Chapter 2 Student Notes

Atoms, Molecules, and Ions

Historical Atomic Theory

The development of atomic theory has shaped our understanding of matter. Early models and laws laid the foundation for modern chemistry.

• Dalton's Atomic Theory: Matter is composed of indivisible atoms; atoms of the same element are identical; atoms combine in simple whole-number ratios to form compounds.

• Law of Conservation of Mass: Mass is conserved in chemical reactions. $\text{Total mass before reaction} = \text{Total mass after reaction}$

• Law of Definite Proportions: A given compound always contains the same proportion of elements by mass.

• Law of Multiple Proportions: When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers.

Example: CO and CO2 both contain carbon and oxygen, but the ratio of oxygen to carbon differs by a small whole number.

Structure of the Atom

Atoms consist of subatomic particles: protons, neutrons, and electrons. The arrangement and number of these particles determine the properties of elements.

• Proton (p+): Positively charged, found in the nucleus, defines atomic number (Z).

• Neutron (n0): Neutral, found in the nucleus, contributes to atomic mass.

• Electron (e-): Negatively charged, found in electron cloud, determines chemical behavior.

Particle	Abbreviation	Mass (amu)	Charge
Proton	p+	1.0073	+1
Neutron	n0	1.0087	0
Electron	e-	0.00055	-1

Isotopes: Atoms of the same element with different numbers of neutrons. Example: $^{12}\text{C}$ and $^{13}\text{C}$ are isotopes of carbon.

Atomic Number and Mass Number

The atomic number (Z) is the number of protons in the nucleus. The mass number (A) is the sum of protons and neutrons.

• Atomic Number (Z): $Z = \text{number of protons}$

• Mass Number (A): $A = \text{number of protons} + \text{number of neutrons}$

Example: For $^{14}\text{C}$, $Z = 6$, $A = 14$.

Modern Atomic Structure

Most of the mass of an atom is concentrated in the nucleus, which is extremely dense. Electrons occupy most of the volume, creating an electron cloud.

• Size: Nucleus is tiny compared to the overall atom; most atomic volume is empty space.

• Electron Arrangement: Determines chemical properties and reactivity.

Periodic Table and Element Classification

The periodic table organizes elements by increasing atomic number and groups elements with similar properties.

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

• Periods: Horizontal rows; elements in the same period have the same number of electron shells.

Type	Properties
Metals	Conduct electricity, malleable, lose electrons
Nonmetals	Poor conductors, gain/share electrons
Metalloids	Intermediate properties

Example: Alkali metals (Group 1) are highly reactive and lose one electron to form +1 ions.

Mole Concept and Chemical Quantities

The mole is a fundamental unit for counting particles in chemistry. It allows conversion between mass, number of particles, and volume.

• Avogadro's Number: $6.022 \times 10^{23}$ particles per mole

• Molar Mass: Mass of one mole of a substance (g/mol)

Key Equations:

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

• $\text{Number of particles} = \text{moles} \times 6.022 \times 10^{23}$

Example: How many atoms are in 25 g of carbon? $\frac{25\,\text{g}}{12\,\text{g/mol}} \times 6.022 \times 10^{23} = 1.26 \times 10^{24}$ atoms

Types of Chemical Bonds

Chemical bonds hold atoms together in compounds. The main types are ionic, covalent, and metallic bonds.

• Ionic Bonds: Formed by transfer of electrons from metals to nonmetals; results in cations and anions.

• Covalent Bonds: Formed by sharing electrons between nonmetals; results in molecules.

• Metallic Bonds: Involves a 'sea' of delocalized electrons among metal atoms.

Bond Length: The distance between nuclei of bonded atoms; shorter bond length means stronger bond.

Classification of Compounds

Compounds are classified as molecular or ionic based on the types of elements involved and the nature of bonding.

• Molecular Compounds: Composed of nonmetals; discrete molecules; low melting points.

• Ionic Compounds: Composed of metals and nonmetals; form crystal lattices; high melting points; conduct electricity when dissolved.

Molecular Properties	Ionic Properties
Low melting/boiling points, non-electrolytes	High melting/boiling points, electrolytes
Liquids, gases, or solids	Usually solids

Naming Compounds

Systematic naming allows clear identification of chemical substances.

• Ionic Compounds: Name cation first, then anion. Use Roman numerals for transition metals with variable charge.

• Molecular Compounds: Use prefixes to indicate number of atoms (mono-, di-, tri-, etc.).

• Acids: Naming depends on the anion:

  • -ide: hydro-...-ic acid

  • -ate: ...-ic acid

  • -ite: ...-ous acid

Prefix	Number
Mono-	1
Di-	2
Tri-	3
Tetra-	4
Penta-	5
Hexa-	6
Hepta-	7
Octa-	8

Example: CO2 is carbon dioxide; N2O4 is dinitrogen tetroxide.

Polyatomic Ions

Polyatomic ions are charged species composed of two or more atoms covalently bonded.

Ion	Formula	Charge
Sulfate	SO42-	-2
Nitrate	NO3-	-1
Phosphate	PO43-	-3
Ammonium	NH4+	+1

Example: NaNO3 is sodium nitrate.

Isotopic Abundance and Atomic Mass

The atomic mass of an element is a weighted average of the masses of its naturally occurring isotopes.

• Calculation: $\text{Atomic mass} = \sum (\text{fractional abundance} \times \text{isotope mass})$

Example: For chlorine: $\text{Atomic mass} = (0.7577 \times 35) + (0.2423 \times 37) = 35.5$ amu

Counting Atoms, Molecules, and Ions

Stoichiometric calculations allow conversion between mass, moles, and number of particles.

• Steps:

  1. Convert mass to moles using molar mass.

  2. Convert moles to number of particles using Avogadro's number.

  3. For compounds, use chemical formula to determine number of atoms/ions per molecule.

Example: How many hydrogen atoms are in 59.2 g of H2O? $\frac{59.2\,\text{g}}{18.02\,\text{g/mol}} \times 2 \times 6.022 \times 10^{23} = 3.96 \times 10^{24}$ hydrogen atoms

Additional info:

• Notes include visual diagrams, worked examples, and practice problems for mastery.

• Periodic table trends, such as group and period properties, are highlighted for classification and prediction of element behavior.

• Practice problems reinforce concepts of atomic structure, chemical formulas, and stoichiometry.