Key Terms

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)

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 $\frac{1}{12}$ 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 $\times$ 10⁻¹⁹ C

group

vertical column of the periodic table

halogen

element in group 17

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 columns 1, 2, and 12–18

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

element in columns 3–11

unified atomic mass unit (u)

alternative unit equivalent to the atomic mass unit