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: MX 2 (e.g. MgCl 2, CaF 2 ) IA-VIA: M 2 O (e.g. H 2 O, H 2 S) 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 HH 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 H 2 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 HCl or H-Cl Notice that Cl has 6 other electrons that are not shared; these are nonbonding electrons.