Atoms, Molecules, and Ions - Key Terms (VOCABULARY Flashcards)

Vocabulary flashcards covering Dalton’s atomic theory, atomic structure, isotopes, chemical formulas, and mole concepts from the chapter.

Atom

The basic unit of an element that participates in chemical changes; composed of a nucleus (protons and neutrons) surrounded by electrons.

Atomos

Greek for 'indivisible,' describing the ancient idea that matter is made of indivisible units.

Dalton's Atomic Theory

Early theory that matter is made of atoms, elements consist of one type of atom, compounds form from fixed whole-number ratios of atoms, and atoms are conserved in chemical changes.

Postulate 1 (Dalton)

Matter is composed of exceedingly small particles called atoms.

Postulate 2 (Dalton)

An element consists of only one type of atom, with a characteristic mass for that element.

Postulate 3 (Dalton)

Atoms of one element differ in properties from atoms of all other elements.

Postulate 4 (Dalton)

A compound consists of atoms of two or more elements in a small, whole-number ratio; the ratio is fixed for a given compound.

Postulate 5 (Dalton)

Atoms are neither created nor destroyed during a chemical change; they are rearranged to form new substances.

Law of Conservation of Matter

Mass is conserved in chemical reactions; total mass before and after remains constant.

Law of Definite Proportions

A pure compound always contains the same elements in the same proportion by mass.

Law of Multiple Proportions

When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in small whole-number ratios.

Isotopes

Atoms of the same element that have different numbers of neutrons and thus different mass numbers.

Atomic Number (Z)

Number of protons in the nucleus; defines the identity of the element.

Mass Number (A)

Total number of protons and neutrons in the nucleus.

Neutral Atom

An atom with equal numbers of protons and electrons, giving no net charge.

Ion

An atom or molecule with a net electric charge due to a loss or gain of electrons.

Cation

A positively charged ion, formed when an atom loses electrons.

Anion

A negatively charged ion, formed when an atom gains electrons.

Chemical Symbol

Abbreviation for an element (e.g., H, He, Fe); capitalization rules: only the first letter is capitalized.

Diatomic Molecules

Molecules composed of two atoms (e.g., H2, N2, O2, F2, Cl2, Br2, I2).

Empirical Formula

The simplest whole-number ratio of atoms in a compound.

Molecular Formula

The exact number of atoms of each element in a molecule.

Formula Mass

Sum of the atomic masses in a chemical formula, expressed in amu.

Molar Mass

Mass of 1 mole of a substance, numerically equal to its formula mass in g/mol.

Avogadro’s Number (NA)

6.022 × 10^23 particles per mole.

Mole

Amount of substance containing exactly NA particles; a bridge between mass and number of particles.

Isotope Notation

Mass number as a superscript and atomic number as a subscript next to the element symbol (e.g., 24Mg).

Nucleus

Dense center of the atom containing protons and neutrons; protons determine identity, neutrons add mass.

Rutherford’s Nuclear Model

Atom has a tiny, dense, positively charged nucleus; most of the atom is empty space with electrons surrounding it.

Thomson’s Plum Pudding Model

Early model where electrons were embedded in a positively charged 'soup' or pudding.

Electron

Subatomic particle with a negative charge (−1) and very small mass; located outside the nucleus.

Proton

Subatomic particle with a positive charge (+1) and mass about 1 amu, located in the nucleus.

Neutron

Uncharged subatomic particle with mass about 1 amu, located in the nucleus.

amu (Atomic Mass Unit)

Unit used to express atomic and molecular masses; 1 amu is 1/12 the mass of a 12C atom.

Average Atomic Mass

Weighted average mass of an element’s isotopes based on their natural abundances.

Averaging Formula

Mass of isotope × its fractional abundance summed over all isotopes.

Common Element Symbols

Two-letter or one-letter abbreviations for elements (e.g., Al, Fe, Hg) with historical origins.

Mass–Mole–Number Relationships

Mass of a substance relates to moles via molar mass; moles relate to number of particles via Avogadro’s number.