studies in science.

Sub-atomic Particles: Discovery aur Properties

Dalton ki atomic theory ke according, atom indivisible tha. Lekin, baad mein scientists ne discover kiya ki atom ke andar bhi chhote particles hote hain, jinhe sub-atomic particles kehte hain.

Electron (e⁻)

Discoverer: J.J. Thomson (1897) Cathode Ray Experiment ke through.
Charge: Negative charge (relative charge -1). Absolute charge \(-1.6 \times 10^{-19}\) C.
Mass: Negligible mass (relative mass 1/1837 amu ya approx 0 amu). Absolute mass \(9.1 \times 10^{-31}\) kg.
Location: Nucleus ke around orbits mein revolve karta hai.

Proton (p⁺)

Discoverer: E. Goldstein (1886) Canal Ray Experiment ke through. Rutherford ne 'proton' naam diya (1919).
Charge: Positive charge (relative charge +1). Absolute charge \(+1.6 \times 10^{-19}\) C.
Mass: 1 amu (atomic mass unit). Absolute mass \(1.672 \times 10^{-27}\) kg.
Location: Atom ke nucleus mein present hota hai.

Neutron (n⁰)

Discoverer: James Chadwick (1932).
Charge: No charge (neutral).
Mass: 1 amu (slightly more than proton). Absolute mass \(1.674 \times 10^{-27}\) kg.
Location: Atom ke nucleus mein present hota hai. Hydrogen atom ke alawa sabhi atoms mein neutrons hote hain.

Sub-atomic Particles ka Comparison

❗Important

Atom electrically neutral hota hai kyunki usme equal number of protons aur electrons hote hain, jinke charges ek doosre ko cancel kar dete hain.

Atomic Models: Evolution of Atomic Structure

Sub-atomic particles ki discovery ke baad, scientists ne atom ke structure ko explain karne ke liye alag-alag models propose kiye. Har model ne pichhle model ki limitations ko address kiya.

1. Thomson's Model of Atom (1904)

Analogy: Watermelon Model ya Plum Pudding Model.
Postulates:
Atom ek positive charged sphere hota hai.
Electrons (negative charge) is positive sphere mein embedded hote hain, jaise watermelon mein seeds ya pudding mein plums.
Positive aur negative charges magnitude mein equal hote hain, isliye atom electrically neutral hota hai.
Merits: Atom ki electrical neutrality ko explain kiya.
Limitations:
Dusre scientists ke experimental results ko explain nahi kar paya, especially Rutherford's alpha-scattering experiment.
Atom ki stability ko explain nahi kar paya.

2. Rutherford's Model of Atom (1911) – Alpha-Particle Scattering Experiment

Experiment Setup:
Fast-moving alpha (α)-particles (doubly-charged Helium ions, He²⁺) ko thin gold foil (approx 1000 atoms thick) par bombard kiya gaya.
Ek fluorescent screen (zinc sulphide) gold foil ke around rakhi gayi thi, jisse α-particles ke impact ko detect kiya ja sake.
Observations:
Most α-particles straight nikal gaye gold foil se, bina kisi deflection ke. (Approx 99.9%)
Kuch α-particles small angles par deflect hue.
Bahut kam α-particles (approx 1 in 12,000) 90° se zyada deflect hue ya wapas bounce back ho gaye (180° deflection).
Conclusions:
Most α-particles un-deflected gaye: Atom ka zyadatar space empty hota hai.
Kuch α-particles deflect hue: Atom ke center mein positive charge concentrated hota hai, jise nucleus kaha gaya. Alpha particles positive hote hain, isliye repulsion hua.
Bahut kam α-particles bounce back hue: Nucleus bahut dense aur bahut chhota hota hai, aur atom ke total volume ka bahut kam hissa occupy karta hai.
Rutherford's Nuclear Model Postulates:
Atom ke center mein ek positively charged nucleus hota hai, jisme atom ka saara mass concentrated hota hai.
Electrons nucleus ke around circular paths (orbits) mein revolve karte hain.
Atom ka size nucleus ke size se bahut bada hota hai (nucleus ka radius atom ke radius ka approx 1/10⁵ hota hai).
Limitations:
Atom ki stability explain nahi kar paya. Classical physics ke according, circular orbit mein revolve karta hua charged particle (electron) energy radiate karega aur spiral karte hue nucleus mein gir jayega, jisse atom unstable ho jayega. Lekin atoms stable hote hain.
Electron ki exact position aur energy levels ke baare mein koi information nahi di.

3. Bohr's Model of Atom (1913)

Rutherford ki limitations ko overcome karne ke liye Neils Bohr ne ye model propose kiya.
Postulates:
Electrons nucleus ke around discrete orbits (ya energy shells) mein hi revolve kar sakte hain. In orbits ko stationary orbits kehte hain.
Jab electrons in discrete orbits mein revolve karte hain, toh wo energy radiate nahi karte. Isliye atom stable rehta hai.
Har orbit ki apni fixed energy hoti hai. Ye orbits energy levels kehlati hain.
Ye energy levels nucleus se distance ke according numbered hote hain: K-shell (n=1), L-shell (n=2), M-shell (n=3), N-shell (n=4), etc., ya 1, 2, 3, 4 se denote karte hain.
Merits:
Atom ki stability ko explain kiya.
Hydrogen atom ke line spectra ko explain kiya.
Limitations:
Complex atoms ke spectra ko explain nahi kar paya.
Electrons ke exact movement ke baare mein koi information nahi di.

💡Tip

Rutherford's experiment ke observations aur conclusions bahut important hain. Direct questions aate hain.

⭐Remember

Rutherford ne nucleus discover kiya. Bohr ne energy shells aur atom ki stability explain ki.

Electron Distribution, Valency, Atomic Number, Mass Number

Atom ke structure ko samajhne ke baad, ab hum dekhenge ki electrons alag-alag shells mein kaise distribute hote hain aur isse atom ki properties kaise determine hoti hain.

Electron Distribution (Bohr-Bury Rules)

Electrons ko different energy shells (K, L, M, N...) mein distribute karne ke kuch rules hain:

Maximum number of electrons in a shell: Kisi bhi shell mein maximum electrons \(2n^2\) formula se calculate hote hain, jahan 'n' shell number hai.

K-shell (n=1): \(2 \times 1^2 = 2\) electrons
L-shell (n=2): \(2 \times 2^2 = 8\) electrons
M-shell (n=3): \(2 \times 3^2 = 18\) electrons
N-shell (n=4): \(2 \times 4^2 = 32\) electrons

Outermost shell limit: Outermost shell mein maximum 8 electrons ho sakte hain, bhale hi uski capacity zyada ho (octet rule).
Shell filling order: Electrons pehle inner shells ko fill karte hain, phir outer shells mein jaate hain. Yani, shells step-by-step fill hoti hain.

Example: Carbon (Atomic Number 6)

Total electrons = 6
K-shell: 2 electrons
L-shell: 4 electrons
Electronic Configuration: (2, 4)

Example: Sodium (Atomic Number 11)

Total electrons = 11
K-shell: 2 electrons
L-shell: 8 electrons
M-shell: 1 electron
Electronic Configuration: (2, 8, 1)

Valency

Definition: Kisi element ke atom ki combining capacity ko uski valency kehte hain.
Reason: Atoms stable electronic configuration (outermost shell mein 8 electrons, ya 2 for H, He) achieve karne ke liye electrons ko lose, gain ya share karte hain.
Calculation:
Agar outermost shell mein 1, 2, ya 3 electrons hain, toh atom unhe lose kar deta hai. Valency = number of valence electrons.
Example: Sodium (2, 8, 1) -> 1 electron lose karega -> Valency = 1.
Agar outermost shell mein 5, 6, ya 7 electrons hain, toh atom electrons gain karta hai. Valency = 8 - number of valence electrons.
Example: Chlorine (2, 8, 7) -> 1 electron gain karega (8-7=1) -> Valency = 1.
Agar outermost shell mein 4 electrons hain, toh atom share karta hai. Valency = 4.
Example: Carbon (2, 4) -> 4 electrons share karega -> Valency = 4.
Agar outermost shell mein 8 electrons hain (ya 2 for Helium), toh atom stable hota hai aur react nahi karta. Valency = 0. (Noble gases).

Atomic Number (Z)

Definition: Atom ke nucleus mein present number of protons ko atomic number kehte hain.
Significance: Atomic number hi kisi element ki identity hoti hai. Different elements ke atomic numbers different hote hain.
Neutral atom mein: Number of protons = Number of electrons = Atomic Number (Z).

Mass Number (A)

Definition: Atom ke nucleus mein present protons aur neutrons ke total number ko mass number kehte hain.
Formula: Mass Number (A) = Number of Protons (Z) + Number of Neutrons (n)
Representation: Element ko \(_{\text{Z}}^{\text{A}}X\) se represent karte hain, jahan X element ka symbol hai.
Example: \(_{11}^{23}Na\) ka matlab hai Sodium ka atomic number 11 aur mass number 23 hai.

🧮Formula

Maximum electrons in a shell = \(2n^2\)

🚧Misconception

Students often confuse atomic number with mass number. Atomic number (Z) protons ka number hota hai, jo identity hai. Mass number (A) protons aur neutrons ka sum hota hai.

Isotopes aur Isobars

Elements ke atoms hamesha identical nahi hote. Kuch variations bhi possible hain, jinhe isotopes aur isobars kehte hain.

Isotopes

Definition: Ek hi element ke atoms, jinka atomic number (Z) same hota hai, lekin mass number (A) different hota hai, unhe isotopes kehte hain.
Reason: Atomic number same hone ka matlab hai number of protons same hain. Mass number different hone ka matlab hai number of neutrons different hain.
Chemical Properties: Same atomic number ki wajah se, isotopes ki chemical properties similar hoti hain (kyunki chemical properties valence electrons par depend karti hain, jo same hote hain).
Physical Properties: Different mass number ki wajah se, isotopes ki physical properties (jaise density, boiling point) different hoti hain.
Examples:
Hydrogen ke isotopes:
Protium (\(_{1}^{1}H\)): 1 proton, 0 neutron
Deuterium (\(_{1}^{2}H\) ya D): 1 proton, 1 neutron
Tritium (\(_{1}^{3}H\) ya T): 1 proton, 2 neutrons
Chlorine ke isotopes:
\(_{17}^{35}Cl\): 17 protons, 18 neutrons
\(_{17}^{37}Cl\): 17 protons, 20 neutrons
Average Atomic Mass: Elements ke atomic mass ko calculate karte waqt, unke isotopes ki relative abundance ko bhi consider kiya jaata hai. Average atomic mass = \(\sum (\text{isotopic mass} \times \text{fractional abundance})\).

Uses of Isotopes

Uranium-235: Nuclear reactors mein nuclear fuel ke roop mein.
Cobalt-60: Cancer ke treatment mein (radiotherapy).
Iodine-131: Goiter (thyroid gland ki bimari) ke treatment mein.

Isobars

Definition: Different elements ke atoms, jinka mass number (A) same hota hai, lekin atomic number (Z) different hota hai, unhe isobars kehte hain.
Reason: Different atomic number ka matlab hai different number of protons. Same mass number ka matlab hai protons aur neutrons ka total sum same hai, lekin individual numbers different ho sakte hain.
Chemical Properties: Different atomic number ki wajah se, isobars ki chemical properties different hoti hain.
Examples:
Argon (Ar) aur Calcium (Ca):
\(_{18}^{40}Ar\): 18 protons, 22 neutrons
\(_{20}^{40}Ca\): 20 protons, 20 neutrons
Iron (Fe) aur Nickel (Ni):
\(_{26}^{58}Fe\)
\(_{28}^{58}Ni\)

💡Tip

Isotopes aur Isobars ki definitions, examples aur differences frequently board exams mein puche jaate hain. Uses of isotopes bhi yaad rakhna.

Easy (15)

Who proposed that the atom was indivisible and indestructible?
- J.J. Thomson
- Ernest Rutherford
- John Dalton (correct)
- Neils Bohr
Why: John Dalton's atomic theory initially proposed that atoms were indivisible and indestructible.
Which subatomic particle has a negative charge?
- Proton
- Neutron
- Electron (correct)
- Nucleus
Why: Electrons are the subatomic particles that carry a negative charge.
J.J. Thomson's model of an atom is often compared to which dessert?
- Chocolate cake
- Christmas pudding (correct)
- Apple pie
- Ice cream sundae
Why: Thomson's model is famously compared to a Christmas pudding, with electrons like currants.
Which scientist discovered the atomic nucleus through the alpha-particle scattering experiment?
- J.J. Thomson
- Ernest Rutherford (correct)
- Neils Bohr
- J. Chadwick
Why: Ernest Rutherford's alpha-particle scattering experiment led to the discovery of the atomic nucleus.
What is the mass of an electron approximately compared to a proton?
- About 2000 times more than a proton
- About the same as a proton
- About 1/2000 times that of a proton (correct)
- About 1/100 times that of a proton
Why: The mass of an electron is significantly smaller, approximately 1/2000 times that of a proton.
Which subatomic particle has no charge?
- Electron
- Proton
- Neutron (correct)
- Ion
Why: Neutrons are electrically neutral subatomic particles.
What is the combining capacity of an atom called?
- Atomic number
- Mass number
- Valency (correct)
- Isotopy
Why: Valency is defined as the combining capacity of an atom.
Which atomic model proposed that electrons revolve in discrete orbits and do not radiate energy?
- Thomson's model
- Rutherford's model
- Bohr's model (correct)
- Dalton's model
Why: Neils Bohr's model introduced the concept of discrete electron orbits where electrons do not radiate energy.
The atomic number of an element is equal to the number of protons in its nucleus.
Why: The atomic number (Z) is indeed defined by the number of protons in an atom's nucleus.
Isotopes are atoms of the same element with different atomic numbers.
Why: Isotopes have the same atomic number but different mass numbers, not different atomic numbers.
The mass of an atom is primarily determined by the sum of the masses of its electrons and protons.
Why: The mass of an atom is determined by protons and neutrons, as electrons have negligible mass.
The K shell can hold a maximum of _____ electrons.
Why: The K shell, being the innermost shell, can accommodate a maximum of 2 electrons.
Atoms of different elements with different atomic numbers but the same mass number are called _____.
Why: Isobars are defined as atoms of different elements having the same mass number but different atomic numbers.
Identify the part of the atom where protons and neutrons are located.
- Electron shell
- Nucleus (correct)
- Orbit
- Valence shell
Why: Protons and neutrons are found together in the central nucleus of an atom.
Which letter represents the outermost electron shell in a typical atomic diagram?
- K shell
- L shell
- M shell (correct)
- N shell
Why: In the provided diagram, 'M' represents the third and often outermost electron shell.

Medium (15)

An atom has 7 protons and 7 neutrons. What is its mass number?
- 7
- 14 (correct)
- 0
- 21
Why: Mass number is the sum of protons and neutrons, so 7 + 7 = 14.
If an atom has an atomic number of 11 and a mass number of 23, how many neutrons does it have?
- 11
- 12 (correct)
- 23
- 34
Why: Number of neutrons = Mass number - Atomic number = 23 - 11 = 12.
What would be the valency of an atom with electronic configuration 2, 8, 7?
- 1 (correct)
- 7
- 8
- 2
Why: An atom with 7 valence electrons needs to gain 1 electron to complete its octet, so its valency is 1.
Which of the following elements has 2 electrons in its K shell and 6 electrons in its L shell?
- Carbon
- Oxygen (correct)
- Neon
- Sulphur
Why: An atom with 2 electrons in K shell and 6 in L shell has a total of 8 electrons, which is oxygen.
Match the scientists with their atomic models or discoveries.
Why: Each scientist is correctly matched with their significant contribution to atomic structure.
Match the subatomic particles with their properties.
Why: Each subatomic particle is correctly matched with its defining characteristic.
Match the terms related to atomic structure.
Why: All terms are correctly matched with their definitions.
Match the electron shells with their maximum electron capacity.
Why: Each shell is correctly matched with its maximum electron capacity based on the 2n² rule.
The number of valence electrons in a sodium atom (atomic number 11) is _____.
Why: Sodium's electronic configuration is 2, 8, 1, so it has 1 valence electron.
The subatomic particle discovered by J. Chadwick is the _____.
Why: J. Chadwick is credited with the discovery of the neutron.
The phenomenon where fast-moving alpha particles are made to fall on a thin gold foil is called Rutherford's alpha-particle _____ experiment.
Why: Rutherford's famous experiment is known as the alpha-particle scattering experiment.
Arrange the following atomic models in chronological order of their proposal: Rutherford's model, Thomson's model, Bohr's model.
Why: The correct chronological order is Thomson, then Rutherford, then Bohr.
Arrange the following subatomic particles by their discovery order: Electron, Neutron, Proton.
Why: Electrons were discovered first, followed by protons, and then neutrons.
In the given diagram of Rutherford's experiment, what does the path labeled 'A' represent?
- Alpha particles passing straight through (correct)
- Alpha particles deflected at small angles
- Alpha particles rebounding back
- Electrons orbiting the nucleus
Why: Path 'A' shows alpha particles passing undeflected, indicating mostly empty space in the atom.
Which part of the given Bohr's model represents the 'L' shell?
- The innermost circle
- The second circle from the center (correct)
- The third circle from the center
- The central nucleus
Why: The L shell is the second energy level or orbit from the nucleus.

Hard (10)

Assertion (A): Rutherford's model of the atom could not explain the stability of the atom. Reason (R): According to classical electromagnetic theory, an orbiting electron would continuously radiate energy and eventually spiral into the nucleus.
Why: Rutherford's model failed to explain atomic stability, and the reason lies in classical physics predicting energy loss for orbiting electrons.
Assertion (A): Isotopes of an element have the same chemical properties. Reason (R): Chemical properties are determined by the number of valence electrons, and isotopes have the same number of electrons.
Why: Isotopes have identical chemical properties because they have the same electron configuration, which dictates chemical behavior.
Assertion (A): Neutrons are present in the nucleus of all atoms. Reason (R): Neutrons contribute to the mass of the atom but not to its charge.
Why: While both statements are true, the reason for neutrons being present in nuclei isn't solely their mass contribution, but also their role in nuclear stability.
A student is analyzing a sample of an unknown element. They determine that the sample contains two types of atoms. Atom X has 17 protons and 18 neutrons. Atom Y has 17 protons and 20 neutrons. Based on this information, what can be concluded about atoms X and Y?
- X and Y are isobars.
- X and Y are isotopes of the same element. (correct)
- X and Y are atoms of different elements.
- X and Y have different atomic numbers.
Why: Since X and Y have the same number of protons (atomic number), they are isotopes of the same element.
Consider an ion with 10 electrons, 8 protons, and 8 neutrons. What is the charge on this ion and its mass number?
- Charge: +2, Mass Number: 18
- Charge: -2, Mass Number: 16 (correct)
- Charge: -2, Mass Number: 18
- Charge: +2, Mass Number: 16
Why: The ion has 8 protons (+8 charge) and 10 electrons (-10 charge), resulting in a -2 charge. Mass number is protons + neutrons = 8 + 8 = 16.
Rutherford's alpha-particle scattering experiment provided evidence for which of the following conclusions?
- Electrons are embedded in a sphere of positive charge.
- The atom has a small, dense, positively charged nucleus. (correct)
- Electrons revolve in discrete orbits without radiating energy.
- Atoms are indivisible and indestructible.
Why: The deflection of alpha particles indicated a small, dense, positive nucleus at the atom's center.
Observe the diagram showing electron distribution in an atom. What is the atomic number and valency of this element?
- Atomic Number: 12, Valency: 2 (correct)
- Atomic Number: 10, Valency: 8
- Atomic Number: 12, Valency: 6
- Atomic Number: 14, Valency: 2
Why: The diagram shows 2 electrons in K, 8 in L, and 2 in M, totaling 12 electrons (atomic number 12). Valency is 2 as it loses 2 electrons.
The diagram shows a simplified model of an atom. If the red spheres represent protons and the blue spheres represent neutrons, what does this diagram depict?
- A Helium atom with 2 protons and 2 neutrons. (correct)
- A Hydrogen atom with 1 proton and 1 neutron.
- A Lithium atom with 3 protons and 3 neutrons.
- A Carbon atom with 6 protons and 6 neutrons.
Why: The diagram shows 2 red spheres (protons) and 2 blue spheres (neutrons), which corresponds to a Helium atom.
Which of the following statements about the distribution of electrons in an atom are correct? (Select all that apply)
- The outermost shell can accommodate a maximum of 8 electrons. (correct)
- Electrons are filled in shells in a stepwise manner, from inner to outer shells. (correct)
- The maximum number of electrons in a shell is given by the formula 2n, where n is the orbit number.
- The K shell is always filled before the L shell. (correct)
Why: The outermost shell can hold 8 electrons, shells fill from inner to outer, and K fills before L. The formula is 2n², not 2n.
Consider two atomic species: A (18 protons, 22 neutrons, 18 electrons) and B (20 protons, 20 neutrons, 20 electrons). Which of the following statements are true about A and B?
- Species A and B are isobars. (correct)
- Species A and B are isotopes.
- Species A is Argon (Ar) and Species B is Calcium (Ca). (correct)
- Species A has a mass number of 38.
Why: A and B have different atomic numbers but the same mass number (40), making them isobars. A is Argon (Z=18) and B is Calcium (Z=20).