Chapter 4 - Covalent Compounds

Chapter Four: Covalent Compounds Introduction to Covalent Bonding • covalent bonds result from the sharing of electrons between two atoms • a covalent bond is a two-electron bond in which the bonding atoms share the two electrons • a molecule is a discrete group of atoms held together by covalent bonds (e.g. H2 molecule) • unshared electron pairs are called the non-bonded electron pairs or lone pairs • atoms share electrons to attain the electron configuration of the noble gas closest to them in the periodic table Covalent Bonding and the Periodic Table lewis structures are electron-dot structures for molecules; they show the location of all valence electrons. covalent bonds are formed when two nonmetals combine or when a metalloid bonds to a nonmetal. • hydrogen forms one bond with its one valence electron • atoms with one, two, or three valence electrons form one, two, or three bonds, respectively • atoms with four or more valence electrons form enough bonds to give an octet; for these atoms, the following formula is used: covalent bonds of common elements: Focus on the Human Body there are many covalent compounds related to the chemistry of the heart Lewis Structures • a molecular formula shows the number and identity of all of the atoms in a compound, but not which atoms are bonded to each other • a lewis structure shows the connectivity between atoms, as well as the location of all bonding and non-bonding valence electrons Drawing Lewis Structures general rules for drawing lewis structures: • draw only valence electrons • give every main group element (except H) an octet of electrons • give each hydrogen two electrons Electron-Dot Symbols • dots representing valence electrons are placed on the four sides of an element symbol • each dot represents one valence electron • for 1-4 valence electrons, single dots are used; with more than 4 electrons, the dots are paired • examples: Molecules With Multiple Bonds Exceptions to the Octet Rule • most of the common elements generally follow the octet rule • H is a notable exception because it only needs two electrons in bonding • elements in group 3A do not have enough valence electrons to form an octet in a neutral molecule (e.g. in BF3, B only has six electrons) • elements in the third row have empty d orbitals available to accept electrons • thus, elements such as P and S may have more than eight electrons around them Drawing Lewis Structures for Polyatomic Ions • add one electron for each negative charge • subtract one electron for each positive charge Drawing Resonance Structures • resonance structures are two lewis structures having the same arrangement of atoms but a different arrangement of electrons • the true structure is a hybrid of the two resonance structures • resonance stabilizes a molecule by spreading out lone pairs and electron pairs in multiple bonds over a larger region of space • a molecule or ion that has two or more resonance structures is resonance-stabilized Focus on the Environment • resonance structures can be drawn for neutral molecules • ozone, O3, can be drawn as two resonance structures • ozone is formed in the upper atmosphere by the reaction of O2 and oxygen atoms • it acts as a shield that protects the earth’s surface from destructive Naming Covalent Compounds • name the first nonmetal by its element name and the second using the suffix “-ide” • add prefixes to show the number of atoms of each element • the prefix “mono-“ is usually omitted when only one atom of the first element is present, but it is retained for the second element • if the combination would place two vowels next to each other, emit the first vowel (e.g. mono + oxide = monoxide) Molecular Shape • the lewis structure gives information about how the atoms are connected, but it implies nothing of the geometry or shape • to determine the shape around a given atom, first determine how many groups surround the atom • a group is either an atom or a lone pair of electrons • use the VESPR (valence shell electron pair repulsion) theory to determine the shape: electron pairs repel each other • the most stable arrangement keeps the groups as far away from each other as possible • in general, there are three possible arrangements of groups around an atom: Two Groups Around an Atom • any atom surrounded by only two groups is linear and has a bond angle of 180 degrees • ignore multiple bonds in predicting geometry; count only atoms and lone pairs Three Groups Around an Atom • any atom surrounded by three groups is trigonal planar and has bond angles of 120 degrees Four Groups Around an Atom • any atom surrounded by four groups is tetrahedral and has bond angles of 109.5 degrees • if the four groups around the atom include one lone pair, the geometry is a trigonal pyramid with bond angles of 107 degrees, close to 109.5 degrees Electronegativity and Bond Polarity • electronegativity is a measure of an atom’s attraction for electrons in a bond • it tells how much a particular atom “wants” electrons • if the electronegativities of two bonded atoms are equal or similar, the bond is nonpolar • the electrons in the bond are being shared equally between the two atoms • bonding between atoms with different electronegativities yields a polar covalent bond or dipole, a partial separation of charge • the electrons are unequally shared between the C and O • electrons are pulled toward O, the more electronegative element; this is indicated by the symbol δ - • electrons are pulled away from C, the less electronegative element; + this is indicated by the symbol δ Polarity of Molecules the classification of a molecule as polar or nonpolar depends on: • the polarity of the individual bonds • the overall shape of the molecule nonpolar molecules generally have: • no polar bonds • individual bond dipoles that cancel polar molecules generally have: • one or more polar bonds • individual bond dipoles that do not cancel to determine the polarity of a molecule with two or more polar bonds: • identify all polar bonds based on electronegativity differences • determine the shape around individual atoms by counting groups • decide if individual dipoles cancel or reinforce