Journey Inside the Atom

Comprehensive vocabulary flashcards covering Chapter 8: Journey Inside the Atom, detailing ancient atomic concepts, subatomic particles, atomic models, electron configuration, valency, isotopes, and isobars.

Parmanu

The smallest indivisible particle of matter (dravya) proposed by Acharya Kanada in the Vaisesika Sutras, which combines into dyads and triads to form the material universe.

Atomos

The Greek word meaning 'indivisible', proposed by Leucippus and Democritus to describe the fundamental particles of matter.

Dalton's Atomic Theory

The first scientific atomic theory proposed in $1808$ by John Dalton, stating that all matter is composed of indivisible fundamental building blocks called atoms.

Radioactivity

The spontaneous emission of invisible energy and particles (radiation) by certain elements, which proved that atoms are composed of smaller particles and are not indivisible.

Cathode Rays

Streams of negatively charged particles observed by J. J. Thomson in $1897$ flowing from the cathode (negative electrode) to the anode (positive electrode) in a low-pressure gas tube under high voltage.

Electron

The first identified subatomic particle, discovered by J. J. Thomson, carrying a negative charge of $-1.602 \times 10^{-19}\,\text{C}$ (conveniently taken as $-1$) and possessing a mass much smaller than an atom.

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, like plums in a pudding or seeds in a watermelon.

Alpha ($\alpha$) Particle

A tiny, positively charged particle emitted by radioactive elements, physically identical to the nucleus of a helium atom containing two protons and two neutrons.

Scattering

The deflection of particles, such as alpha particles in Geiger and Marsden's gold foil experiment, away from their straight paths upon encountering matter.

Planetary Model of the Atom

The atomic model proposed by Ernest Rutherford in $1911$, where electrons revolve around a tiny, dense, positively charged nucleus like planets orbiting the Sun.

Nucleus

The dense, centrally located region of an atom with a diameter of about $10^{-15}\,\text{m}$ (compared to the atom's diameter of $10^{-10}\,\text{m}$) containing all the positive charge and nearly all the atom's mass.

Limitations of Rutherford's Model

Its failure to explain atomic stability, because according to classical physics an accelerating electron in circular motion would continuously radiate energy, spiral inward, and collapse into the nucleus.

Proton

A positively charged subatomic particle discovered and named by Ernest Rutherford, located in the nucleus with a relative charge of $+1$ and a mass much heavier than that of an electron.

Bohr's Model of the Atom

The atomic model proposed by Niels Bohr in $1913$, proposing that electrons revolve only in discrete, non-radiating circular orbits called stationary states or energy levels.

Stationary States

The specific, allowed circular orbits (energy levels labelled K, L, M, N or $n = 1, 2, 3, 4$) in which an electron has fixed energy and does not radiate or lose energy while moving.

Neutron

An uncharged subatomic particle ($n^0$) discovered by James Chadwick in $1932$, residing in the nucleus of all atoms except hydrogen and possessing a mass nearly equal to that of a proton.

Nuclear Force

The strong force within the nucleus that binds protons and neutrons together, counteracting the electrostatic repulsion between positively charged protons.

Bhabha Atomic Research Centre (BARC)

An Indian scientific research institution in Mumbai, founded by Homi Jehangir Bhabha, which conducts advanced neutron-scattering experiments using research reactors such as Dhruva.

Berzelius's Notation

The chemical symbol system introduced in $1813$ by Jöns Jacob Berzelius, which derived alphabetic symbols of elements from their names (often their Latin names).

IUPAC

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

Atomic Number ($Z$)

The total number of protons present in the nucleus of an atom of an element, which uniquely determines the element's identity and chemical properties.

Mass Number ($A$)

The total number of protons and neutrons present in the nucleus of an atom ($A = \text{protons} + \text{neutrons}$).

Nucleons

The subatomic particles that reside together inside the atomic nucleus, specifically protons and neutrons.

Bohr-Bury Rules

The rules dictating electron distribution: a shell $n$ holds a maximum of $2n^2$ electrons (K holds $2$, L holds $8$, M holds $18$), the outermost shell cannot hold more than $8$ electrons, and shells fill stepwise.

Electronic Configuration

The specific arrangement and stepwise distribution of electrons across the different energy levels or shells (K, L, M, N) of an atom.

Valence Shell

The outermost electron-containing shell of an atom.

Valence Electrons

The electrons located in the outermost shell (valence shell) of an atom that participate in chemical bonding.

Octet

A stable electronic arrangement in which the outermost valence shell of an atom contains eight electrons (or two in the case of helium).

Valency

The combining capacity of an atom, defined as the number of electrons gained, lost, or shared by the atom to complete a stable octet in its outermost shell.

Isotopes

Atoms of the same element that have the same atomic number ($Z$) but different mass numbers ($A$) due to varying numbers of neutrons, exhibiting identical chemical properties but different physical properties.

Protium, Deuterium, and Tritium

The three naturally occurring isotopes of hydrogen: Protium ($^1_1\text{H}$, $0$ neutrons, ~$99.98\%$), Deuterium ($^2_1\text{H}$, $1$ neutron, ~$0.015\%$), and Tritium ($^3_1\text{H}$, $2$ neutrons, trace amounts).

Unified Atomic Mass Unit ($\text{u}$)

A standard unit of mass used by scientists to express atomic and molecular masses, formerly abbreviated as amu.

Weighted Average Atomic Mass

The average atomic mass of an element calculated by summing the products of each isotope's mass and its fractional natural abundance (e.g., chlorine's mass of $35.5\,\text{u}$ from $75\%$ $^{35}\text{Cl}$ and $25\%$ $^{37}\text{Cl}$).

Isobars

Atoms of different chemical elements that have different atomic numbers ($Z$) but identical mass numbers ($A$), such as Argon-40, Potassium-40, and Calcium-40.