Ch 8Atomic and Nuclear Physics — Class 12 Physics, concept-first.

Everything around us that occupies space is matter, and matter appears in the forms solid, liquid and gas.

Under normal atmospheric conditions, gases are poor conductors of electricity because they contain essentially no free electrons available to carry current.

J.J. Thomson's 1887 experiment measured the "specific charge" of the electron - its charge-to-mass ratio - and is regarded as one of the landmark experiments marking the birth of modern physics.

Once the specific charge of the electron was known from Thomson's experiment, the next step was to measure the electron's actual charge itself. This was accomplished by R.A.

Around 400 BC, the Greek philosophers Leucippus and Democritus first proposed that matter, if repeatedly subdivided, would eventually yield indivisible atoms.

J.J. Thomson's 1904 atom model - nicknamed the "watermelon model" - pictures the atom as a homogeneous sphere of uniformly distributed positive charge, with the negatively charged electrons embedded i…

In 1911, acting on Rutherford's advice, his students Hans Geiger and Ernest Marsden carried out what became one of the most famous experiments in physics: firing alpha particles at a thin gold foil an…

To overcome Rutherford's twin failures - instability and continuous emission - Niels Bohr introduced quantisation into the atomic model. His postulates for the hydrogen atom are:

Heated solids, liquids and gases generally emit a continuous spectrum of electromagnetic radiation - all wavelengths represented, as seen when white light is passed through a spectrometer.

Having discussed the theoretical models of the atom as a whole in the previous sections - Thomson's watermelon model, Rutherford's alpha-scattering-derived nuclear model, and Bohr's quantised orbital…

The nucleus at the centre of every atom is built from two kinds of particles, collectively called nucleons: protons, which carry a positive charge (equal in magnitude to the electron's charge), and ne…

Nuclei can be classified and compared along three different axes, depending on which of , or they share:

Nuclear masses, expressed in kilograms, are inconveniently tiny numbers (of order kg or smaller), so it is far more practical to use a specially defined unit called the atomic mass unit (u).

The alpha-particle scattering experiment, together with many other independent measurement techniques applied to a wide range of nuclei, shows that nuclei are approximately spherical, and that for nuc…

A remarkable experimental fact underlies the entire concept of nuclear binding energy: the measured mass of any nucleus is always slightly less than the sum of the masses of its individual, separated…

Rather than just the total binding energy of a nucleus, it is far more informative to compute the average binding energy per nucleon, which represents, roughly, the energy needed to pull a single typi…

Since like charges repel, and protons inside a nucleus are packed only a few fermi ( m) apart, simple electrostatics predicts an enormous mutual repulsion between them.

The binding-energy curve shows that nuclear stability begins to decline once the atomic number exceeds ; nuclei beyond this point, together with certain lighter isotopes, are collectively called unsta…

In alpha decay, an unstable nucleus emits an alpha particle - a nucleus, i.e. two protons and two neutrons bound together.

In beta decay, a radioactive nucleus emits either an electron or a positron; both are collectively called beta particles, with the positron being the electron's antiparticle - identical mass, but oppo…

In both alpha and beta decay, the resulting daughter nucleus is very often left in an excited energy state rather than settling directly into its true ground state; such excited nuclear states typical…

A real radioactive sample contains an enormous number of nuclei, and not all of them decay simultaneously - each individual nucleus decays at a random, unpredictable moment (like the toss of a coin),…

It is generally very hard to say exactly when all of the nuclei in a sample will have decayed (since the decay law implies this technically takes infinite time), but it is straightforward to calculate…

One of the most striking practical applications of beta decay is radiocarbon (carbon) dating, a technique for estimating the age of ancient organic material.

In 1930, the German physicists Walther Bothe and Herbert Becker discovered that bombarding beryllium with alpha particles produced a highly penetrating, uncharged radiation, one able to pass through t…

In 1939, the German scientists Otto Hahn and Fritz Strassmann discovered that when a uranium nucleus is bombarded with a neutron, it can break apart into two smaller nuclei of roughly comparable mass,…

Nuclear fusion is the process by which two or more light nuclei (typically each with mass number ) combine to form a single, heavier nucleus.

- A device used to study the conduction of electricity through gases is known as a gas discharge tube.

The chapter's concept map ties every topic together under the single title "Atomic and Nuclear Physics," branching into two broad halves that mirror the unit's two main themes.

This section is the textbook's own end-of-chapter evaluation set for the unit, covering all the material from the electric discharge through gases up to elementary particles and the fundamental forces…

The chapter closes with a short reading list of six standard modern-physics textbooks that the authors recommend to students who wish to explore atomic and nuclear physics in greater depth than the NC…

The chapter ends with an "ICT Corner" activity box directing students to a free online virtual-lab simulation of Millikan's oil drop experiment, hosted at the Amrita Vishwa Vidyapeetham virtual labs p…

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