Concepts

Class 9
Chapter 4 Class 9 - Structure of Atom

As per Rutherford Nuclear Model of Atom ,

• Every atom has a nucleus .
• The size of the nucleus is very small . It is 1/10 the size of an atom .
• This nucleus is positively charged .
• Even though the nucleus is of small size, it has a very high mass . Nearly all the mass of an atom is inside the nucleus.
• Electrons revolve around the nucleus in a circular path .

How was the Rutherford Model formed?

• It was formed by the scientist Ernest Rutherford .

He designed the model after performing Alpha Particle Scattering Experiment on a gold foil .

What is an alpha particle?

• Alpha particles are nucleus of helium atoms . It has a charge of +2 . The fast moving alpha particles have a good amount of energy.

• The mass of an alpha - particle is 4u .

Why did Rutherford use a gold foil?

• He selected a gold foil because he wanted as thin a layer as possible . This gold foil was about 1000 atoms thick .

What did Rutherford expect before the experiment?

• Mass of the alpha particles was 4u while that of the proton is 1u. Hence, they were much heavier than the proton.

• Rutherford expected that alpha particles would deflect a little by the subatomic particles (protons and electrons) in the gold atoms.

• But since alpha particles were much heavier than protons , he did not expect to see large deflections.

What was Rutherford's Alpha Particle Scattering Experiment?

• In this experiment, Rutherford made fast moving alpha particles to fall on a gold foil .

• He observed that:
• Many of fast moving alpha particles pass straight through the gold foil with no deflection at all

• Some of the alpha particles were deflected by foil at small angles.

• Very less alpha particles(1 out of every 12000) were reflected back  at 180 degree (rebound)

Analysis

Rutherford’s observations from his experiment: -

1. Most of spaces inside atom was empty
This is because most of the fast moving alpha particles pass straight through the gold foil.

2. Positive charge of atom occupies very little space
This is because only some of the alpha particles were deflected by the foil at small angles.

3. All mass of atom and positive charged was concentrated in very small volume of atom called Nucleus
This is because very less alpha particles were reflected back at 180 degree (rebound).

Note: From his experiment, he estimated that, radius of an atom is about 10 5   more than the radius of a nucleus.

What was Rutherford's model of an atom?

Rutherford's model of an atom stated that:

1. There is a positively charged centre in an atom called the nucleus .
Nearly all the mass of an atom resides in the nucleus .

2. The electrons (negatively charged particles) revolve around the nucleus in circular paths.

3. The size of the nucleus is very small as compared to the size of the atom .

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