Question

(i) Describe the Rutherford's model of an atom. State one drawback of Rutherford's model of the atom.
​(ii) The mass number of an element is 23 and it contains 11 electrons. What is the number of protons and neutrons in it? What is the atomic number of the element?

Answer 1

(i) Rutherford bombarded gold foil with alpha particles.
Set up for Rutherford's experiment:
1. A thin gold foil, approximately 1000 atoms thick, was taken. Gold was chosen for its high density.
2. A detector screen with a small slit was placed around the foil.
3. A source of alpha particles was kept in front of the foil.
4. The foil was bombarded with fast moving alpha particles.
The set up for the Rutherfords gold foil experiment is shown in the figure.

Rutherford's expectation and observations

Rutherford expected that alpha particles would pass straight through the foil and only a small fraction of alpha particles would be deflected. This expectation was in compliance with Thomson's atomic model.
Rutherfords findings were contrary to his expectation. He observes that:
1. Most of the fast-moving alpha particles passed straight through the gold foil.
2. Some particles were deflected through the foil by small angles.
3. One out of every 12000 particles rebounded, i.e. they got deflected by an angle of 180°.

Rutherford then carefully studied his observation and made the following conclusions:
1. Most alpha particles pass through the gold foil without any deflection. This indicates that most of the space inside an atom is empty.
2. Very few particles suffered a deflection from their path. This means that positive charge occupies very little space inside an atom.
3. Only a few fractions of particles underwent a 180° deflection. This shows that the entire positive charge and mass of an atom are present within a very small volume inside the atom.

The drawback of Rutherford's model of the atom:
a) Model is applicable for one electron system.
b) According to Rutherford as the electron revolves around the nucleus it liberates energy regularly so when the electron loses all its energy it should fall in the nucleus which was impossible.


(ii)
The mass number of the element = 23
Number of electrons = 11
Now,
Number of electrons = Number of protons
So, the number of protons = 11
Number of neutrons = Mass number of element - Number of protons
Number of neutrons = 23 - 11
Number of neutrons = 12

As the number of electrons are 11, so the atomic number of the element will be 11.