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Structure of the Atom - Thomson’s and Rutherford’s Model

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Thomson’s Model (Plum Pudding Model): Proposed that an atom consists of a positively charged sphere with electrons (e−e^-) embedded in it like seeds in a watermelon.

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According to Thomson, the negative and positive charges are equal in magnitude, so the atom as a whole is electrically neutral.

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Rutherford’s α\alpha-particle Scattering Experiment: Fast-moving α\alpha-particles (24He2+^4_2He^{2+}) were directed at a thin gold foil (about 10001000 atoms thick).

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Observations of Rutherford's experiment: Most α\alpha-particles passed straight through, some were deflected by small angles, and very few (11 in 12,00012,000) rebounded at 180∘180^\circ.

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Rutherford’s Nuclear Model: An atom has a positively charged, dense center called the nucleus where nearly all the mass resides. Electrons revolve around the nucleus in circular paths.

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The radius of the nucleus is approximately 10510^5 times smaller than the radius of the atom (Ratom≈10−10 mR_{atom} \approx 10^{-10} \text{ m} vs Rnucleus≈10−15 mR_{nucleus} \approx 10^{-15} \text{ m}).

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Drawback of Rutherford’s Model: It could not explain the stability of the atom. Electrons moving in circular orbits undergo acceleration and should radiate energy, eventually spiraling into the nucleus.

📐Formulae

Alpha Particle=24He2+\text{Alpha Particle} = ^4_2He^{2+}

Charge of Electron (e−)=−1.6×10−19 C\text{Charge of Electron } (e^-) = -1.6 \times 10^{-19} \text{ C}

Radius of AtomRadius of Nucleus≈105\frac{\text{Radius of Atom}}{\text{Radius of Nucleus}} \approx 10^5

💡Examples

Problem 1:

Why did Rutherford select a gold foil for his α\alpha-particle scattering experiment?

Solution:

Rutherford wanted a layer as thin as possible. Gold is highly malleable and can be hammered into a very thin foil, approximately 10001000 atoms thick.

Explanation:

A thicker material would have absorbed the α\alpha-particles (24He2+^4_2He^{2+}) or caused too many multiple scatterings, making it impossible to observe the structure of a single nucleus.

Problem 2:

On the basis of Thomson's model, how is the atom neutral?

Solution:

Thomson proposed that the atom is a sphere of positive charge with negatively charged electrons (e−e^-) embedded in it. Since the total positive charge equals the total negative charge, the net charge is 00.

Explanation:

If QpositiveQ_{positive} is the charge of the sphere and QnegativeQ_{negative} is the sum of charges of all electrons, then Qtotal=Qpositive+Qnegative=0Q_{total} = Q_{positive} + Q_{negative} = 0.

Problem 3:

What conclusion did Rutherford draw from the fact that most α\alpha-particles passed through the gold foil undeflected?

Solution:

He concluded that most of the space inside the atom is empty.

Explanation:

If the positive charge or mass were distributed uniformly (as Thomson suggested), almost all α\alpha-particles would have been deflected by at least a small amount.

Thomson’s and Rutherford’s Model - Revision Notes & Key Formulas | CBSE Class 9 Science