Unit 1: Atomic Theory - History, Models, and Nuclear Concepts

Vocabulary flashcards covering key terms from the history, models, and nuclear concepts of atomic theory.

Democritus

Ancient Greek philosopher who proposed the atomos (atomos means indivisible) and suggested matter is composed of solid, indivisible spheres.

atom

The basic unit of matter; the smallest unit that retains the identity of a chemical element.

atomos

Greek word meaning indivisible; name Democritus used for the fundamental particle.

Plum Pudding Model

Thomson’s model where the atom is a sphere of positive charge with electrons embedded throughout.

Billiard Ball Model

Dalton’s early view of the atom as a solid, indivisible sphere.

Rutherford

Scientist who proposed the Nuclear Model after the Gold Foil Experiment, showing a dense nucleus and mostly empty space.

Nuclear Model

Rutherford’s model with a dense, positively charged nucleus at the center and electrons surrounding it.

Bohr Model

Planetary model in which electrons occupy fixed energy levels around the nucleus and can move between levels by absorbing or emitting energy.

Electron Cloud Model

Schrödinger’s view where electrons occupy probabilistic regions around the nucleus (orbitals) rather than fixed paths.

Quantum Mechanical Model

Current atomic model; electrons occupy orbitals described by probability distributions in three dimensions.

Dalton

English chemist who proposed the first modern atomic theory in 1803, describing atoms as indivisible particles.

Dalton's Atomic Theory

Postulates: atoms are indivisible, identical within elements, rearranged in reactions, combine in whole-number ratios; atoms are solid spheres.

Proton

Positively charged subatomic particle in the nucleus with about 1 amu of mass; determines the identity of the element.

Neutron

Electrically neutral subatomic particle in the nucleus with about 1 amu of mass; contributes to mass and helps hold the nucleus together.

Electron

Negatively charged subatomic particle surrounding the nucleus; very small mass and involved in chemical bonding.

Atomic Number (Z)

Number of protons in the nucleus; identifies the element; equals the number of electrons in a neutral atom.

Mass Number (A)

Total number of protons and neutrons in the nucleus; used to describe isotopes.

Isotope

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

Ion

A charged atom formed when electrons are gained or lost.

Cation

Positively charged ion formed when electrons are lost; common in metals.

Anion

Negatively charged ion formed when electrons are gained; often ends with -ide.

A-Z Notation

Notation for isotopes using mass number (A) and atomic number (Z), e.g., 40/19K or Potassium-40.

Neutral Atom

An atom with equal numbers of protons and electrons; overall charge is zero.

Relative Abundance

The fraction or percentage of a given isotope that occurs naturally in a sample.

Average Atomic Mass

Weighted average mass of all naturally occurring isotopes of an element, measured in amu.

Mass Spectrometry

Analytical technique that measures mass-to-charge ratios to determine isotopic composition and abundances.

Fission

Nuclear process where a heavy nucleus splits into smaller nuclei, releasing energy and often causing a chain reaction.

Fusion

Nuclear process where light nuclei combine to form a heavier nucleus, releasing large amounts of energy.

Half-life

The time required for half of a radioactive sample to decay; used to calculate remaining quantities.

Transmutation

Conversion of one element into another through nuclear reactions.

Nuclear Reaction

A reaction that involves changes in the nucleus, often emitting particles or radiation and possibly changing the element.

Law of Conservation of Mass

Mass cannot be created or destroyed in a chemical or nuclear reaction; total mass of reactants equals total mass of products.

Atomic Mass Unit (amu)

Unit used to express atomic and molecular masses; defined relative to carbon-12.

Identity of an Atom

Determined by the number of protons (the atomic number Z), which defines the element.

Nucleus

Dense, positively charged center of the atom containing protons and neutrons; contains most of the atom’s mass.

Electron Orbitals

Regions in space around the nucleus where electrons are likely to be found; described by quantum mechanics.

Energy Levels

Quantized levels in early models (and in Bohr’s work) where electrons reside around the nucleus.

Chadwick

Scientist who discovered the neutron in 1932.

Proton-Neutron Balance

Protons and neutrons in the nucleus contribute to mass; protons determine identity while neutrons help hold the nucleus together.

Beta Particle

High-energy electron emitted during beta decay; symbol β or e−.

Alpha Particle

Helium-4 nucleus (2 protons, 2 neutrons) emitted during alpha decay.

Gamma Ray

High-energy photon emitted during nuclear decay; no mass or charge.

Ionization

Process of forming ions by losing or gaining electrons.

Isotope Notation Variants

Names can be written as element-mass number, symbol-mass number, or AZ notation (e.g., Potassium-40, K-40, 40/19K).

Democritus

Ancient Greek philosopher who proposed the atomos (atomos means indivisible) and suggested matter is composed of solid, indivisible spheres.

atom

The basic unit of matter; the smallest unit that retains the identity of a chemical element.

atomos

Greek word meaning indivisible; name Democritus used for the fundamental particle.

Plum Pudding Model

Thomson’s model where the atom is a sphere of positive charge with electrons embedded throughout.

Billiard Ball Model

Dalton’s early view of the atom as a solid, indivisible sphere.

Rutherford

Scientist who proposed the Nuclear Model after the Gold Foil Experiment, showing a dense nucleus and mostly empty space.

Nuclear Model

Rutherford’s model with a dense, positively charged nucleus at the center and electrons surrounding it.

Bohr Model

Planetary model in which electrons occupy fixed energy levels around the nucleus and can move between levels by absorbing or emitting energy.

Electron Cloud Model

Schrödinger’s view where electrons occupy probabilistic regions around the nucleus (orbitals) rather than fixed paths.

Quantum Mechanical Model

Current atomic model; electrons occupy orbitals described by probability distributions in three dimensions.

Dalton

English chemist who proposed the first modern atomic theory in 1803, describing atoms as indivisible particles.

Dalton's Atomic Theory

Postulates: atoms are indivisible, identical within elements, rearranged in reactions, combine in whole-number ratios; atoms are solid spheres.

Proton

Positively charged subatomic particle in the nucleus with about 1 amu of mass; determines the identity of the element.

Neutron

Electrically neutral subatomic particle in the nucleus with about 1 amu of mass; contributes to mass and helps hold the nucleus together.

Electron

Negatively charged subatomic particle surrounding the nucleus; very small mass and involved in chemical bonding.

Atomic Number (Z)

Number of protons in the nucleus; identifies the element; equals the number of electrons in a neutral atom.

Mass Number (A)

Total number of protons and neutrons in the nucleus; used to describe isotopes.

Isotope

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

Ion

A charged atom formed when electrons are gained or lost.

Cation

Positively charged ion formed when electrons are lost; common in metals.

Anion

Negatively charged ion formed when electrons are gained; often ends with -ide.

A-Z Notation

Notation for isotopes using mass number (A) and atomic number (Z), e.g., \text{40/19K} or Potassium-40.

Neutral Atom

An atom with equal numbers of protons and electrons; overall charge is zero.

Relative Abundance

The fraction or percentage of a given isotope that occurs naturally in a sample.

Average Atomic Mass

Weighted average mass of all naturally occurring isotopes of an element, measured in amu.

Mass Spectrometry

Analytical technique that measures mass-to-charge ratios to determine isotopic composition and abundances.

Fission

Nuclear process where a heavy nucleus splits into smaller nuclei, releasing energy and often causing a chain reaction.

Fusion

Nuclear process where light nuclei combine to form a heavier nucleus, releasing large amounts of energy.

Half-life

The time required for half of a radioactive sample to decay; used to calculate remaining quantities.

Transmutation

Conversion of one element into another through nuclear reactions.

Nuclear Reaction

A reaction that involves changes in the nucleus, often emitting particles or radiation and possibly changing the element.

Law of Conservation of Mass

Mass cannot be created or destroyed in a chemical or nuclear reaction; total mass of reactants equals total mass of products.

Atomic Mass Unit (amu)

Unit used to express atomic and molecular masses; defined relative to carbon-12.

Identity of an Atom

Determined by the number of protons (the atomic number Z), which defines the element.

Nucleus

Dense, positively charged center of the atom containing protons and neutrons; contains most of the atom’s mass.

Electron Orbitals

Regions in space around the nucleus where electrons are likely to be found; described by quantum mechanics.

Energy Levels

Quantized levels in early models (and in Bohr’s work) where electrons reside around the nucleus.

Chadwick

Scientist who discovered the neutron in 1932.

Proton-Neutron Balance

Protons and neutrons in the nucleus contribute to mass; protons determine identity while neutrons help hold the nucleus together.

Beta Particle

High-energy electron emitted during beta decay; symbol \beta or \text{e-} .

Alpha Particle

Helium-4 nucleus (2 protons, 2 neutrons) emitted during alpha decay.

Gamma Ray

High-energy photon emitted during nuclear decay; no mass or charge.

Ionization

Process of forming ions by losing or gaining electrons.

Isotope Notation Variants

Names can be written as element-mass number, symbol-mass number, or AZ notation (e.g., Potassium-40, K-40, \text{40/19K} ).

Gold Foil Experiment (Procedure)

Rutherford's experiment where a beam of positively charged \alpha \text{-particles} was directed at a very thin sheet of gold foil.

Gold Foil Experiment (Observations)

1. Most \alpha \text{-particles} passed straight through the foil. 2. A few \alpha \text{-particles} were deflected at large angles. 3. A very small number of \alpha \text{-particles} bounced back towards the source.

Gold Foil Experiment (Conclusions)

1. Atoms are mostly empty space (from most particles passing through). 2. Atoms have a small, dense, positively charged nucleus (from deflections and bounce-backs). These conclusions led to the Nuclear Model.