Chapter 2: Atoms, Molecules, & Ions

Key terms for Chapter 2: Atoms, Molecules, & Ions (OpenStax).

**actinide**

inner transition metal in the bottom of the bottom two rows of the periodic table

**alkali metal**

element in group 1

**alkaline earth metal**

element in group 2

**alpha particle (α particle)**

\n positively charged particle consisting of two protons and two neutrons

**anion**

negatively charged atom or molecule (contains more electrons than protons)

**atomic mass**

average mass of atoms of an element, expressed in amu

**atomic mass unit (amu)**

(also, unified atomic mass unit, u, or Dalton, Da) unit of mass equal to 1/12 of the mass of a 12C atom

**atomic number (Z)**

number of protons in the nucleus of an atom

**binary acid**

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

**binary compound**

compound containing two different elements.

**cation**

positively charged atom or molecule (contains fewer electrons than protons)

**chalcogen**

element in group 16

**chemical symbol**

one-, two-, or three-letter abbreviation used to represent an element or its atoms

**covalent bond**

attractive force between the nuclei of a molecule’s atoms and pairs of electrons between the atoms

**covalent compound**

(also, molecular compound) composed of molecules formed by atoms of two or more different elements

**Dalton (Da)**

alternative unit equivalent to the atomic mass unit

**Dalton’s atomic theory**

set of postulates that established the fundamental properties of atoms

**electron**

negatively charged, subatomic particle of relatively low mass located outside the nucleus

**empirical formula**

formula showing the composition of a compound given as the simplest whole-number ratio of atoms

**fundamental unit of charge**

(also called the elementary charge) equals the magnitude of the charge of an electron (e) with e = 1.602 × 10^−19 C

**group**

vertical column of the periodic table

**halogen**

element in group 17

**hydrate**

compound containing one or more water molecules bound within its crystals

**inert gas**

(also, noble gas) element in group 18

**inner transition metal**

(also, lanthanide or actinide) element in the bottom two rows; if in the first row, also called lanthanide, or if in the second row, also called actinide

**ion**

electrically charged atom or molecule (contains unequal numbers of protons and electrons)

**ionic bond**

electrostatic forces of attraction between the oppositely charged ions of an ionic compound

**ionic compound**

compound composed of cations and anions combined in ratios, yielding an electrically neutral substance

**isomers**

compounds with the same chemical formula but different structures

**isotopes**

atoms that contain the same number of protons but different numbers of neutrons

**lanthanide**

inner transition metal in the top of the bottom two rows of the periodic table

**law of constant composition**

(also, law of definite proportions) all samples of a pure compound contain the same elements in the same proportions by mass

**law of definite proportions**

(also, law of constant composition) 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 element**

(also, representative element) element in groups 1, 2, and 13–18

**mass number (A)**

sum of the numbers of neutrons and protons in the nucleus of an atom

**metal**

element that is shiny, malleable, good conductor of heat and electricity

**metalloid**

element that conducts heat and electricity moderately well, and possesses some properties of metals and some properties of nonmetals

**molecular compound**

(also, covalent compound) composed of molecules formed by atoms of two or more different elements

**molecular formula**

formula indicating the composition of a molecule of a compound and giving the actual number of atoms of each element in a molecule of the compound.

**monatomic ion**

ion composed of a single atom

**neutron**

uncharged, subatomic particle located in the nucleus

**noble gas**

(also, inert gas) element in group 18

**nomenclature**

system of rules for naming objects of interest

**nonmetal**

element that appears dull, poor conductor of heat and electricity

**nucleus**

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

**oxyacid**

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

**oxyanion**

polyatomic anion composed of a central atom bonded to oxygen atoms

**period**

(also, series) horizontal row of the periodic table

**periodic law**

properties of the elements are periodic function of their atomic numbers.

**periodic table**

table of the elements that places elements with similar chemical properties close together

**pnictogen**

element in group 15

**polyatomic ion**

ion composed of more than one atom

**proton**

positively charged, subatomic particle located in the nucleus

**representative element**

(also, main-group element) element in columns 1, 2, and 12–18

**series**

(also, period) horizontal row of the period table

**spatial isomers**

compounds in which the relative orientations of the atoms in space differ

**structural formula**

shows the atoms in a molecule and how they are connected

**structural isomer**

one of two substances that have the same molecular formula but different physical and chemical properties because their atoms are bonded differently

**transition metal**

elements in groups 3–12

**unified atomic mass unit (u)**

alternative unit equivalent to the atomic mass unit