Ch. 8 Bonding: General Concepts

chemical bond - force that holds groups of atoms together group function as a unit

bond NRG – NRG required to break bond indicates strength of bond

bond length – distance between two atoms bonded together indicates most stable(least amount of NRG)

state between two atoms

ionic bond – electromagnetic force that holds two oppositely charged ions together formed between cations and anions cations formed when metals lose e-‘s

▪ Na Na+ + e- (oxidation) anions formed when nonmetals gain e-‘s

▪ Cl + e- Cl- (reduction) opposite charges attract(electromagnetic force)

▪ Na+ + Cl- Na+Cl-

why do substances form ionic bonds??? lowest possible NRG for the

system(substances involved with bonding) see figure 8.9 on page 355

1. change Li(s) to Li(g) endothermic

Li(s) + 161 kJ/mol Li(g)

2. Li(g) oxidizes endothermic

Li(g) + 520 kJ/mol Li+ + e-

3. fluorine molecules separate and form fluorine atoms endothermic

½ F2(g) + 77 kJ/mol F(g)

4. fluorine reduces exothermicF(g) + e- F-(g) + 328 kJ/mol

5. ionic bond formed extremely exothermicLi+(g) + F-(g) Li+F- + 1047 kJ/mol

total endothermic processes = 758 kJ/moltotal exothermic processes = 1375 kJ/mol

NET NRG = 617 kJ/mol overall process is exothermic so an ionic bond

forms

metals lose e- (oxidation) nonmetals gain e- (reduction)

ionic compound formula total oxidation = total reduction

Mg2+ F- MgF2 utilize criss-cross method

Mg2+ F- MgF2

dissolving ionic crystals

covalent bond – force of attraction between 2 atoms when e-‘s are shared each nucleus attracts the other atoms e-‘s balance between attraction and repulsion

bonds result from system trying to attain lowest possible NRG state two driving forces in nature

1) lowest NRG2) highest entropy(disorder/chaos)

ionic and covalent bonds generally form to attain the lowest NRG state for atoms involved

single covalent bond – 1 pair e- shared H-H, F-F, Cl-Cl, Br-Br, I-I

double covalent bond – 2 pair e- shared O=O

triple covalent bond – 3 pair e- shared N≡N

molecular orbital most probable location of e-‘s when covalently

bonded

sigma () bond centers along the internuclear axis. single covalent bond

pi () bond occupies space above and below internuclear axis.

2nd or 3 covalent bond

electronegativity attraction an atom has for another atom’s e- ‘s arbitrary scale – based on F varies periodically (click here)

▪ generally increases across and decreases down

electronegativity difference can generally predict type of bond if diff. > 1.7 = ionic bond if 1.7 > diff. > 0.3 = polar covalent bond if diff. < 0.3 = nonpolar covalent bond

nonpolar covalent bond pure covalent bond sharing of e- ‘s is equal no poles/charges created

• Lewis structures localized e- model for diagramming bonds

and molecular shapes

1) total valence e- of all atoms in molecule• HCl = 8 valence e-

2) write symbols for each element• least number of = interior atom• hydrogen is always exterior atom• H Cl

3) add a pair of e- between atoms bonding together

• H : Cl valence e- remaining = 6

4) add remaining e- in pairs to exterior atoms to form octets

• if e- remain add them to interior atoms to form octets

• not H, H forms duets

• if necessary, move e- pairs to create octets

• H : Cl

5) all shared pair of e- become dashes

• H - Cl

::

:

:

::

6) follow VSEPR for shape• VSEPR – valence shell electron pair

repulsion theory• model used to predict geometry/shape of a

molecule based on the repulsion of e- pairs• e- pairs repel each other to maximum distance

• in 2-dimension = 90o

• in 3-dimension = 109.5o

Molecular Geometry• 3-dimensional shapes determine the

physical and chemical properties of molecules• example – sucrose- its molecular shape fits the

nerve receptors of the tongue for sweetness• sugar substitutes(Splenda, Nutrasweet, …) have similar

shapes as sucrose

1) linear• all atoms lie in a straight line• HCl, CO2

2) bent• atoms not in straight line• e- pairs point to 4 corners of tetrahedron

3) trigonal pyramidal• 3 atoms bonded to central atom and a pair

of nonbonding e-

4) trigonal planar• 3 groupings of e- around central atom• all atoms lie in same plane

5) tetrahedral• 4 groupings of e- around central atom

resonance ability to draw more than one acceptable

shape for a molecule originally believed molecule resonated

between different shapes▪ benzene, nitrate ion

actual structure is an average of all resonant images

exceptions to octet rule C, N, O, F always follow octet rule

Be and B often have less than 8

3rd period and heavier usually follow▪ some may exceed by putting e- in unoccupied d-

orbitals▪ when writing Lewis structures follow octet rule

▪ if e- remain add them to elements with d orbitals

• polar molecules• molecule with oppositely, partially charged

atoms on opposite sides• aka – dipoles, dipole moments

• molecule that has an asymmetrical distribution of charge

• partial charges not evenly distributed around central atom

• polar molecules must have:• polar bonds(partial charges)• unevenly distributed partial charges

• HCl• H = 2.1, Cl = 3.0• (electroneg diff = 0.9) = bond is polar• H = δ+, Cl = δ-• linear shape• polar molecule

• N2 • N = 3.0• (electroneg diff = 0) = bond is nonpolar• nonpolar molecule

• H2O• H = 2.1, O = 3.5• (electroneg diff = 1.4) = bonds are polar• H = δ+, O = δ-• bent shape• polar molecule

• NH3

• N = 3.0, H = 2.1• (electroneg diff = 0.9) = bonds are polar• H = δ+, N = δ-• trigonal pyramidal shape• polar molecule

• CCl4• C = 2.5, Cl = 3.0• (electroneg diff = 0.5) = bonds are polar• C = δ+, Cl = δ-• tetrahedral shape• nonpolar molecule

hybridization formation of hybrid orbitals from atomic orbitals of

similar NRG sp3 hybridization

sp2 hybridization

sp hybridization

naming binary molecules1) determine # of 1st element

▪ use prefix if more than one▪ 1=mono- 2=di-▪ 3=tri- 4=tetra-▪ 5=penta- 6=hexa-▪ 7=hepta- 8=octa-▪ 9=nona- 10=deca-

2) name element

3) determine # of 2nd element▪ use prefix except if bonded to H

4) use root of element name

5) end with -ide

polar covalent bond bond in which e- ‘s are shared unequally

▪ electronegativity of one atom is higher than other