Ch 12 Chemical Bonding

1. Ch 12 Chemical Bonding Bonds formed betw atoms depend on e- config & attraction the atoms have for e-’s - varies systematically

2. 12.1 Electronegativity • - the relative tendency of an atom to attract e-’s to itself when it’s bonded to another atom • Values assigned to elems on the basis of experiments

3. 12.1 Electronegativity • Electroneg diff betw atoms incr as bond strength incr • Electroneg.’s are influenced by same factors that affect ionization energies & e- affinities • Follow the same trends • Fr – lowest electroneg, F – highest • \ in bonds w/ F, bonding e-’s are drawn closer to F than the other atom

4. 12.1 Electronegativity • In a rxnbetw 2 elems, relative attraction for e-’s determines how they react • Electroneg scale determines this attraction

5. 12.2 Bond Character • If electroneg diff is high, atoms transfer e-’s to form ionic bonds • Held together by electrostatic (+ & - ) attraction • If electroneg diff is small, atoms share e-’s to form covalent bonds. • Amt of transfer depends on electroneg diff betw 2 atoms • If electroneg diff < 1.67, for covalent bonds • If electroneg diff > or = 1.67, form ionic bonds • All bonds have some charact. of both ionic & covalent

6. 12.3 Ionic Bonds • When e-’s are tranferred from 1 atom to the other, (+) & (-) ions are formed • When brought close together, the attractive force betw them holds the ions together • Ionic Bond – the electrostatic force that holds 2 ions together due to differing charges

7. 12.3 Ionic Bonds • Ionic comps have high melting pts, can conduct electricity in molten state, are usually soluble in water, & usually crystallize as sharply defined particles.

8. 12.4 Ionic Radii • - found from experimental data & simplifying assumptions • Internuclear Distance – in a crystal – found by adding the radii of 2 ions in a comp • Can also be used as bond length in a molec • Or bond axis • Radii are not fixed values • 1. fuzziness of e- cloud • 2. effect ea ion has on neighboring ions

9. 12.5 Covalent Bonds • - shared pair of e-’s • Covalent comps have low melting pts, do not conduct electricity, & are brittle • Molecule – particle resulting from 2 or more atoms bonding covalently • Bond Axis – line joining the nuclei of 2 bonded atoms in a molecule • Bond Angle – angle betw 2 bond axes in a molec.

10. 12.5 Covalent Bonds • Bond Length – distance betw nuclei along bond axis • Not fixed – bond acts like a stiff spring • Atoms vibrate as though bonds were strectching & shrinking • Bonds also undergo bending, wagging, & rotational vibrations • All cause bond angles & lengths to vary • \ bond lengths & angles are average values

11. 12.5 Covalent Bonds • Chemists use infra-red spectroscopy to determine molecular structure & vibrations of molecules. • Using IR spectrum, can compute bond strengths & determine much about how & where specific atoms are bonded in a molec. • Microwave radiation affects rotation of molecs • Can also determine bond lengths & bond angles

12. 12.6 Covalent Radii • It’s possible to determine internucleardistbetw 2 bonded atoms • For IClit is 230 • Cl2 is 199, I2 is 266 – ½ of ea of these may be used for the radii of ea atom & their sums are the bond dist of ICl. • Covalent radii are only approximate • Useful in predicting bond lengths in molecs

13. 12.7 Polyatomic Ions • - consist of 2 or more atoms covalently bonded together and possessing an overall charge • Form ionic bonds like other ions • Most are negative (exception – NH4)

14. 12.8 Van der Waals Radii • There is a certain minimum distance maintained betw atoms which are not bonded to ea other • E- clouds of ea atom repel ea other • When free atoms or molecs collide, they act as if they had a rigid outer “shell”. • The “shell” limits how close other atoms or molecs may come • Bonded atoms are closer together than unbonded atoms.

15. 12.8 Van der Waals Radii • The radius of this imaginary rigid shell is called Van der Waals Radius • The minimum dist maintained betwnonbonded atoms or atoms on adjacent molecs • Larger than covalent radius of bonded atoms

16. 12.9 Summary of Radii • Studied 4 types of radii: • Atomic • Ionic • Covalent • Van der Waals

17. 12.9 Summary of Radii • Atomic Radii – measured in 2 ways • 1. Meas on individual atoms in gaseous state • Atoms unaffected by neighboring atoms • 2. Meas on atoms in metallic crystals • Neighboring atoms have large effect • These 2 methods won’t give the same results

18. 12.9 Summary of Radii • Ionic Radii – differ from atomic radii bec of loss or gain of e-’s

19. 12.9 Summary of Radii • Covalent & Van der Waals Radii – vary greatly bec of wide range of atoms which can be bonded together • Covalent radii is expected to be less than atomic radii • When an atom is bonded to more than 1 other atom, e- cloud may be distorted • May make covalent radius larger than atomic radius • Same thing happens w/ Van der Waals radii • Radii are used to predict internucleardistbetw atoms.

20. 12.10 Special Properties of Metals • Metals form crystals in which ea metal atom is surrounded by 8 or 12 neighboring metal atoms • Form when atoms crowd together and outer level orbitals overlap • E-’s can easily move from 1 atom to another • These e-’s are called delocalized electrons. • Not held in 1 place – “float” around the crystal

21. 12.10 Special Properties of Metals • If an electric field is applied, e-’s will flow thru the metal, creating electric current • Delocalized e-’s interact w/ light creating luster. • Metalic Bond – constituted by the delocalized e-’s holding metallic atoms together

22. 12.10 Special Properties of Metals • Properties of metals are determined by the # of outer e-’s available • Group 1 - 1 e- available - soft • Group 2 - 2 e-’s available - harder • Transition metals – d e-’s take part in metallic bond • Many are very hard & stron • Groups 3-6 – 3 to 6 delocalized e-’s • Groups 7-10 - 6 delocalized e-’s • Not all d e-’s are involved in the bond

23. 12.10 Special Properties of Metals • It is possible to strengthen some metals by combining them w/ others to form alloys.