By the nineteenth century, the atomic hypothesis of matter was well-supported. In 1897, J.J. Thomson discovered electrons, revealing that atoms have negatively charged components. Since atoms are neutral, they must also contain positive charges. Thomson proposed the "plum pudding model," where electrons are embedded in a positively charged sphere. This model was later replaced by Rutherford's nuclear model.
Did You Know?
The plum pudding model compared electrons to seeds in a watermelon, but this model was soon found to be incorrect.
In 1911, Geiger and Marsden, under Rutherford's guidance, conducted the alpha-particle scattering experiment. They directed a beam of alpha-particles at a thin gold foil and observed the scattering patterns.
Common Misconception:
It is often thought that alpha particles easily penetrate the nucleus, but they actually scatter off due to the repulsive force of the nucleus.
Rutherford's model led to the understanding that:
Niels Bohr modified Rutherford’s model by introducing quantum concepts:
NEET Tip:
Remember Bohr's quantization rules for solving problems related to atomic spectra.
The energy of an electron in the nth orbit is given by: En=−13.6/n2,eV
Mnemonic:
"Higher n, higher energy" - As the principal quantum number n increases, the energy level rises.
Atoms emit light at specific wavelengths, forming an emission spectrum. The spectrum of hydrogen, for instance, includes the Balmer series in the visible region.
Example:
The transition from n=3 to n=2 in hydrogen emits a photon of wavelength 656 nm (red light).
Real-life Application:
Spectral lines are used in astronomy to identify elements in stars.
While Bohr's model explained hydrogen spectra well, it had limitations:
Concept Connection:
Bohr's model was a stepping stone towards quantum mechanics, which provides a more comprehensive understanding of atomic structure.
The nucleus, discovered by Rutherford, contains protons and neutrons. The number of protons (atomic number) defines the element, while the sum of protons and neutrons (mass number) determines its isotopes.
Did You Know?
Hydrogen has three isotopes: Protium (1 proton), Deuterium (1 proton + 1 neutron), and Tritium (1 proton + 2 neutrons).
Nuclear forces are strong, short-range forces that hold protons and neutrons together in the nucleus, overcoming the repulsive electromagnetic force between protons.
Common Misconception:
Nuclear forces are often misunderstood as gravitational forces, but they are distinct and much stronger at short distances.
Example:
Uranium-238 undergoes alpha decay to form Thorium-234.
The half-life of a radioactive substance is the time taken for half of its atoms to decay. It is given by: T1/2=λ0.693 where λ is the decay constant.
Real-life Application:
Carbon-14 dating is used to determine the age of archaeological samples.
Nuclear fission involves splitting a heavy nucleus into lighter nuclei, releasing a large amount of energy. Example: Uranium-235 fission in nuclear reactors.
Nuclear fusion is the process where light nuclei combine to form a heavier nucleus, releasing energy. Example: Fusion of hydrogen nuclei in the sun to form helium.
NEET Problem-Solving Strategy:
Understand the mass-energy equivalence principle given by Einstein’s equation E=mc2 to solve problems related to nuclear energy.
Link to NEET Physics: Atomic and Nuclear Physics Understanding atomic models and nuclear processes is essential for NEET Physics, as it forms the basis for numerous applications in medical imaging, energy production, and radiometric dating.