Atomic Theory, Subatomic Particles, and Isotopes

Comprehensive vocabulary flashcards covering the history of atomic theory, atomic models, subatomic particles, isotopic notations, and nuclear properties.

Atomos

The Greek word meaning indivisible, proposed by the ancient Greek philosopher Democritus to describe the fundamental particles that make up all matter.

Atomic Theory

The scientific theory stating that all matter is composed of tiny particles called atoms.

Law of Conservation of Mass

A foundational chemical law discovered by Antoine Lavoisier stating that matter cannot be created or destroyed during a chemical reaction, meaning the total mass remains constant.

Law of Definite Proportions

A chemical law established by Joseph Proust stating that a chemical compound always contains the same elements in the exact same proportions by mass, regardless of source or quantity.

Law of Multiple Proportions

A chemical law discovered by John Dalton stating that when two elements form different compounds, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers.

Dalton's Atomic Theory

The first truly scientific theory of the atom proposed by John Dalton, asserting that all matter is made of indivisible atoms, atoms of the same element have identical weights, atoms of different elements have different weights, and atoms combine in simple whole-number ratios to form compounds.

Avogadro's Law

The gas law stating that equal volumes of any two gases, measured at the same temperature and pressure, contain equal numbers of molecules.

Brownian Motion

The constant, random movement of tiny particles suspended in a fluid (such as dust inside pollen grains floating in water) caused by continuous collisions with fluid molecules, which Albert Einstein mathematically described in 1905.

Cathode Ray

A stream of negatively charged particles produced inside a vacuum tube when an electrical voltage is applied across two electrodes.

Plum Pudding Model

An early atomic model proposed by J. J. Thomson describing the atom as a sphere of positive charge with negatively charged electrons scattered throughout, analogous to raisins in plum pudding.

Gold Foil Experiment

An experiment conducted by Ernest Rutherford, Hans Geiger, and Ernest Marsden where positively charged alpha particles were fired at thin gold foil; deflections at angles greater than $90^\text{o}$ revealed the presence of a dense, positively charged nucleus.

Nuclear Model of the Atom

Ernest Rutherford's atomic model proposing that an atom has a tiny, dense, positively charged nucleus at its center containing most of the atom's mass, surrounded by electrons in mostly empty space.

Quantum Atom Theory

The modern theory stating that the atom is mostly empty space consisting of two primary regions: a dense, positively charged nucleus composed of protons and neutrons, and an electron cloud where there is a $90\%$ chance of finding an electron.

Electron Cloud

The expansive space surrounding the atomic nucleus where there is a high probability ($90\%$ chance) of finding electrons.

Subatomic Particles

Particles smaller than an atom that constitute its internal structure: protons, neutrons, and electrons.

Electron

A negatively charged subatomic particle ($e^-$) discovered in 1899 by J. J. Thomson (initially termed "corpuscles") with a charge of $-1$, a relative mass of $0\,\text{amu}$, and an actual mass of $9.11 \times 10^{-28}\,\text{g}$, where $1840$ electrons equal the mass of one proton.

Proton

A positively charged subatomic particle ($p^+$) located in the nucleus, discovered by Ernest Rutherford in 1917 and named in 1920, with a charge of $+1$, a relative mass of $1\,\text{amu}$, and an actual mass of $1.6726 \times 10^{-24}\,\text{g}$.

Neutron

An uncharged subatomic particle ($n^0$) residing in the nucleus, discovered by James Chadwick in 1932, with a charge of $0$, a relative mass of $1\,\text{amu}$, and an actual mass of $1.6749 \times 10^{-24}\,\text{g}$ (slightly heavier than a proton).

Atomic Number ($Z$)

The number of protons in an atom's nucleus that uniquely identifies an element; no two elements possess the same atomic number, and in a neutral atom, it equals the number of electrons.

Mass Number ($A$)

The total number of protons and neutrons contained within an atom's nucleus.

Atomic Mass Unit ($\text{amu}$)

A standard unit of relative mass defined as $\frac{1}{12}$ the mass of a carbon-12 atom, which is approximately the average mass of a single proton and neutron.

Cation

A positively charged ion formed when an atom has fewer electrons than protons (excess protons bring an overall positive charge).

Anion

A negatively charged ion formed when an atom gains electrons, resulting in more electrons than protons (excess electrons bring an overall negative charge).

Isotope

Atoms of the same element that have the same number of protons (same atomic number) but different numbers of neutrons (different mass numbers), resulting in identical chemical properties but different masses and physical properties.

Nuclide

A specific type of atom characterized by a particular number of protons and neutrons, focusing specifically on nuclear properties rather than comparing atoms of the same element.

Nuclear Notation (AZE Notation)

A standardized format for depicting an isotope using the element symbol ($\text{X}$), its mass number ($A$) as a superscript on the upper left, and its atomic number ($Z$) as a subscript on the lower left ($^A_Z\text{X}$).

Hyphenated Notation

A method of writing an isotope using the element name or chemical symbol followed by a hyphen and its mass number (e.g., Fluorine-19, $\text{C-14}$, or $\text{U-238}$).

Average Atomic Mass

The weighted average mass of all naturally occurring isotopes of an element, calculated by multiplying each isotope's mass by its percent occurrence in nature and summing the products.

Nuclear Stability

The physical condition of an atomic nucleus governed by its proton-to-neutron ratio; neutrons stabilize the nucleus by providing attractive nuclear forces that counter the electrostatic repulsion between positively charged protons.

Stable Isotope

An isotope with a balanced proton-to-neutron ratio that has not been observed to undergo radioactive decay.

Radioactive Isotope (Radioisotope)

An unstable isotope that spontaneously undergoes radioactive decay over time to achieve a more stable nuclear state.

Mass Spectrograph

An analytical instrument used by Francis Aston to separate and identify multiple stable isotopes in various chemical elements.

Radiometric Dating

A technique that uses radioactive isotopes and their known half-lives (such as using carbon-14 in carbon dating) to determine the absolute age of once-living materials or ancient objects.