Textbook Ch.7: Covalent Bonding

Lewis suggested that nonmetal atoms, by sharing electrons to form an electron-pair bond, can acquire a stable noble-gas structure

electrons in the outermost principal energy level • Shown as dot symbols around the element

occur in pairs

a pair of electrons shared between two atoms • Shown by a straight line between bonded atoms in a Lewis structure

a pair of electrons owned entirely by one atom • Shown as a pair of dots on that atom in a Lewis structure

a single electron pair shared between two bonded atoms • 1 line between two atoms on a Lewis structure

when bonded atoms share two electron pairs • 2 lines between two atoms on a Lewis structure

when bonded atoms share three electron pairs • 3 lines between two atoms on a Lewis structure

atoms in covalently bonded species tend to have noble-gas electronic structures • Nonmetals (except H) achieve a noble-gas structure by sharing an octet of electrons (eight) • Hydrogen atoms, in molecules or polyatomic ions, are surrounded by a duet of electrons (two)

• Count the number of valence electrons. (the last digit of the group number) • Draw a skeleton structure for the species, joining atoms by single bonds - The central atom is usually the one written first in the formula, put this in the center. Terminal atoms are often H, O, or a halogen • Determine the number of valence electrons still available for distribution • Determine the number of valence electrons required to fill out an octet for each atom (except H)

distribute the available electrons as unshared pairs, satisfying the octet rule for each atom

must modify the skeletal structure by changing single bonds to multiple bonds

distribute leftover electrons around the central atom

C, N, O, and S

hydrogen and halogen atoms

two structures separated by a double-headed arrow when the single Lewis structure does not adequately reflect the properties of the substance • Ex: in SO₂, you don’t know which oxygen the sulfur will be double-bonded to, it could be double-bonded to either

the distribution of electrons, not n the arrangement of atoms

a way to choose the more plausible of two resonance structures. It is the difference between the number of valence electrons in the free atom and the number of valence electrons assigned to the atom in the Lewis structure. The assigned electrons include • All the unshared electrons owned by that atom • One half of the bonding electrons shared by that atom

Cբ = X-(Y + Z/2) X= number of valence electrons in the free atom in the Lewis structure Y=the number of bonding electrons shared by the atom in the Lewis structure Z=the number of bonding electrons shared by the atom in the Lewis structure

• The formal charges are as close to zero as possible - Any negative formal charge is located on the most strongly electronegative atom

• NO • NO₂ • Species in which the central atom is surrounded by more than 4 pairs of valence electrons

the angles between bonds. Must be considered when dealing with molecules with three or more atoms

a model in which the valence electron pairs surrounding an atom repel one another. Consequently the orbitals containing those electron pairs are oriented to be as far apart as possible

helps in determining the arrangement of various electron groups