Key Terms

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

cation

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

chemical symbol

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

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

ion

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

isomers

compounds with the same chemical formula but different structures

isotopes

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

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

mass number (A)

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

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.

neutron

uncharged, subatomic particle located in the nucleus

nucleus

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

proton

positively charged, subatomic particle located in the nucleus

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

unified atomic mass unit (u)

alternative unit equivalent to the atomic mass unit