Covalent Bonding Chemistry Chapter 9

The Covalent Bond Covalent bond – chemical bond that results from the sharing of valence electrons Generally occurs when elements are relatively close to each other on the periodic table Majority form between nonmetallic elements Molecule – formed when 2 or more atoms bond covalently

Single covalent bonds (sigma bonds) Group 7A form a single covalent bond Ex: F2 Group 6A share 2 electrons to form 2 covalent bonds Ex: H2O Group 5A form 3 covalent bonds with atoms of nonmetals Ex: ammonia NH3 Group 4A form 4 covalent bonds Ex: methane CH4

Multiple covalent bonds Atoms of C, N, O, and S most often form multiple bonds O molecule shares 2 electron pairs, forming a double bond: O2 N molecule shares 3 electron pairs, forming a triple bond: N2 Pi bond – parallel orbitals overlap to share electrons Double covalent bond has 1 sigma and 1 pi bond

Strength of covalent bonds Bond length – distance between 2 bonding nuclei at the position of maximum attraction As the number of shared electron pairs increases, bond length decreases Shorter bond length makes stronger bond Bond dissociation energy – amount of energy required to break a specific covalent bond As 2 atoms are bonded closer together, more bond energy is needed to separate them

Naming Molecules Naming binary molecular compounds – prefixes indicate number of atoms Exception – first element never uses mono- Common names 1-mono 6-hexa 2-di 7-hepta 3-tri 8-octa 4-tetra 9-nona 5-penta 10-deca

Naming acids – if compound produces H+ ions in solution, it is an acid Binary acids – contains H and one other element Use prefix hydro-, root word of other element, suffix –ic, followed by acid Ex: HBr is hydrobromic acid Oxyacids – contains H and a oxyanion If anion suffix is –ate, it is replaced w/ -ic If anion suffix is –ite, it is replaced w/ -ous Ex: HNO3 is nitric acid HNO2 is nitrous acid Considered acids only when in solution (i.e., dissolved in water) Ex: HCl at STP is hydrogen chloride gas but in solution is hydrochloric acid

Molecular structures Structural formulas – letter symbols and bonds show relative positions of atoms H is always on the end Central atom is usually closer to the left on the periodic table If central atom is not surrounded by 4 e- pairs, it does not have an octet Must have double or triple bond Ex. problems 9-3 – 9-5

Exceptions to Octet Rule Resonance structures – more than one valid Lewis structure can be written; differ only in position of e- pairs and never atom positions Exceptions to Octet Rule Uneven number of valence electrons - some molecules have an odd number of valence electrons and can’t form an octet Ex: NO2 Suboctet - some compounds form w/ fewer than 8 electrons present Reactive and can share an entire pair of electrons donated by another atom Coordinate covalent bond – when one atom donates a pair of electrons to be shared w/ an atom or ion that needs 2 electrons to become stable Ex: BH3 Expanded octet - some compounds have central atoms that contain more than 8 valence electrons Ex: PCl5

Molecular shape Valence Shell Electron Pair Repulsion (VSEPR) model – used to determine molecular shape Based on arrangement that minimizes repulsion of shared and unshared pairs of electrons around the central atom Bond angle – angle formed by any 2 terminal atoms and the central atom Lone pairs of electrons are not shared between 2 nuclei, therefore they occupy a slightly larger orbital than shared electrons Shared bonding orbitals are pushed together slightly by lone pairs, therefore they occupy a slightly smaller orbital than lone pairs Table 9-3

Hybridization – atomic orbitals are mixed to form new, identical hybrid orbitals Hybrid orbitals – contain one electron that it can share w/ another atom Ex: methane CH4

Electronegativity and polarity Polar covalent bond – unequal sharing of electrons due to differences in electronegativity Ex: water H2O Microwave ovens Water molecules are constantly moving as they align themselves w/ the changing polarity of the microwave energy field How it Works, p. 270