General Chemistry 1A Exam 2 Study Guide: Atomic Structure, Periodicity, Chemical Reactions, and Solution Chemistry

Atomic Structure and Electron Configuration

Valence Electrons and Core Electrons

Understanding the distinction between valence electrons and core electrons is essential for predicting chemical behavior and bonding.

• Valence electrons are the electrons in the outermost shell of an atom that are involved in chemical reactions and bonding.

• Core electrons are those found in inner shells; they are not typically involved in bonding.

• For main group elements, the number of valence electrons corresponds to the group number.

Example: Oxygen (O) has 6 valence electrons (Group 16).

Electron Configuration and Orbital Filling Order

Electron configurations describe the arrangement of electrons in an atom's orbitals, following specific rules:

• Aufbau Principle: Electrons fill the lowest energy orbitals first.

• Pauli Exclusion Principle: No two electrons in an atom can have the same set of quantum numbers.

• Hund's Rule: Electrons occupy degenerate orbitals singly before pairing.

Orbital filling order: $1s \rightarrow 2s \rightarrow 2p \rightarrow 3s \rightarrow 3p \rightarrow 4s \rightarrow 3d \rightarrow 4p$

Example: The electron configuration for sodium (Na): $1s^2 2s^2 2p^6 3s^1$

Electron Configurations for Ions

When atoms form ions, electrons are added or removed from the highest energy orbitals.

• Cations (positive ions): Electrons are removed from the outermost shell.

• Anions (negative ions): Electrons are added to the outermost shell.

Example: $\text{Na}^+$ loses one electron: $1s^2 2s^2 2p^6$

Periodic Properties of the Elements

Trends in Atomic Radius, Ionization Energy, and Electron Affinity

Periodic trends help predict element properties and reactivity.

• Atomic radius decreases across a period and increases down a group.

• Ionization energy increases across a period and decreases down a group.

• Electron affinity generally becomes more negative across a period.

Example: Fluorine has a higher ionization energy than sodium.

Shielding and Effective Nuclear Charge

Inner electrons shield outer electrons from the full charge of the nucleus, affecting atomic size and ionization energy.

• Shielding increases with more inner electrons.

• Effective nuclear charge (Zeff) is the net positive charge experienced by valence electrons.

Chemical Bonding and Lewis Structures

Lewis Dot Structures

Lewis structures represent valence electrons as dots around element symbols, helping visualize bonding and lone pairs.

• Count valence electrons for each atom.

• Arrange atoms to satisfy the octet rule (or duet for hydrogen).

Example: Water (H2O): Oxygen has two lone pairs and forms two single bonds with hydrogen.

Chemical Reactions and Stoichiometry

Types of Chemical Reactions

Chemical reactions can be classified by the changes in chemical bonds and the transfer of electrons.

• Acid-base reactions: Involve transfer of protons (H+).

• Gas evolution reactions: Produce a gas as a product.

• Precipitation reactions: Form an insoluble solid (precipitate).

• Redox reactions: Involve transfer of electrons; oxidation and reduction occur.

Balancing Chemical Equations

Balanced equations have equal numbers of each atom on both sides.

• Adjust coefficients to balance atoms.

• Check charge balance for ionic equations.

Example: $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$

Net Ionic Equations and Spectator Ions

Net ionic equations show only the species that participate in the reaction, omitting spectator ions.

• Spectator ions do not change during the reaction.

• Write the full ionic equation, then remove spectator ions to get the net ionic equation.

Solution Chemistry

Molarity and Solution Calculations

Molarity (M) is a measure of concentration, defined as moles of solute per liter of solution.

• $M = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Use molarity in dilution calculations: $M_1V_1 = M_2V_2$

Example: To prepare 0.5 L of 1.0 M NaCl, dissolve 0.5 mol NaCl in 0.5 L water.

Conductivity and Measurement

Conductivity measures a solution's ability to conduct electricity, which depends on the presence of ions.

• Strong electrolytes dissociate completely in water.

• Weak electrolytes partially dissociate.

• Measured using a Hertz meter.

Thermochemistry

Types of Energy

Energy in chemical systems can be kinetic, thermal, potential, or chemical.

• Kinetic energy: Energy of motion.

• Thermal energy: Related to temperature.

• Chemical energy: Stored in chemical bonds.

Endothermic and Exothermic Reactions

Reactions can absorb or release energy.

• Endothermic: Energy is absorbed; $\Delta H > 0$

• Exothermic: Energy is released; $\Delta H < 0$

Example: Melting ice is endothermic; combustion is exothermic.

Heat Capacity Calculations

Heat capacity is the amount of heat required to change a substance's temperature by one degree.

• $q = m c \Delta T$

• Where $q$ is heat, $m$ is mass, $c$ is specific heat, and $\Delta T$ is temperature change.

Gas Laws and Calculations

Major Gas Laws

Gas behavior is described by several laws:

• Boyle's Law: $P_1V_1 = P_2V_2$ (at constant T)

• Charles's Law: $\frac{V_1}{T_1} = \frac{V_2}{T_2}$ (at constant P)

• Ideal Gas Law: $PV = nRT$

Example: Calculate the volume of 1 mol of gas at STP: $V = \frac{nRT}{P}$

Acid-Base and Redox Chemistry

Acid-Base Reactions

Acid-base reactions involve the transfer of protons.

• Acid: Proton donor

• Base: Proton acceptor

Example: $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$

Redox Reactions and Oxidation States

Redox reactions involve electron transfer. Assigning oxidation states helps identify oxidizing and reducing agents.

• Oxidation: Loss of electrons

• Reduction: Gain of electrons

• Oxidizing agent is reduced; reducing agent is oxidized.

Example: In $2\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl}$, Na is oxidized, Cl2 is reduced.

Assigning Oxidation States

• Elemental form: Oxidation state is 0.

• Monatomic ion: Oxidation state equals the charge.

• Oxygen is usually -2, hydrogen is +1.

Additional Topics

Frequency, Wavelength, and Electromagnetic Spectrum

Light and electromagnetic waves are characterized by frequency and wavelength.

• High frequency = short wavelength

• Low frequency = long wavelength

• $c = \lambda \nu$ (speed of light = wavelength × frequency)

Double-Slit Experiment

The double-slit experiment demonstrates the wave-particle duality of light and electrons.

• Shows interference patterns, evidence for wave nature.

Calculations Involving Solutions

• Molarity: $M = \frac{\text{mol}}{\text{L}}$

• Dilution: $M_1V_1 = M_2V_2$

Summary Table: Types of Chemical Reactions

Type	Description	Example
Acid-Base	Transfer of H+ ions	$\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$
Precipitation	Formation of insoluble solid	$\text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} + \text{NaNO}_3$
Gas Evolution	Formation of a gas	$\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{CO}_2 + \text{H}_2\text{O}$
Redox	Transfer of electrons	$2\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl}$

Additional info: Some content inferred and expanded for completeness, including definitions, examples, and equations for clarity and academic context.