CHM1045 CH.2 Terms: Atoms, Molecules, and Ions

Alpha particle (α particle)

A positively charged particle consisting of two protons and two neutrons.

Anion

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

Atomic mass

The average mass of atoms of an element, expressed in amu.

Atomic mass unit (amu)

Unit of mass equal to the mass of a carbon-12 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

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 given compound as the simplest whole-number ratio of atoms.

Fundamental unit of charge

(aka 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; atom with an unequal number of protons and electrons.

Isomers

Compounds with the same chemical formula but different structures.

Isotopes

Atoms of the same element with different masses; atoms that contain the same number of protons but different numbers of neutrons.

Law of constant composition

(aka law of definite proportions) 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)

The 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.