Atomic Structure

Democritus

Proposed the idea of "atoms in a void"

Aristotle

Believed matter was made of 4 "elements"

John Dalton

Proposed a modern atom model based on experimentation, first scientist to have a theory about matter being composed of atoms

Law of Conservation of Mass

States that atoms are neither created nor destroyed during chemical reactions, the number of atoms remains constant

Law of Definite Proportions

In a given compound, the elements are in a fixed ratio, atoms are neither lost nor gained in a chemical reaction

Atom

The smallest particle of a substance that can exist by itself or be combined with other atoms to form a molecule

JJ Thompson

Discovered electrons through the discovery of cathode rays, proposed the "plum pudding" model of the atom

James Chadwick

Discovered neutrons in the nucleus of an atom

Neils Bohr

Proposed that electrons move around the nucleus in specific layers or energy levels

Atomic number

The number of protons in an atom of an element, equal to the number of electrons

Mass number

The total number of protons and neutrons in an atom

Isotopes

Atoms of the same element with different numbers of neutrons

Relative Abundance

The percentage of atoms with a specific atomic mass found in a naturally occurring sample of an element

Radioactivity decay

The emission of ionizing radiation from nuclear decay

Ions

Charged particles formed when a neutral atom gains or loses electrons

Bohr's Atomic Theory

Electrons can exist only in certain circular orbits or energy levels, electron energy is quantized

Ground state

The lowest energy state of an atom

Excited state

A state where the atom has a higher potential energy than in its ground state

Neon signs

Excited atoms returning to their ground state and emitting energy as electromagnetic radiation

Electron Cloud Model

Represents all orbitals in an atom, consists of a dense nucleus surrounded by a cloud of electrons at various energy levels

Pauli Exclusion Principle

Each orbital can hold a maximum of 2 electrons with opposite spins

Hund's Rule

Electrons occupy equal-energy orbitals singly before pairing up

Aufbau Principle

Electrons are added one at a time to the lowest energy orbitals available

Electron configuration

The probable distribution of electrons around the nucleus of an atom.