Chapter 8: Journey Inside the Atom

Comprehensive vocabulary flashcards covering subatomic particles, historical atomic models, electronic configurations, isotopes, and nuclear concepts from Chapter 8: Journey Inside the Atom.

Parmanu

The infinitely small, indivisible particles of matter (dravya) proposed by Acharya Kanada in the Sanskrit text Vaisesika Sutras, which combine into dyads and triads to build the material universe.

Atomos

An ancient Greek term meaning 'indivisible', proposed by the philosophers Leucippus and Democritus to describe the fundamental particles of matter.

Dalton's Atomic Theory

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

Radioactivity

The phenomenon discovered in the late 19th century where certain elements emit invisible energy and particles (radiation), demonstrating that atoms are divisible into smaller subatomic parts.

Cathode Rays

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

Electron

The first identified subatomic particle, discovered by J. J. Thomson, which carries a negative charge of $-1.602 \times 10^{-19}\,\text{C}$ (assigned a relative charge of $-1$) and has a negligible mass compared to the atom.

Thomson's Model of an Atom

An early atomic model (also called the plum pudding or watermelon model) proposing that an atom is a sphere of positive charge with electrons distributed throughout it to maintain electrical neutrality.

Alpha ($\alpha$) Particle

A positively charged particle emitted by certain radioactive elements, consisting of the nucleus of a helium atom containing two protons and two neutrons.

Gold Foil Experiment

The 1911 experiment conducted by Geiger and Marsden under Ernest Rutherford in which alpha particles were fired at a thin sheet of gold foil, revealing that most particles passed through undeflected while a few were scattered at large angles or bounced back.

Scattering

The deflection of moving particles (such as alpha particles) from their straight path upon interacting with matter.

Nucleus

The extremely small, dense, positively charged region at the center of an atom discovered by Ernest Rutherford, which contains nearly all of the atom's mass and has a diameter of approximately $10^{-15}\,\text{m}$.

Planetary Model of the Atom

Rutherford's atomic model which proposes that the atom consists mostly of empty space, with electrons revolving around a tiny, dense central nucleus like planets orbiting the Sun.

Limitation of Rutherford's Model

Its inability to explain atomic stability; according to classical physics, an accelerating charged electron in circular motion should continuously radiate energy, spiral inward, and collapse into the nucleus.

Proton

A subatomic particle discovered and named by Ernest Rutherford that resides in the nucleus, carries a relative positive charge of $+1$, and has a mass much greater than that of an electron.

Bohr's Model of the Atom

The 1913 atomic model proposed by Niels Bohr stating that electrons revolve only in specific, non-radiating circular orbits called stationary states or energy levels around the nucleus.

Stationary States (Energy Levels)

The fixed circular paths (designated K, L, M, N… or $n = 1, 2, 3, 4\dots$) in which an electron has a definite energy and revolves around the nucleus without radiating or losing energy.

Neutron

A neutral subatomic particle with no electrical charge and a mass roughly equal to that of a proton, discovered in 1932 by James Chadwick and located in the nucleus of all atoms except standard hydrogen.

Nuclear Force

The strong binding force in the nucleus that overcomes electrostatic repulsion between positively charged protons and holds all nucleons tightly together.

Bhabha Atomic Research Centre (BARC)

A premier scientific research facility in Mumbai, India, established by Homi Jehangir Bhabha, that conducts advanced neutron-scattering experiments using research reactors like Dhruva.

International Union of Pure and Applied Chemistry (IUPAC)

The international scientific organisation responsible for formally approving and standardising 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 combined in the nucleus of an atom ($\text{Mass number} = \text{Number of protons} + \text{Number of neutrons}$).

Nucleons

The collective term used for both protons and neutrons located together within the atomic nucleus.

Bohr-Bury Rules

The rules governing the distribution of electrons in an atom: the maximum capacity of a shell is given by $2n^2$, the outermost shell can hold at most 8 electrons, and shells fill in a stepwise manner from innermost to outermost.

Electronic Configuration

The 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 present in the outermost (valence) shell of an atom, which primarily govern the atom's chemical properties and bonding.

Octet

A stable configuration consisting of 8 electrons in the outermost shell of an atom (or 2 electrons in the case of helium), making the atom largely unreactive.

Valency

The combining capacity of an atom, defined as the number of electrons gained, lost, or shared by an atom to attain 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 containing different numbers of neutrons in their nuclei.

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

The standard unit used to measure the mass of atoms and subatomic particles, replacing the earlier abbreviation atomic mass unit ('amu').

Weighted Average Atomic Mass

The average mass of an element calculated by multiplying the mass of each naturally occurring isotope by its fractional abundance and summing the results.

Isobars

Atoms of different chemical elements that have the same mass number ($A$) but different atomic numbers ($Z$), such as calcium-40, potassium-40, and argon-40.

Scanning Tunnelling Microscope (STM)

An advanced electron microscope that can image materials at atomic-level resolution by specifically studying their surface structures.