Chemistry Unit 2 Key Equations and Key Terms

Average mass of atoms

Σ(Fractional Abundance x Isotopic Mass)

Acids

Subsances that produce H3O+ when dissolved in water

Alpha (α) particle

Positively charged particle consisting of two protons and two neutrons

Anion

A negatively charged ion

Atomic mass

Average mass of atoms of an element, expressed in atomic mass units (amu)

Atomic number

The number of protons in the nucleus of an atom, represented by Z

Atomic theory

The theory that all matter is composed of minute particles called atoms

Avogadro’s number

Defined value of the number of entities comprising 1 mole of substance, equal to 6.022 × 10²³ per mole

Ball-and-stick model

A representation that shows the geometric arrangement of the atoms of a given substance with atomic sizes not to scale

Binary acid

Compound containing two different elements

Bionary ionic compound

Ionic compound consisting of a monatomic cation and a monatomic anion

Cation

A positively charged ion

Constant composition

A pure substance always contains the same elements in the same proportions

Coordination compounds

Substances containing one or more metal complexes

Covalent compound

Molecule formed by covalent bonds, which are chemical bonds that share rather than transfer electrons

Covalent substance

Substance whose molecules are formed by covalent bonds, which are chemical bonds that share rather than transfer electrons

Diatomic molecules

Molecules consisting of only two atoms

Electron

A negatively charged subatomic particle

Empirical formula

Formula showing the simplest whole-number ratio of atoms present in a compound

Fixed mass

The ratio of the element's masses within a molecule does not change—it is constant

Formula mass

Sum of the average masses for all atoms represented in a chemical formula; for covalent compounds, this is also the molecular mass

Hydrate

Compound containing one or more water molecules bound within its crystals

Inner transition metals

Elements in the bottom two rows. Those in the first row are also called lanthanides, and those in the second row are also called actinides

Inorganic molecular compound

A chemical compound that has no carbon–hydrogen bonds

Ion

An atom that has gained or lost electrons and therefore is electrically charged

Ionic compound

Compound held together by ionic bonds

Isomers

Molecules that have identical molecular formulas but different arrangements of atoms

Isotopes

Atoms of the same element that have different masses due to different numbers of neutrons

Law of definite proportions

Also called the law of constant composition, this law states that all samples of a pure compound contain the same elements in the same proportions by mass

Law of multiple proportions

When two elements react to form more than one compound, a fixed mass of one element will react with masses of the other element in a ratio of small whole numbers

Main group elements

Elements in groups 1, 2, and 13 through 18 (also called representative elements)

Mass number

The total number of protons and neutrons in an atom, represented by A

Mass ratio

The proportion of the masses of each element within a molecule

Mass spectrometry

A method used to analyze and identify the chemical composition of a substance by separating gaseous ions according to their differing mass and charge

Molar mass

Mass in grams of 1 mole of a substance

Mole

Amount of substance containing the same number of atoms, molecules, ions, or other entities as the number of atoms in exactly 12 grams of C-12

Molecular formula

A representation of a molecule that uses chemical symbols to indicate the types of atoms followed by subscripts to show the actual number of atoms of each type in the molecule

Monatomic

Formed from one atom

Natural abundance

Natural abundance is the average amount of an isotope that naturally occurs on Earth

Neutron

Uncharged, subatomic particle located in the nucleus

Nucleus

Massive, positively charged center of an atom made up of protons and neutrons

Organic compounds

Chemical compounds that contain carbon–hydrogen or carbon–carbon bonds

Oxyacids

Compounds that contain hydrogen, oxygen, and one other element, bonded in a way that imparts acidic properties to the compound (the ability to release H⁺ ions when dissolved in water)

Oxyanion

Polyatomic ion that contains one or more oxygen atoms

Periodic law

The properties of the elements are periodic functions of their atomic numbers

Polyatomic ion

Ion composed of more than one atom

Postulate

A hypothesis that is accepted as true for the purposes of following a particular line of reasoning

Proton

Positively charged subatomic particle located in the nucleus of the atom

Space-filling model

A representation that shows the relative sizes of the atoms of a given substance

Structural formula

A representation of a molecule that gives the same information as its molecular formula (the types and numbers of atoms in the molecule) but also shows how the atoms are connected in the molecule

Transition metals

Element in groups 3 through 12 (more strictly defined, 3–11)