Rutherford’s Alpha Scattering Experiment

The plum pudding model of J. J. Thomson could not able to explain certain experimental results about the atomic structure of elements. still, there was no clear model that defines atoms, therefore in 1909, Ernest Rutherford, a British scientist conducted an experiment and based on the experiment he observed and proposed the atomic structure of elements and Rutherford Atomic Model.

Atomic model

Basic Constituents of an Atom

An atom consists of Electrons, Protons, and Neutrons are the fundamental particles or sub-atomic particles that build the structure of an atom. Let us understand each term.

• Electron: In 1897, J. J. Thomson discovered negatively charged particles towards the anode, these rays are emitted by the cathode in a cathode ray experiment. Then these negatively charged particles are proposed as Electrons.
• Protons: In 1886, Ernest Goldstein discovered that anode emitted positively charged particles with a different condition in the same tube,  known as Canal rays or as Protons.
• Neutrons: A subatomic particle with no charge and a mass equivalent to protons in the nucleus of all atoms were discovered by J. Chadwick. These neutrally charged particles are termed Neutrons.  

Isotopes are the elements that have the same atomic number but different mass. e.g. Isotopes of the Hydrogen atoms are Protium (¹H₁), Deuterium (²H₁) and Tritium(³H₁). Isotopes of the Carbon atoms are ¹²C₆, ¹³C₆, ¹⁴C₆.

Isobars are the elements that have different atomic number but have same mass number. e.g. ¹⁹K₄₀, ¹⁸Ar₄₀, ²⁰Ca₄₀, here all the elements having same mass number hence they are isobars.

Rutherford’s  Alpha Scattering Experiment

set-up of Rutherford’s experiment

He conduct an experiment by bombarding alpha particles into a thin sheet of gold and then notices their interaction with the gold foil and trajectory or path followed by these particles. 

In the experiment, Rutherford passes very high streams of alpha-particles from a radioactive source i.e. alpha-particle emitter, at a thin sheet of100 nm thickness of gold. In order to examine the deflection produced by the alpha particles, he placed a screen of fluorescent zinc sulphide around the thin gold foil. Rutherford made certain observations that oppose Thomson’s atomic model.

Observations of Rutherford’s Alpha Scattering Experiment

The observations of Rutherford’s Alpha Scattering Experiment are:

1. First, he observe that most of the α-particles that are bombarded towards the gold sheet pass away the foil without any deflection, and hence it shows most of the space is empty.
2. Out of all, some of the α-particles were deflected through the gold sheet by very small angles, and hence it shows the positive charge in an atom is non-uniformly distributed. The positive charge is concentrated in a very small volume in an atom.
3. Very few of the alpha-particles(1-2%) were deflected back, i.e. only a very less amount of α-particles had nearly 180° angle of deflection. this shows that the volume occupied by the positively charged particles is very small as compared to the total volume of an atom.

Rutherford Atomic Model

Rutherford proposed the atomic structure of elements, on the basis of his experiment. According to Rutherford’s atomic model:

• The positively charged particle was concentrated in an extremely small volume and most of the mass of an atom was also in that volume. He called this a nucleus of an atom.
• Rutherford proposed that there is negatively charged electrons around the nucleus of an atom. the electron surrounding the nucleus revolves around it in a circular path with very high speed. He named orbits to these circular paths.
• Nucleus being a densely concentrated mass of positively charged particles and electrons being negatively charged are held together by a strong force of attraction called electrostatic forces of attraction.

Observation of Rutherford’s model

Limitations of Rutherford Atomic Model

The Rutherford atomic model is failed to explain certain things.

1. According to Maxwell, an electron revolving around the nucleus should emit electromagnetic radiation due to accelerated charged particles emit electromagnetic radiation. but Rutherford model says that the electrons revolve around the nucleus in fixed paths called orbits. The radiation would carry energy from the motion which led to the shrinking of orbit. Ultimately electrons would collapse inside the nucleus.
2. As per the Rutherford model, calculations have shown that an electron would collapse in the nucleus in less than 10^-8 seconds. So Rutherford model has created a high contradiction with Maxwell’s theory and Rutherford later could not explain the stability of an atom.
3. Rutherford also did not describe the arrangement of electrons in the orbit as one of the other drawbacks of his model.

Regardless of seeing the early atomic models were inaccurate and failed to explain certain experimental results, they were the base for future developments in the world of quantum mechanics.

Sample Questions

Question 1: Name the atom which has one electron, one proton and no neutron.

Answer:

It is true for hydrogen atom ¹H₁.

The atomic number of Hydrogen = No. of Proton = 1

The mass number of hydrogen = 1

No. of neutrons = 0

Therefore hydrogen atom has one electron, one proton and no neutron.

Question 2: Define the term ground state of an atom?

Answer:

It is the state of an atom where all the electrons in the atom are in their lowest energy state or levels is called the ground state.

Question 3: Represent the element ‘X’ which contains 15 electrons and 16 neutrons.

Answer:

Atomic number of element = no. of electron = 15

Mass number of element = no. of electrons + no. of neutrons

                                       = 15 + 16

                                       = 31

The correct representation of element X is ³¹X₁₅.

Question 4: Name the particle and give its location in the atom which has no charge and has a mass nearly equal to that of a proton.

Answer:

The particle which has no charge and has a mass nearly equal to that of a proton is a neutron and it is present in the nucleus of the atom.

Question 5: An atom has both electron attribute negative charge and protons attribute positive charge but why there is no charge?

Answer:

The positive and negative charges of protons and electrons are equal in magnitude, they cancel the effect of each other. So, the atom as a whole is electrically neutral.

Question 6: What is the valency of the Sodium atom (Na)?

Answer:

The atomic number of sodium = 11

electronic configuration (2, 8, 1).

by losing one electron it gains stability hence its valency is 1.

Question 7: Which property do the following pairs show?

²⁰⁹X₈₄ and ²¹⁰X₈₄

Answer:

The atomic number of X is the same hence the pair shows an isotopic property.

So, ²⁰⁹X₈₄ and ²¹⁰X₈₄ are isotopes.

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