Chapter 8: Journey Inside the Atom Vocabulary

Vocabulary flashcards covering key terms, concepts, subatomic particles, historical atomic models, and chemical notation from Chapter 8.

Parmanu

The smallest, indivisible particles of matter proposed by Acharya Kanada in the Vaisesika Sutras, which combine to form dyads and triads that create the material universe.

Atomos

A Greek term meaning indivisible, proposed by philosophers Leucippus and Democritus to describe the ultimate indivisible particles of matter.

Dalton's Atomic Theory

A scientific theory proposed by John Dalton in $1808$ stating that all matter is composed of indivisible particles called atoms, which serve as the fundamental building blocks of matter.

Cathode Rays

Streams of negatively charged particles observed in $1897$ moving from the cathode to the anode inside a glass discharge tube operating under low gas pressure and high voltage.

Electron

The first identified subatomic particle, discovered by J. J. Thomson in $1897$, carrying a negative charge of $-1.602 \times 10^{-19}\text{ C}$ (taken conventionally as $-1$).

Plum Pudding Model

An early atomic model proposed by J. J. Thomson depicting the atom as a sphere of positive charge with electrons distributed throughout it, similar to seeds in a watermelon.

Alpha ($\alpha$) Particle

A tiny, positively charged particle emitted from certain radioactive elements, consisting of a helium nucleus with $2$ protons and $2$ neutrons.

Gold Foil Experiment

An experiment conducted in $1911$ by Geiger and Marsden under Ernest Rutherford, where alpha particles were shot at a thin gold foil, revealing the existence of the atomic nucleus.

Nucleus

The extremely small, dense central region of an atom discovered by Ernest Rutherford, containing all positive charge and nearly all of the atom's mass, with a diameter of approximately $10^{-15}\text{ m}$.

Planetary Model of the Atom

Rutherford's atomic model proposing that an atom consists mostly of empty space with electrons revolving around a central, positively charged nucleus like planets orbiting the Sun.

Proton

A positively charged subatomic particle found in the nucleus, discovered and named by Ernest Rutherford, carrying a relative charge of $+1$.

Stationary States

Fixed circular paths around the nucleus proposed by Niels Bohr in $1913$, in which revolving electrons maintain constant energy without collapsing into the nucleus.

Energy Levels

Atomic shells designated by letters K, L, M, N… or quantum numbers $n = 1, 2, 3, 4...$, holding electrons of specific energy amounts that increase with distance from the nucleus.

Neutron

An uncharged subatomic particle present in the atomic nucleus of all elements except hydrogen, discovered by James Chadwick in $1932$ with a mass nearly equal to a proton.

Nuclear Force

The strong force binding protons and neutrons together inside the nucleus, overcoming the electrostatic repulsion between positively charged protons.

IUPAC

The International Union of Pure and Applied Chemistry, an international scientific organisation that standardises and approves the names and symbols of chemical elements.

Atomic Number ($Z$)

The total number of protons present in the nucleus of an atom of an element, designated by the symbol $Z$.

Mass Number ($A$)

The total number of protons and neutrons (nucleons) present in the nucleus of an atom, denoted by the symbol $A$.

Nucleons

The collective name given to both protons and neutrons situated within the atomic nucleus.

Electronic Configuration

The systematic arrangement or distribution of electrons among the various energy shells (K, L, M, N…) of an atom.

Valence Shell

The outermost electron-containing shell of an atom.

Valence Electrons

The electrons residing in the outermost (valence) shell of an atom.

Octet

A stable state of an atom having eight electrons in its outermost valence shell.

Valency

The combining capacity of an atom, defined as the number of electrons gained, lost, or shared to achieve a stable octet or duet configuration.

Isotopes

Atoms of the same element that share the same atomic number ($Z$) but possess different numbers of neutrons, resulting in different mass numbers ($A$).

Unified Atomic Mass Unit ($u$)

A special standard unit used by scientists to measure and express the mass of individual atoms and subatomic particles.

Weighted Average Atomic Mass

An element's average atomic mass calculated by taking into account the mass and fractional natural abundance of each of its isotopes.

Isobars

Atoms of different chemical elements that have different atomic numbers ($Z$) but possess the same mass number ($A$).

Bhabha Atomic Research Centre (BARC)

An advanced nuclear research facility in Mumbai, India, leading neutron-scattering experiments on materials using reactors such as Dhruva.