Unit 2 - Atoms and Elements Key Terms

Vocabulary flashcards covering key terms related to atoms, elements, and fundamental chemical laws from Unit 2 of the AP Chemistry curriculum.

Law of Conservation of Mass

States that matter is neither created nor destroyed in a chemical reaction; mass of reactants equals mass of products.

Law of Definite Proportions

States that all samples of a given chemical compound contain the same elements in the same proportions by mass.

Law of Multiple Proportions

States that when two elements form two or more compounds, the ratio of the masses of the second element that combine with a fixed mass of the first element is always a ratio of small whole numbers.

Atomic Theory

A scientific theory that describes matter as being composed of discrete units called atoms, often referring to John Dalton's postulates.

Cathode Ray

A beam of electrons observed in a vacuum tube.

Cathode Ray Tube

A vacuum tube that produces a visual image, historically used by J.J. Thomson to discover the electron.

Electrical Charge

A fundamental property of matter that describes how it interacts with an electromagnetic field; it can be positive or negative.

Electron

A stable, negatively charged subatomic particle found in all atoms.

Radioactivity

The process by which an unstable atomic nucleus loses energy by emitting radiation.

Nuclear Theory

Ernest Rutherford's model of the atom, proposing a small, dense, positively charged nucleus surrounded by negatively charged electrons.

Nucleus

The dense, positively charged center of an atom, containing protons and neutrons.

Proton

A subatomic particle with a positive charge, found in the nucleus of an atom.

Neutron

A subatomic particle with no electrical charge, found in the nucleus of an atom.

Atomic Mass Unit (amu)

A unit of mass used to express atomic and molecular masses, defined as 1/12 the mass of a carbon-12 atom.

Atomic Number

The number of protons found in the nucleus of an atom, defining the element.

Chemical Symbol

A one or two-letter abbreviation for a chemical element.

Isotope

Atoms of the same element that have the same number of protons but different numbers of neutrons.

Nuclear Symbol

A notation that represents an isotope, showing the chemical symbol, mass number (A), and atomic number (Z). Example: A/Z X.

Hyphen Notation

A notation that represents an isotope by writing the element name followed by a hyphen and the mass number (e.g., Carbon-12).

Natural Abundance

The average percentage of a particular isotope in a naturally occurring sample of an element.

Mass Number

The total number of protons and neutrons in an atomic nucleus.

Ions

An atom or molecule that has gained or lost one or more electrons, resulting in a net electrical charge.

Cation

A positively charged ion, formed by losing electrons.

Anion

A negatively charged ion, formed by gaining electrons.

Periodic Law

States that when elements are arranged in order of increasing atomic number, there is a periodic repetition of their chemical and physical properties.

Metal

Elements typically shiny, malleable, ductile, good conductors of heat and electricity, and tend to lose electrons.

Nonmetal

Elements that generally lack metallic properties, are poor conductors, and tend to gain or share electrons.

Metalloid

Elements that have properties intermediate between those of metals and nonmetals, sometimes called semiconductors.

Semiconductor

A material with electrical conductivity between that of a conductor and an insulator, often referring to metalloids.

Main-Group Elements

Elements in groups 1-2 and 13-18 of the periodic table.

Transition Elements (Transition Metals)

Elements in groups 3-12 of the periodic table.

Family (Group)

A vertical column of elements in the periodic table, sharing similar chemical properties.

Noble Gas

Elements in Group 18 of the periodic table, characterized by high stability and low reactivity due to a full valence shell.

Alkali Metals

Elements in Group 1 of the periodic table (excluding hydrogen), highly reactive metals.

Alkaline Earth Metals

Elements in Group 2 of the periodic table, reactive metals but less so than alkali metals.

Halogens

Elements in Group 17 of the periodic table, highly reactive nonmetals.

Atomic Mass

The weighted average mass of an atom of an element, calculated from the masses of its isotopes and their natural abundances.

Mass Spectrometry

An analytical technique that measures the mass-to-charge ratio of ions to determine isotopic masses and abundances.

Mole (mol)

The SI unit for amount of substance, equal to Avogadro's number of particles.

Avogadro

s Number

The number of constituent particles (atoms or molecules) per mole of a substance, approximately 6.022 x 10^23 particles/mol.

Molar Mass

The mass in grams of one mole of a substance.