CHAPTER 3: Part 2

Covalent Bonds

Electron Sharing

PERIODIC TABLE (1)

• The most powerful tool in chemistry

• Elements are in order of increasing Z

• Used to understand phys and chem properties of each element

• Used to predict undiscovered elements and their properties (Feb 2004)

• Electronic configurations of electrons in atoms explain periodicity

PERIODIC TABLE (2)

• Elements in the same column or group have the same number of valence electrons and have similar properties

• Main Groups: Roman numeral at top of column = # valence electrons in atom; note “A” in group number; 8 columns of Main Group elements

• Transition Metals: note “B” in group number; 10 columns

• Elements in the same row or period. As we move from left to right across the row, the Z (# valence electrons) increases by 1.

USING the PERIODIC TABLE

• All IA metal atoms form a diatomic cmp with a VIIA nonmetal atom: MX (e.g. NaCl, KBr, RbF)

• All IIA metal atoms form a triatomic cmp with a VIIA nonmetal atom: MX2 (e.g. MgCl2, CaF2)

• IA-VIA: M2O (e.g. H2O, H2S)

• What about IIA-VIA?

MOLECULAR BONDING (1)

• Chemical bonding involves electrostatic attractions between electrons (negative) and the nucleus (positive) in the atom.

• Electrons are shared between atoms in covalent bonds.

• Electrons are transferred between atoms in ionic bonds.

• Other types: metallic, hydrogen bonding

MOLECULAR BONDING (2)

• The driving force for chemical bond formation is the drive to minimize total energy; to seek the lowest energy level (like water on an incline).

• This minimum energy state is the most stable one.

• Recall that atoms with an octet of valence electrons are particularly stable. (H, He have a duet of electrons)

COVALENT BOND (1)

• Main group atoms will readily share or electrons to achieve an octet or duet.

• H has 1 valence electron (ve) and wants one more. This is possible by sharing one ve with another H atom to give H:H or H••H or H-H.

• The : or •• or - indicates a shared pair of ve's called the bonding pair. The ve's are the glue that hold the two nuclei together.

COVALENT BOND (2)

• This sharing of ve's to form the chemical bond is called a covalent bond.

• The formation of the H-H bond is so much lower in energy than two separate H atoms that hydrogen exists as H2 and not as H.

• He is unreactive (does not form chemical bonds) because it has a duet of ve's.

COVALENT BOND (3)

• Consider the bond between H• and the Group VIIA chlorine atom with its 7 ve's

• H and Cl each want one more ve to achieve a duet or octet; they can each gain this additional ve by sharing one ve to give the molecule H••Cl or H-Cl

• Notice that Cl has 6 other electrons that are not shared; these are nonbonding electrons.