Ch 15Structure of Atoms and Nuclei — Class 12 Physics, concept-first.

The idea that matter is built from tiny, indivisible pieces goes back to the Greek philosophers Leucippus and Democritus in the 5th century BC, who first proposed 'atoms' -- literally 'uncuttable' --…

J.J. Thomson's discovery of the electron came from studying the rays produced inside an evacuated glass tube when a high voltage was applied between two electrodes.

To probe how an atom's positive charge is actually distributed, Rutherford proposed bombarding atoms with alpha particles -- positively charged particles (each carrying a charge of +2e, equal to that…

Rutherford reasoned carefully from the Geiger-Marsden results. In Thomson's plum-pudding model, the positive charge is spread over a sphere the size of the whole atom, so its charge density is very lo…

Elegant as it was at explaining the scattering data, Rutherford's model ran into a serious theoretical problem the moment classical physics was applied to it rigorously.

When an ordinary solid object is heated, it glows and emits light across a broad, continuous range of wavelengths -- passing that light through a prism produces a smooth, unbroken rainbow (a continuou…

Niels Bohr resolved the contradiction between Rutherford's mostly-correct nuclear picture and the classical prediction of atomic collapse by borrowing ideas from the newly emerging quantum physics of…

Bohr's first two postulates together are enough to work out the size of each allowed orbit. Let the electron (mass ) move with speed in a circular orbit of radius , the nth stable orbit.

The electron's total energy in the nth orbit is the sum of its kinetic energy and its (negative) electrostatic potential energy, .

Bohr's model was a triumph for hydrogen, but it had real limits. First and most seriously, it could not explain the spectral lines of any atom OTHER than hydrogen (or other single-electron, hydrogen-l…

Chapter 14 established that matter, not just light, has a dual wave-particle nature: every material particle has an associated wave, with wavelength given by the de Broglie relation .

Having built up a working picture of how electrons are arranged and behave around the nucleus, the chapter now turns to the nucleus itself.

The atomic nucleus is built from two types of subatomic particles, together called nucleons: protons and neutrons.

Just as different atoms have different overall sizes depending on how many electron orbits they occupy, different nuclei have different sizes depending on how many nucleons they contain.

Physics recognises four fundamental forces of nature, and the one responsible for holding the nucleus together -- overcoming the mutual electrostatic repulsion between its positively charged protons -…

Section 15.7.1's list of proton and neutron masses can be used to check a striking fact: the actual measured mass of any real nucleus is always slightly LESS than the sum of the masses its individual…

Most naturally occurring nuclei are perfectly stable, remaining unchanged indefinitely. But many others are not: they spontaneously transform, emitting some particle in the process and changing into a…

Even though an individual radioactive nucleus's decay is intrinsically unpredictable -- there is no way to say in advance exactly when any ONE particular nucleus will decay -- the behaviour of a large…

The half-life of a radioactive species, denoted , is defined as the time required for the number of parent nuclei to fall to exactly HALF its initial value.

Different individual nuclei of the very same radioactive species do not all decay at the same moment -- some decay almost immediately, others survive for a very long time -- so it is natural to ask fo…

Everyday energy sources -- fossil fuels such as coal, petroleum and natural gas, along with firewood -- release energy through CHEMICAL reactions, typically involving only a few electron-volts (eV) of…

Nuclear fission exploits the DECREASING part of the binding-energy curve (Fig.15.6) on its heavy-nucleus side: a very heavy nucleus, with mass number around A = 230, has a lower binding energy per nuc…

Nuclear fusion exploits the OTHER, rising part of the binding-energy curve (Fig.15.6), on its light-nucleus side: light nuclei (with mass number below about A = 40) have a lower binding energy per nuc…

Sample papers and previous-year board questions for this subject.