General Chemistry I: Foundations, Atomic Theory, Periodic Table, and Moles

Properties and Changes of Matter

Classification of Matter

Matter is anything that has mass and occupies space. It can be classified by its state (solid, liquid, gas) and its properties.

• Physical Properties: Properties that can be measured or observed without changing the identity of the substance. Examples: Color, melting point, density.

• Chemical Properties: Properties that describe the way a substance may change or react to form other substances. Examples: Flammability, reactivity with acids.

Matter can also be classified as pure substances or mixtures:

• Pure Substance: Has a constant composition and a fixed set of physical and chemical properties. Examples: Water (H2O), sodium chloride (NaCl).

• Mixture: A combination of two or more substances in which each substance retains its own identity and properties. Mixtures can be physically separated into their components. Examples: Air, saltwater.

Modern Atomic Theory

Law of Conservation of Mass

The total mass of substances present before a chemical reaction is equal to the total mass after the reaction.

• Equation: $\text{Total mass before reaction} = \text{Total mass after reaction}$

Law of Definite Proportions

A chemical compound always contains exactly the same proportion of elements by mass.

• Example: Water (H2O) always contains 2 parts hydrogen to 16 parts oxygen by mass.

Dalton's Atomic Theory

John Dalton proposed a theory to explain the nature of matter:

1. All matter is composed of indivisible atoms. Atoms are extremely small particles that retain their identity during chemical reactions.

2. An element is a type of matter composed of only one kind of atom; all atoms of an element have the same properties.

3. A compound is a type of matter composed of atoms of two or more elements chemically combined in fixed proportions.

4. A chemical reaction consists of the rearrangement of atoms present in the reacting substances to form new chemical combinations in the products.

Modern View: Atoms are divisible into subatomic particles (protons, neutrons, electrons), but Dalton's theory laid the foundation for modern chemistry.

The Periodic Table

Organization and Patterns

The periodic table arranges elements by increasing atomic number and groups elements with similar chemical properties into columns (groups/families).

• Groups: Vertical columns; elements in the same group have similar properties.

• Periods: Horizontal rows; properties change progressively across a period.

• Metals, Nonmetals, Metalloids: Elements are classified based on their physical and chemical properties.

The periodic table allows prediction of element properties and chemical behavior.

Ions and the Periodic Table

Formation of Ions

Atoms can gain or lose electrons to form ions, achieving electron configurations similar to the nearest noble gas.

• Cations: Positively charged ions formed by loss of electrons (e.g., Na+).

• Anions: Negatively charged ions formed by gain of electrons (e.g., Cl-).

Elements tend to form ions that result in a stable electron configuration.

Masses and Moles

Atomic Mass Unit (amu)

The atomic mass unit is defined as one-twelfth the mass of a carbon-12 atom.

• Equation: $1\ \text{amu} = 1.66054 \times 10^{-27}\ \text{kg}$

• One carbon atom weighs 12 amu.

Molecular Mass and Formula Mass

• Molecular Mass: The sum of the atomic masses of all atoms in a molecule.

• Formula Mass: The sum of the atomic masses of all atoms in a formula unit of an ionic compound.

The Mole and Avogadro's Number

The mole is a unit for counting particles in chemistry. One mole contains Avogadro's number ($N_A$) of entities.

• Avogadro's Number: $N_A = 6.022 \times 10^{23}$ entities/mol

• Molar Mass ($M$): The mass of one mole of a substance (units: g/mol).

• Equation: $\text{Number of moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$

For many substances, the molar mass in grams per mole is numerically equal to the formula weight in atomic mass units.

Sample Calculations

• Calculating Mass from Moles: $\text{mass (g)} = \text{number of moles} \times \text{molar mass (g/mol)}$

• Calculating Number of Atoms: $\text{number of atoms} = \text{number of moles} \times N_A$

Example: To find the mass of 43.9 mol of Xe, multiply by the molar mass of Xe.

Summary Table: Key Terms and Definitions

Term	Definition	Example
Physical Property	Measured without changing substance identity	Melting point
Chemical Property	Describes substance's ability to change/react	Flammability
Pure Substance	Constant composition, fixed properties	Water (H2O)
Mixture	Combination of substances, variable composition	Air
Mole	6.022 × 1023 entities	1 mol H2O
Molar Mass	Mass of 1 mole (g/mol)	H2O: 18.02 g/mol

Additional info:

• Significant figures, precision, and accuracy are important in chemical measurements (see Section 1.4).

• Atomic structure includes electrons, protons, neutrons, mass number, and atomic number (see Section 2.4).