Compounds: How Elements Combine

Electron Shells and Energy Levels

Atoms contain electrons arranged in shells, each with a specific energy level. Understanding how these shells fill is crucial for predicting chemical behavior.

• Electron Shells: Electrons occupy discrete energy levels (shells) around the nucleus. Each shell is quantized, meaning it has a fixed energy.

• Order of Filling: Electrons fill shells in order of increasing atomic number, starting with hydrogen.

• Maximum Electrons per Shell:

  • n = 1 (1st period): Maximum 2 electrons

  • n = 2 (2nd period): Maximum 8 electrons

  • n = 3 (3rd period): Maximum 8 electrons (focus on these for now)

• Energy Levels: Each successive shell is at a higher energy level.

• Quantization: Energy levels are discrete, not continuous.

Periodic Table Groups and Valence Electrons

The periodic table is organized into groups (columns) where elements share similar valence electron configurations, leading to similar chemical properties.

• Valence Electrons: Electrons in the outermost shell; responsible for chemical interactions.

• Group Properties: Elements in the same group have similar reactivity due to their valence electrons.

The Octet Rule

The octet rule states that atoms tend to gain, lose, or share electrons to achieve eight electrons in their valence shell, mimicking the stable configuration of noble gases.

• Main Group Elements: Groups 1A to 8A (except H and He) follow the octet rule.

• Electron Loss: Groups 1A, 2A, 3A tend to lose electrons, forming cations (e.g., Na+, Mg2+).

• Has 1A=+1, 2A=+2, 3A=+3, 5A=-3, 4A=-2, 3A=-1

• Electron Gain/Sharing: Groups 4A–7A tend to gain or share electrons, forming anions or covalent bonds (e.g., O2−, N3−).

• Cations: Positively charged ions. usually metals

• Anions: Negatively charged ions. Usually non metals

• Transition Metals: Can form ions with multiple charges (e.g., Fe2+, Fe3+).

• Just ensure that they will equal out both sides

Polyatomic Ions

Polyatomic ions are groups of covalently bonded atoms that act as a single charged unit.

• Examples: Hydroxide (OH−), Hydronium (H3O+), Hydrogen(HCO3-), Ammonium (NH4+), Nitrate (NO3−), Chlorate (ClO3−), Carbonate (CO3^2−), Bicarbonate (HCO3−), Cyanide (CN−), Sulfate (SO4^2−), Phosphate (PO4^3−).

• Practice: Memorize names and formulas; use flashcards.

Naming Ions and Ionic Compounds

Combining cations and anions forms ionic compounds. The compound must be electrically neutral.

• Examples: Sodium chloride (NaCl), magnesium chloride (MgCl2), iron oxide (Fe2O3), copper(II) oxide (CuO).

• Roman Numerals: Used for metals with multiple ionic states (e.g., iron(II) vs. iron(III)).

• Formula Writing: Balance charges to achieve neutrality.

Covalent Bonds

Covalent bonds involve the sharing of electrons between nonmetal atoms to achieve an octet.

• Strength: Covalent bonds are strong and hold molecules together.

• Bond Types:

  • Single bond: 2 electrons shared

  • Double bond: 4 electrons shared

  • Triple bond: 6 electrons shared

• Bonding Patterns:

  • Carbon: 4 bonds

  • Nitrogen: 3 bonds (plus one lone pair)

  • Oxygen: 2 bonds (plus two lone pairs)

  • Sulfur: 2 bonds

  • Hydrogen and halogens: 1 bond

Electron Dot Symbols and Lewis Structures

Electron dot symbols (Lewis symbols) represent valence electrons around an atom. Lewis structures show how atoms share electrons in molecules.

• Practice: Draw electron dot symbols for H, C, N, O, F (or any halogen).

• Lewis Structures: Draw for methane (CH4), ammonia (NH3), ethylene (C2H4), tetrachloromethane (CCl4).

Molecular Shapes and VSEPR Model

The VSEPR (Valence Shell Electron Pair Repulsion) model predicts molecular shapes based on electron pair repulsion.

• Tetrahedral: Four single bonds, bond angles 109.5° (e.g., methane).

• Planar: One double bond, bond angles ~120° (e.g., ethylene).

• Linear: Two double bonds or a triple bond, bond angles 180° (e.g., CO2).

• Lone Pairs: Push covalent bonds closer together, reducing bond angles (e.g., water: 104.5°, ammonia: 107°).

Naming Covalent Compounds

Covalent compounds are named using Greek prefixes to indicate the number of each atom.

• Prefixes: mono-, di-, tri-, tetra-, penta-, hexa-, etc.

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

The Mole Concept

The mole is a counting unit in chemistry, representing 6.022 × 1023 entities (Avogadro's number).

• Definition: 1 mole = particles

• Mass Relationship: 1 mole of an element has a mass in grams equal to its atomic mass.

• Conversions:

  • Moles to atoms:

  • Moles to grams:

  • Grams to moles:

• (one mole of atoms of a certain element has a mass in grams equal to its atomic mass!)

• Formula Weight: Sum atomic masses for all atoms in a compound.

• Example: Glucose (C6H12O6): Calculate molar mass by adding atomic masses.

• Atomic Mass Unit: 1 amu = 1 Dalton (Da)

Electronegativity and Bond Polarity

Electronegativity is an atom's ability to attract electrons in a bond. The difference in electronegativity determines bond polarity.

• Key Values: F = 4.0 (most electronegative), O = 3.5, N = 3.0, C = 2.5, H = 2.1

• Bond Polarity:

  • Same atoms: Nonpolar

  • Difference < 0.4: Nonpolar

  • Difference 0.5–1.9: Polar

  • Difference ≥ 2.0: Ionic bond (electrons transferred)

Molecular Polarity

The polarity of a molecule depends on both the presence of polar bonds and the molecular shape.

• Hydrocarbons: Molecules of only C and H are nonpolar.

• Polar Molecules: Must have polar bonds that do not cancel out (e.g., H2O is polar, CO2 is nonpolar).

• Shape: Use VSEPR to determine if polar bonds cancel.

Key Tables

Several tables are referenced for memorization and practice. Here are reconstructed versions:

Table: Polyatomic Ions (Main Purpose: Memorization)

Table: Number of Bonds Formed by Common Elements (Main Purpose: Bonding Patterns)

Table: Greek Prefixes for Naming Covalent Compounds (Main Purpose: Naming)

Practice and Application

• Work through sample and practice problems in the textbook.

• Struggle with problems before consulting solutions to deepen understanding.

• Use flashcards for polyatomic ions and naming conventions.

Additional info: Academic context was added to clarify electron shell filling, the octet rule, VSEPR model, and the mole concept, as well as to reconstruct tables for polyatomic ions, bonding patterns, and naming prefixes.