Chapter 3: Atoms, Molecules, and Ions

Outline

• 3.1 Prelude to Atoms, Molecules, and Ions

• 3.2 Atomic Theory

• 3.3 Molecules and Chemical Nomenclature

• 3.4 Masses of Atoms and Molecules

• 3.5 Ions and Ionic Compounds

• 3.6 Acids – NOT COVERED

3.1 Prelude to Atoms, Molecules, and Ions

Historical Context and Measurement

The study of atoms, molecules, and ions is foundational to chemistry. Anders Jonas Ångström, a Swedish physicist, contributed to our understanding of atomic dimensions by studying the sun and its emissions. The size of atoms is measured in a unit called the angstrom (Å), where:

• 1 Å = cm

This unit is commonly used to express atomic and molecular dimensions.

3.2 Atomic Theory

Modern Atomic Theory

Atomic theory describes the nature and behavior of atoms, the smallest stable units of matter. Key principles include:

• Matter is composed of atoms.

• Atoms of the same element are identical.

• Atoms of different elements are distinct.

• Atoms combine in whole-number ratios to form compounds.

Subatomic Particles

Atoms are made up of three fundamental subatomic particles:

• Protons (p+): Positively charged particles located in the nucleus.

• Electrons (e-): Negatively charged particles found in the electron cloud surrounding the nucleus.

• Neutrons (n0): Neutral particles also located in the nucleus.

Overall, atoms are electrically neutral because the number of protons equals the number of electrons.

Structure of an Atom

• Nucleus: Contains protons and neutrons, accounting for nearly all the atom's mass.

• Electron Cloud: Region around the nucleus where electrons are found; most of the atom's volume is empty space.

The electron cloud is so named because electrons are constantly moving and difficult to pinpoint.

Atomic Mass and Units

• Atoms are weighed in atomic mass units (amu).

• 1 amu is defined as one-twelfth the mass of a carbon-12 atom.

• Proton and neutron each have a mass of approximately 1 amu.

• The mass of an electron is about 1/2000 that of a proton or neutron and is considered negligible (0 amu).

Atomic Number and Mass Number

• Atomic Number (Z): Number of protons in the nucleus; determines the element's identity.

• Mass Number (A): Sum of protons and neutrons in the nucleus.

For a neutral atom, the number of electrons equals the number of protons.

Isotopic Notation

Atoms can be represented using symbolic (isotopic) notation:

• where A is the mass number, Z is the atomic number, and X is the atomic symbol.

Isotopes

• Isotopes: Atoms of the same element with different numbers of neutrons (and thus different mass numbers).

• Isotopes are indicated by symbolic notation or by stating the mass number after the element name (e.g., carbon-12, oxygen-16).

3.3 Molecules and Chemical Nomenclature

Molecules and Elements

A molecule consists of two or more atoms bonded together. Some elements naturally exist as molecules (e.g., O2, N2).

Diatomic Molecules

• Diatomic molecules: Molecules composed of two atoms (e.g., H2, O2, N2, F2, Cl2, Br2, I2).

Binary Covalent Molecules

Binary covalent compounds are formed when two nonmetal atoms of different elements bond together.

Naming Binary Covalent Compounds

1. Name the first element using its elemental name.

2. Name the second element, changing its ending to -ide.

3. Indicate the number of each element present using Greek prefixes:

Examples: carbon dioxide (CO2), dinitrogen tetroxide (N2O4), phosphorus pentachloride (PCl5).

3.4 Masses of Atoms and Molecules

Molecular Mass

The molecular mass is the mass of one molecule, measured in atomic mass units (amu). It is calculated by summing the atomic masses of all atoms in the molecule:

Examples:

• Cl2: 208.22 amu

• S2: 76.13 amu

• N2O: 44.02 amu

• SiO2: 60.09 amu

• N2O5: 108.02 amu

• Br2: 159.8 amu

3.5 Ions and Ionic Compounds

Ions

• Ions: Atoms that have gained or lost electrons.

• Cation: Formed when an atom loses electrons; becomes positively charged.

• Anion: Formed when an atom gains electrons; becomes negatively charged.

Naming Ions

• For metal ions (cations), add the word "ion" to the metal name (e.g., sodium ion).

• For transition metals with multiple charges, use Roman numerals (e.g., iron(II) ion).

• For anions, replace the ending of the element name with -ide (e.g., chloride ion).

Ionic Compounds and Formulas

• Ionic compounds are formed by the combination of metal cations and nonmetal anions.

• The compound must have a net charge of zero; the sum of the charges of cations and anions equals zero.

• Subscripts in chemical formulas indicate the number of each ion present.

• Charges are not shown in the formula.

Examples of Ionic Compounds

• Sodium oxide: Na2O

• Barium chloride: BaCl2

• Copper(I) nitride: Cu3N

Naming Ionic Compounds

• Name the cation first, followed by the anion (e.g., lithium chloride, calcium bromide).

• For transition metals, include the Roman numeral to indicate charge (e.g., chromium(III) oxide).

Determining the Naming System

• First, determine if the compound is ionic (metal + nonmetal) or covalent (all nonmetals).

• For ionic compounds, check if a Roman numeral is needed for transition metals.

• For covalent compounds, use prefixes to indicate the number of atoms.

Examples:

• Calcium oxide (ionic)

• Magnesium bromide (ionic)

• Copper(II) sulfide (ionic, transition metal)

• Carbon dioxide (covalent)

• Sodium nitride (ionic)

Additional info: This guide omits acids as per the original outline. For further study, students should refer to the periodic table for atomic numbers and masses, and practice writing chemical formulas and names for both ionic and covalent compounds.