Ch 4

• Covalent bonds occur when elements share electrons
• The octet rule states that the noble gas configuration will be achieved if the Lewis structure shows eight valence electrons
• Hydrogen, Beryllium, and Boron are exceptions to the octet rule
• Magnetic properties of atoms, ions and molecules arise from unpaired electrons. The more unpaired electrons, the stronger the magnetic effect. Using the magnetic quantum number, the spin of electrons cancel when paired, which cancels the magnetic effect
• Substances that have unpaired electrons are called paramagnetic
• Substances that have all their electrons paried are called diamagnetic and have very small magnetic fields
• Electron-deficient molecules require less than a full octet of electrons
• Equally probable Lewis structures are called resonance structures
• Some compounds have an odd number of valence electrons. Unpaired valence electrons are called free radicals
• In an effort to pair up the single electron, free radicals may form dimers or pairs of molecules. For example, NO2 dimerizes to N2O4 in order to pair up the free radical
• The formal charge is the difference between the number of electrons in the Lewis structure to the actual number of valence electrons (period number)
• Formal charges predict which structure is most accurate. The structure with the most 0s and (-) charge on the most electronegative element will be most correct
• Resonance structures occur when there are multiple correct variations of a chemical's structure (even in regards to formal charge)
• The benzene ring is the most popular resonance structure in both the structural formula (lines) and abbreviated resonance structure (hexagon)
• Aromatic molecules contain one or more benzene rings
• Linus Pauling developed the concept of electronegativity
• Covalent bond polarity is dependant on the difference in electronegativities between the two elements
• Covalent bond polarity and electronegativity are diagonal trends on the periodic table
• Elements closest to fluorine will be most electronegative
• The dipole moment is a measure of the difference in charge of two covalently bonded atoms and the distance between them.

  Diploe movement = q x r
• EN = 0 is nonpolar covalent
• 1.7 > EN > 0 is polar covalent
• EN = 1.7 is 50% covalent and 50% ionic
• EN > 1.7 is ionic
• A double bond is twice as strong as a single bond, but much shorter
• Bond energies are measured by infrared spectroscopy and combustion
• Bond length is measured by X-ray crystallography and infrared spectroscopy
• In resonance structures, all bonds are not 100% single or double bonds, but rather a mix of all as they have the same bond length
• The greater the bond order, the shorter the bond length
• Nomenclature (naming) of covalent molecules use prefixes
• VSEPR: valence shell electron pair repulsion theory
• Bonding domain: bonding electrons
• Nonbonding domain: lone pairs
• AXE system: A represents the central atom, X represents bonding electrons, E represents nonbonding electrons
• A molecule that is symmetrical is nonpolar
• A nonsymmetrical molecule is polar only if the bonds are polar
• A molecule with more than one type of peripheral atom will most likely be asymmetrical, therefore polar
• A central atom with lone pairs is often nonsymmetrical and polar
• Valence Bond Theory (VB Theory) suggests that atoms approach and interact with an overlap of atomic orbitals with electrons of opposing spin laying on the internuclear axis
• Molecular orbital theory (MO Theory) suggests that a molecule is similar to an atom in that they both have distinct energy levels that can be populated by electorns