7.1 Structure and Bonding in Main Group Compounds

Main group elements, found in groups 1, 2, and 13-18, have valence electrons in s and p orbitals. Their electronic configurations determine reactivity and bonding behavior, with trends in electronegativity, atomic radius, and ionization energy influencing compound formation.

Main group compounds exhibit various bonding types and structures. Ionic, covalent, and polar covalent bonds form based on electronegativity differences. VSEPR theory predicts molecular geometry, while polarity depends on bond types and overall structure.

Electronic Configurations and Periodic Trends

Main Group Elements and Their Valence Electrons

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• Main group elements are found in groups 1, 2, and 13-18 of the periodic table (alkali metals, alkaline earth metals, boron group, carbon group, pnictogens, chalcogens, halogens, and noble gases)
• They have valence electrons in s and p orbitals
• The number of valence electrons increases from left to right across a period (from 1 to 8) and determines the element's reactivity and bonding behavior

Trends in Electronegativity, Atomic Radius, and Ionization Energy

• Electronegativity increases from left to right across a period due to increasing effective nuclear charge and decreases down a group due to increasing atomic radius, influencing the type of bonding in main group compounds
• Atomic radius decreases from left to right across a period due to increasing effective nuclear charge and increases down a group due to the addition of electron shells
• Ionization energy increases from left to right across a period as valence electrons become more tightly held and decreases down a group as the valence electrons become farther from the nucleus

Bonding and Structures in Main Group Compounds

Types of Bonding in Main Group Compounds

• Main group elements can form ionic, covalent, or polar covalent bonds depending on the electronegativity difference between the bonded atoms
• Ionic bonds form between metals and nonmetals with a large electronegativity difference (sodium chloride, NaCl), resulting in the transfer of electrons and the formation of ions
  • Ionic compounds have high melting points, are brittle, and conduct electricity when molten or in aqueous solution
• Covalent bonds form between nonmetals with similar electronegativities (methane, CH4), resulting in the sharing of electrons
  • Covalent compounds have lower melting points, are often gases or liquids at room temperature, and do not conduct electricity
• Polar covalent bonds form between atoms with an intermediate electronegativity difference (hydrogen fluoride, HF), resulting in an unequal sharing of electrons and a dipole moment

Structures of Main Group Compounds

• Main group compounds can form various structures, including linear (carbon dioxide, CO2), bent (sulfur dioxide, SO2), trigonal planar (boron trifluoride, BF3), tetrahedral (methane, CH4), trigonal bipyramidal (phosphorus pentachloride, PCl5), and octahedral (sulfur hexafluoride, SF6) geometries
• The structure depends on the number of bonding and lone electron pairs around the central atom

Geometry and Polarity of Main Group Compounds

VSEPR Theory and Molecular Geometry

• VSEPR (Valence Shell Electron Pair Repulsion) theory predicts the geometry of molecules based on the number of electron domains (bonding and lone pairs) around the central atom
• Electron domains arrange themselves to minimize repulsion, leading to specific geometries:
  • 2 domains: linear (beryllium chloride, BeCl2)
  • 3 domains: trigonal planar (boron trifluoride, BF3)
  • 4 domains: tetrahedral (methane, CH4)
  • 5 domains: trigonal bipyramidal (phosphorus pentachloride, PCl5)
  • 6 domains: octahedral (sulfur hexafluoride, SF6)
• The presence of lone pairs can distort the geometry, leading to bent (water, H2O), trigonal pyramidal (ammonia, NH3), and seesaw (sulfur tetrafluoride, SF4) geometries

Polarity of Main Group Compounds

• Polarity is determined by the geometry and the presence of polar bonds
• Molecules with polar bonds and an asymmetric geometry will be polar (water, H2O)
• Molecules with polar bonds and a symmetric geometry will be nonpolar (carbon tetrachloride, CCl4)
• Nonpolar molecules have no net dipole moment, while polar molecules have a net dipole moment

Lone Pair Influence on Structure and Reactivity

Lone Pairs and Molecular Geometry

• Lone pairs are non-bonding valence electrons that occupy more space than bonding pairs, causing greater repulsion and distorting the geometry of the molecule
• The presence of lone pairs leads to decreased bond angles compared to the ideal geometry without lone pairs
  • Ammonia (NH3) has a trigonal pyramidal geometry with a bond angle of 107°, less than the ideal tetrahedral angle of 109.5°
  • Water (H2O) has a bent geometry with a bond angle of 104.5°, less than the ideal tetrahedral angle of 109.5°

Lone Pairs and Reactivity

• Lone pairs can increase the reactivity of main group compounds by serving as electron donors in chemical reactions
  • Compounds with lone pairs, such as ammonia and water, can act as Lewis bases and form coordinate covalent bonds with Lewis acids (boron trifluoride, BF3)
• The stereochemically active lone pair in compounds like ammonia and water can invert, leading to a rapid interconversion between enantiomers (nitrogen inversion in ammonia)