Q 7
d) What is Thomson's plum pudding model?
Answer:
Thomson knew that atoms had an overall neutral charge. Therefore, he reasoned that there must be a source of positive charge within the atom to counterbalance the negative charge on the electrons. This led Thomson to propose that atoms could be described as negative particles floating within a soup of diffuse positive charge. This model is often called the plum pudding model of the atom, due to the fact that its description is very similar to plum pudding, a popular English dessert (see image below).
Q- 8
a) Explain the discovery of electrons.
Ans:
What are Electrons?
Electrons are subatomic particles that hold an elementary charge of magnitude -1. The charge of an electron is equal in magnitude to the charge held by a proton (but has an opposite sign). Therefore, electrically neutral atoms/molecules must have an equal number of electrons and protons. Although the magnitude of the charges held by protons and electrons are the same, the size and mass of an electron are much smaller than that of a proton (the mass of an electron is roughly 1/1836 the mass of a proton).
Charge of Electrons
An electron is a negatively charged particle. The negative charge is equal to 1.602 × 10-19 coulomb in magnitude. The mass of an electron is 1/1837 of a proton.
Mass of Electrons
The mass of an electron is 9.10938356 × 10-31 kilograms. The mass of the electron is negligible compared to the mass of the proton.
Owing to their small size and mass, the properties exhibited by electrons can be studied better with the help of quantum mechanics rather than classical mechanics. This is because matter behaves differently at the quantum scale. For example, the uncertainty associated with the position and the velocity of an electron is much greater than that associated with a proton or a neutron, as per Heisenberg’s uncertainty principle. Electrons are distributed around the nuclei of atoms in atomic orbitals, which can be simply visualized as regions around the nucleus in which the probability of finding a specific electron is the highest.
Who Discovered the Electron?
The electron was discovered by the English physicist J.J.Thomson in the year 1897, via his experiments with cathode ray tubes.
Charge of Electrons
An electron is a negatively charged particle. The negative charge is equal to 1.602 × 10-19 coulomb in magnitude. The mass of an electron is 1/1837 of a proton.
Mass of Electrons
The mass of an electron is 9.10938356 × 10-31 kilograms. The mass of the electron is negligible compared to the mass of the proton.
b) Explain Rutherford's alpha-scattering experiment and the conclusions drawn from it.
Answer:
Rutherford's model of an atom :
Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. Rutherford designed an experiment for this. In this experiment, fast-moving alpha (α)-particles were made to fall on a thin gold foil.
He selected a gold foil because he wanted as thin a layer as possible. This gold foil was about 1000 atoms thick.
α-particles are doubly-charged helium ions. Since they have a mass of 4µ, the fast-moving α-particles have a considerable amount of energy.
It was expected that α-particles would be deflected by the sub-atomic particles in the gold atoms. Since the α-particles were much heavier than the protons, he did not expect to see large deflections. But, the α-particle scattering experiment gave totally unexpected results.
Observations of Rutherford's scattering experiment:
Most of the fast-moving α-particles passed straight through the gold foil.
Some of the α-particles were deflected by the foil by small angles.
Surprisingly one out of every 12,000 alpha particles appeared to rebound.
Conclusion of Rutherford's scattering experiment:
- Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.
- Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.
- A very small fraction of α-particles were deflected by very large angles, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.
c) Explain the discovery of protons by Eugen Goldstein.
Answer:
He was examining cathode tubes, in which energy rays were observed traveling between the cathode and anode inside a cathode tube. He measured various properties of the rays, which were later understood to be electrons. During these experiments, he also observed rays traveling in the opposite direction. The properties of these rays seemed to vary depending on the residual gas in the tube, and it was the measurements of the properties of ionized hydrogen gas in the tube that was in fact measuring data concerning the proton. He had no idea what they were but were later known to be protons.
d) Explain the arrangement of electrons in an atom as suggested by Bohr and bury.
Answer:
- The electrons are arranged around the nucleus in different energy levels or energy shells. The electrons first occupy the shell with the lowest energy i.e., closest to the nucleus.
- The first or the innermost energy shell (K or n = 1) can take only two electrons.
- The second shell (L or n = 2) can contain upto 8 electrons.
- From third shell (M or n = 3) onwards, the shells become bigger. The third shell can accommodate as many as 18 electrons. In general, the maximum number of electrons that can be present in any shell is 2n2 where n is the number of energy shell. Thus, the first orbit (n = 1, known as K shell) can contain 2 × 12 = 2 electrons, the second orbit (n = 2, known as L shell) can contain 2 × 22 = 8 electrons.
Maximum No. of electrons in different orbits
Orbit | Value of n | Maximum no. of electrons in the orbit |
K | 1 | 2 × 12 = 2 |
| L | 2 | 2 × 22 = 8 |
M | 3 | 2 × 32 = 18 |
| N | 4 | 2 × 42 = 32 |
The outermost shell of an atom cannot have more than 8 electrons and the shell next to the outermost shell cannot have more than 18 electrons.
e) Explain the discovery of neutrons.
Answer:
Neutrons are the uncharged, neutral particles present in the nucleus of an atom. The British physicist, James Chadwick discovered neutrons in the year 1932 through the following experiment :
James Chadwick fired alpha radiation at a beryllium sheet from a polonium source. This led to the production of uncharged, penetrating radiation.
This radiation was made incident on paraffin wax, a hydrocarbon having a relatively high hydrogen content.
The protons ejected from the paraffin wax (when struck by the uncharged radiation) were observed with the help of an ionization chamber.
The range of the liberated protons was measured, and the interaction between the uncharged radiation and the atoms of several gases was studied by Chadwick.
He concluded that the unusually penetrating radiation consisted of uncharged particles having (approximately) the same mass as a proton. These particles were later termed as ‘neutrons’.
Comments
Post a Comment