Chapter 3: Atoms – Vocabulary Flashcards

A set of vocabulary flashcards covering key terms from Chapter 3: Atoms, including atomic theory, periodic trends, subatomic particles, isotopes, atomic models, and ion formation.

Atom

The smallest unit of an element that retains the element’s chemical identity.

Atomos

Greek term meaning “indivisible,” first used by Democritus (~400 B.C.E.) to describe the ultimate particles of matter.

Law of Conservation of Mass

Principle stating that matter is neither created nor destroyed in a chemical reaction.

Antoine Lavoisier

18th-century chemist who established the Law of Conservation of Mass.

John Dalton

English scientist (1803-1808) who developed the first modern atomic theory.

Dalton’s Atomic Theory

Set of postulates stating that elements are made of atoms, each element’s atoms are unique, atoms combine in whole-number ratios, and atoms are unchanged in reactions.

Whole-Number Ratios

Fixed, small integer proportions in which atoms combine to form compounds.

Periodic Table

Chart organizing the elements by increasing atomic number and recurring chemical properties.

Dmitri Mendeleev

Russian chemist who arranged the first widely recognized periodic table (1869).

Metal

Element that is typically shiny, malleable, conducts heat/electricity, and lies on the left side of the periodic table.

Nonmetal

Element lacking metallic luster, poor conductor, found on the upper-right side of the periodic table.

Metalloid

Element with properties intermediate between metals and nonmetals; located along the stair-step line.

Alkali Metals

Group 1A elements; very reactive, soft metals like Li, Na, K.

Alkaline Earth Metals

Group 2A elements; reactive metals such as Be, Mg, Ca.

Halogens

Group 7A elements; highly reactive nonmetals like F, Cl, Br, I.

Noble Gases

Group 8A elements; inert gases including He, Ne, Ar, Kr, Xe, Rn.

Subatomic Particle

Particle smaller than an atom: proton, neutron, or electron.

Proton

Positively charged subatomic particle located in the nucleus; mass ≈ 1 u.

Neutron

Electrically neutral subatomic particle in the nucleus; mass ≈ 1 u.

Electron

Negatively charged subatomic particle in the electron cloud; mass ≈ 0.00055 u.

Atomic Number (Z)

Number of protons in an atom; defines the element.

Mass Number (A)

Total number of protons plus neutrons in an atom’s nucleus.

Isotope

Atoms of the same element (same Z) with different numbers of neutrons (different A).

Protium

Hydrogen-1 isotope with 1 proton and 0 neutrons.

Deuterium

Hydrogen-2 isotope with 1 proton and 1 neutron.

Tritium

Hydrogen-3 isotope with 1 proton and 2 neutrons.

Atomic Symbol

Notation showing an element’s symbol with its atomic and mass numbers (e.g., ²³⁵₉₂U).

Atomic Mass Unit (u)

Standard unit of atomic mass; 1 u = 1.66 × 10⁻²⁷ kg.

Average Atomic Mass

Weighted average of the masses of an element’s naturally occurring isotopes.

Weighted Average Formula

average = (mass₁×fraction₁)+(mass₂×fraction₂)+…

Plum Pudding Model

J. J. Thomson’s 1904 model: negative electrons embedded in a positive sphere.

Gold Foil Experiment

Rutherford’s alpha-particle scattering study that revealed the atomic nucleus.

Rutherford Nuclear Model

Atom consists of a tiny, dense, positively charged nucleus surrounded by electrons in mostly empty space.

Bohr Model

1913 model where electrons orbit the nucleus in quantized energy levels.

Quantum Model

Modern view describing electrons as wave-like and existing in orbitals rather than fixed orbits.

Ion

Charged particle formed when an atom gains or loses electrons.

Cation

Positively charged ion produced by loss of electrons.

Anion

Negatively charged ion produced by gain of electrons.

Sulfide Ion

S²⁻ anion formed when a sulfur atom (Z = 16) gains two electrons.

X-ray Crystallography

Technique that determines atomic arrangement in crystals by analyzing X-ray diffraction patterns.

Electrochemical Cell

Device (battery) that produces electricity through spontaneous redox reactions; invented by Volta in 1800.

Alpha Particle

Helium-4 nucleus (2 protons, 2 neutrons) emitted in some radioactive decays; used by Rutherford in scattering experiments.