Chemistry: Atoms First

# Key Terms

Chemistry: Atoms FirstKey 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 $112112$ 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 $××$ 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
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