Atomic Theory, Matter, and the Periodic Table

Vocabulary practice flashcards covering the history of atomic theory, subatomic particles, isotopes, states and classifications of matter, physical and chemical changes, electronic structure, and periodic trends.

Democritus

A 4th century B.C. philosopher who first proposed that matter is made of indivisible particles called "atomos."

John Dalton

A scientist who proposed an atomic theory stating that atoms of the same element are identical, atoms of different elements differ, atoms combine in whole-number ratios to form compounds, and atoms are rearranged (not created or destroyed) in chemical reactions.

J.J. Thomson

A scientist who discovered the electron in 1897 using cathode ray tubes and proposed the "plum pudding" model of the atom.

Ernest Rutherford

A scientist who conducted the gold foil experiment, demonstrating that atoms have a small, dense, positively charged nucleus because most alpha particles passed through while a few deflected or bounced back.

James Chadwick

A scientist who discovered the neutron in 1932.

Proton

A subatomic particle with a positive charge located in the nucleus, possessing a mass of approximately $1\,\text{amu}$.

Neutron

A subatomic particle with no charge (neutral) located in the nucleus, possessing a mass of approximately $1\,\text{amu}$.

Electron

A subatomic particle with a negative charge found in the electron cloud outside the nucleus, with a negligible mass of approximately $\frac{1}{1836}\,\text{amu}$.

Nucleus

The small, dense, positively charged central region of an atom that holds almost all of the atom's mass in a tiny volume.

Atomic Number

The number of protons found in the nucleus of an atom, which also equals the number of electrons in an electrically neutral atom.

Mass Number

The total number of protons plus neutrons in an atom's nucleus.

Isotopes

Atoms of the same element that have the same number of protons but different numbers of neutrons, resulting in different mass numbers.

Average Atomic Mass

The weighted average of the masses of all naturally occurring isotopes of an element, calculated using their percent abundances converted to decimals.

Period

A horizontal row (left to right) on the periodic table that matches the highest occupied energy level (principal quantum number) of an element.

Group (Family)

A vertical column on the periodic table containing elements that share similar chemical properties and valence electron counts.

Metals

Elements that tend to lose electrons easily, are typically malleable and ductile, display a shiny luster, and act as good conductors of heat and electricity.

Nonmetals

Elements that tend to gain electrons easily, are typically brittle in solid form, lack metallic luster, and act as poor conductors (insulators) of heat and electricity.

Element

A pure substance that cannot be broken down into simpler substances by normal chemical means.

Compound

A substance composed of two or more elements chemically combined in a fixed proportion that can only be separated by chemical means and exhibits properties different from its constituent elements.

Mixture

A physical blend of two or more substances that has a variable composition and can be separated by physical means.

Homogeneous Mixture

A mixture that features a uniform composition throughout, such as saltwater.

Heterogeneous Mixture

A mixture that is not uniform in composition, often allowing distinct parts to be visibly seen.

Filtration

A physical separation method used to separate an insoluble solid from a liquid.

Filtrate

The liquid that passes through the filter paper during the process of filtration.

Solid

A state of matter characterized by tightly packed particles, a definite shape, and a definite volume.

Liquid

A state of matter that has a definite volume but an indefinite shape, conforming to the shape of its container.

Gas

A state of matter that has an indefinite shape, an indefinite volume, low density, and high compressibility.

Physical Property

A characteristic that can be observed or measured without altering the identity of the substance, such as color, mass, melting point, or hardness.

Chemical Property

A characteristic describing the ability of a substance to undergo a chemical change, such as flammability, reactivity, or the ability to spoil and rust.

Physical Change

A change affecting the form of a chemical substance without altering its chemical identity or forming a new substance, such as melting or dissolving.

Chemical Change

A process in which one or more new substances are formed, indicated by clues such as color change, precipitate formation, gas production, or temperature change.

Law of Conservation of Mass

The scientific principle stating that in any chemical reaction, atoms are rearranged and are never created or destroyed.

Scientific Law

A summary of observed behavior that describes what happens in nature.

Scientific Theory

A well-substantiated explanation of natural behavior that explains why phenomena occur.

Net Charge

The electrical charge of a particle determined by the number of protons minus the number of electrons.

Valence Electrons

The electrons located in the outermost energy level of an atom that determine how the atom bonds and reacts chemically.

Electron Configuration

A representation showing how electrons occupy energy sublevels in order ($1s, 2s, 2p, 3s, 3p, 4s, 3d, \dots$), where the superscript indicates the number of electrons in each sublevel.

Noble Gas Shorthand

A notation method that replaces the filled inner electron configuration with the chemical symbol of the preceding noble gas enclosed in brackets (e.g., $[\text{Ne}]\,3s^2 3p^3$).

Alkali Metals

Group 1 elements (excluding hydrogen) consisting of very reactive metals that typically form $1+$ ions.

Alkaline Earth Metals

Group 2 elements consisting of reactive metals that typically form $2+$ ions.

Halogens

Group 17 elements consisting of reactive nonmetals that typically form $1-$ ions.

Noble Gases

Group 18 elements consisting of very unreactive (inert) gases with full valence shells.

Metalloids

Elements positioned along the "staircase" line of the periodic table between the metals to the left and nonmetals to the right.

Atomic Radius

The measure of the size of an atom, which generally decreases across a period (left to right) due to increasing nuclear charge and increases down a group due to the addition of energy levels.

Ionization Energy

The energy required to remove the outermost electron from a gas-phase atom, which generally increases across a period and decreases down a group due to increased distance and electron shielding.

Electronegativity

An atom's tendency to attract electrons toward itself within a chemical bond, which generally increases across a period and decreases down a group.