Summarizing Atomic Structure and Chemical Formulas

Summary of lessons so far

Atomic Structure and Symbolism

An atom consists of a small, positively charged nucleus surrounded by electrons. The nucleus contains protons and neutrons; its diameter is about 100,000 times smaller than that of the atom. The mass of one atom is usually expressed in atomic mass units (amu), which is referred to as the atomic mass. An amu is defined as exactly \(\frac{1}{12}\) of the mass of a carbon-12 atom and is equal to 1.6605 × 10⁻²⁴ g.

Protons are relatively heavy particles with a charge of 1+ and a mass of 1.0073 amu. Neutrons are relatively heavy particles with no charge and a mass of 1.0087 amu. Electrons are light particles with a charge of 1− and a mass of 0.00055 amu. The number of protons in the nucleus is called the atomic number (Z) and is the property that defines an atom’s elemental identity. The sum of the numbers of protons and neutrons in the nucleus is called the mass number and, expressed in amu, is approximately equal to the mass of the atom. An atom is neutral when it contains equal numbers of electrons and protons.

Isotopes of an element are atoms with the same atomic number but different mass numbers; isotopes of an element, therefore, differ from each other only in the number of neutrons within the nucleus. When a naturally occurring element is composed of several isotopes, the atomic mass of the element represents the average of the masses of the isotopes involved. A chemical symbol identifies the atoms in a substance using symbols, which are one-, two-, or three-letter abbreviations for the atoms.

Chemical Formulas

A molecular formula uses chemical symbols and subscripts to indicate the exact numbers of different atoms in a molecule or compound. An empirical formula gives the simplest, whole-number ratio of atoms in a compound. A structural formula indicates the bonding arrangement of the atoms in the molecule. Ball-and-stick and space-filling models show the geometric arrangement of atoms in a molecule. Isomers are compounds with the same molecular formula but different arrangements of atoms.

Glossary of words

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

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

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.

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)