Atoms, Molecules, and Ions Practice Flashcards

These vocabulary flashcards cover early atomic theory, the evolution of atomic structure through key historical experiments, isotopic notation, chemical formulas, and the concept of the mole.

Atomos

A Greek word meaning "indivisible," used by Leucippus and Democritus to describe small, finite particles that compose all matter.

Aristotle's Four Elements

The philosophical concept that matter consists of various combinations of fire, earth, air, and water.

John Dalton’s Atomic Theory (1807)

The theory stating that matter is composed of atoms, which are the smallest units of an element that can participate in chemical change, and that atoms of an element are identical in mass but differ from atoms of other elements.

Law of Definite Proportions

Also known as the Law of Constant Composition, it states that all samples of a pure compound contain the same elements in the same proportion by mass.

Law of Multiple Proportions

The principle that when two elements react to form more than one compound, a fixed mass of one element reacts with masses of the other in a ratio of small, whole numbers.

Electron

A negatively charged subatomic particle discovered by J.J. Thomson in 1897 using cathode-ray tube experiments; its charge/mass ratio is $1.759 \times 10^{11}\text{ C/kg}$.

Robert A. Millikan (1909)

The scientist who determined the fundamental charge of an electron as $1.6 \times 10^{-19}\text{ C}$ and its mass as $9.11 \times 10^{-31}\text{ kg}$ via oil drop experiments.

Plum Pudding Model

J.J. Thomson's proposed atomic structure where the atom resembles an English dessert with raisins (electrons) embedded in a moist cake (positive charge).

Radioactivity

The spontaneous emission of radiation by uranium, characterized by Becquerel, Rontgen, and Curie as consisting of gamma rays, beta particles, and alpha particles.

Alpha particles ($\alpha$)

Radioactive particles characterized by a $2+$ charge.

Beta particles ($\beta$)

High-speed radioactive particles consisting of an electron with a negative charge.

Gamma rays ($\Upsilon$)

High-energy light emissions with no charge.

Nucleus

A small, relatively heavy, positively charged body at the center of each atom, discovered by Ernest Rutherford in his 1913 gold foil experiment.

James Chadwick (1932)

The scientist who discovered the neutron, a subatomic particle with no charge and a mass of $1.67493 \times 10^{-24}\text{ g}$, located in the nucleus.

Atomic Number ($Z$)

The number of protons in an atom, which determines the specific identity of the element.

Mass Number ($A$)

The sum of the number of protons ($p$) and the number of neutrons ($n$) in an atom.

Isotopes

Atoms of the same element that have the same number of protons and electrons but a different number of neutrons.

Anion

An ion formed when an atom gains one or more electrons, resulting in more electrons than protons and an overall negative charge.

Cation

An ion formed when an atom loses one or more electrons, resulting in more protons than electrons and an overall positive charge.

Atomic Mass Unit (amu)

A unit of measure defined as exactly $\frac{1}{12}$ of the mass of one carbon-12 atom, equivalent to $1.6605 \times 10^{-24}\text{ g}$.

Atomic Mass

A weighted average mass of all the isotopes present in a naturally occurring sample of an element.

Molecular Formula

A representation of a molecule that indicates the exact number and types of atoms present using chemical symbols and subscripts.

Structural Formula

A representation that shows how the atoms in a molecule are connected to one another.

Empirical Formula

The simplest whole-number ratio of the atoms of each element present in a compound.

Isomers

Compounds that share the same molecular formula but have different molecular structures and properties.

Mole

The unit of amount equal to the number of carbon atoms in exactly $12\text{ g}$ of pure ${}^{12}\text{C}$, or $6.022 \times 10^{23}$ discrete entities.

Avogadro’s number ($N_A$)

The constant value $6.022 \times 10^{23}$, representing the number of entities in one mole.

Molar Mass

The mass in grams of one mole of a substance, expressed in units of $\text{g/mol}$, which is numerically equivalent to its atomic or molecular mass in amu.