AS VII Periodic Trends

Transcription

1 AS VII Periodic Trends 7-1 Shielding recall: Z = nuclear charge = # of protons (p + s) (= atomic #) 7-2 Shielding & Repulsion; Z eff Trends: size ionization energy, IE electron affinity, EA magnetic properties chemical properties Ref 8: 4, 5 Prob 9: in-text: 1-4 end: 9-23, 31-35, 40, 43 Adv Rdg 2: 8 ; 3 - all new: Z eff = effective nuclear charge = nuclear charge as seen by outer e s, always < Z, b/c inner e s cancel (cover up) some of the pos. charge of nucleus, i.e., they shield shielding... generally, e s with lower n shield shielding... General Trends e s with same n don t shield (but there are subtleties, see later) L R in P.T. results in shielding Z eff crude illustration: 1) Li has 1s 2 2s 1 config n the two 1s e s cancel two of the p + s Z eff for 2s e ) Be has 1s 2 2s 2 config n the two 1s e s cancel two of the p + s, but the other 2s e does not Z eff for 2s e causes atomic size ionization energy

2 Size Trends I.) as n, size simply b/c larger n larger orbital T B in P.T: size Size of Ions T B size of ion (w/ same charge) (larger n, larger orbital) II.) L R a.) main group elements (s & p blocks) (1, 2, 13,... 18) Z eff, size b.) transition metals (d block) size remains same b/c 4s e s remain outer e s 3d e s effectively shield, Z eff remains same cations, relative to neutral parent atom size (remaining e s held more tightly by same # of p + s) (shrink) anions, relative to neutral parent atom (important for main group non-metals) size ( add l (puff up) e s held by same # of p + s) size of ions Illustrations stable cations (possessing N.G. config n ) size when going L R e.g., Na + > Mg 2+ > Al 3+ (all have same # of e s, but Z ) stable anions (possessing N.G. config n ) size when going L R e.g., N 3 > O 2 > F (all have same # of e s, but Z ) Ionization Energy, IE Def. energy requ d to remove 1 e in the gas state (from ground state) IE ( = ΔE sys ) always pos. successive IE s possible: first IE (IE 1 ) described by M(g) M + (g) + e second IE (IE 2 ) M + (g) M 2+ (g) + e third IE (IE 3 )... M 2+ (g) M 3+ (g) + e... (where M = symbol for a general element)

3 Successive IE s Pet. Table 9.4 IE as more and more e s are removed IE abruptly if core e s are removed (Z eff suddenly as we go to lower n) Ex. Mg process e transition energy (kj/mol) IE 1 Mg(g) Mg + (g) + e 3s e 738 IE 2 Mg + (g) Mg 2+ (g) + e 3s e 1451 IE 3 Mg 2+ (g) Mg 3+ (g) + e 2p e 7733 more extensive listing, see Pet. Table Definition of Radius Pet. Fig.9.3 Definition of covalent, metallic & ionic radii vague term since electrons are undefined cloud different terms exist: covalent, metallic, ionic experimental evidence by X-ray and IR spectroscopy see Pet. Fig. 9.3 for details of definition generally, the definition for metallic radius is meant when referring to metals, covalent radius is meant when referring to nonmetals, and ionic radius... cations & anions

4 Pet. Fig. 9.4 Trends in Atomic Radii Pet. Fig. 9.8 Atomic & Ionic Radii Add l Details on Z eff 1.) ns e s shield np e s Z eff & IE when going 2s 2p (i.e., Be B) 3s 3p (i.e., Mg Al) 2.) e s in same orbital repel each other (esp. in p, d,... orbitals; minor for s orbitals) is equivalent to Z eff & IE when e s start to pair in a particular orbital e.g., going from N to O Z eff Details this is the reason why half filled p and d subshells are relatively stable; (of course, completely filled ones are also stable) e.g., remember e config n for Cr is [Ar] 4s 1 3d 5 (3d subshell half filled) reflected well in IE trends see Pet. Fig. 9.9 Note especially the kinks : Be/B, N/O, Mg/Al, P/S easier to remove e from O than from N or, in other words, N atom is more stable than O atom 2p orbitals

5 Pet. Fig. 9.9 Def. Electron Affinity, EA system energy change when 1 e is added to an atom in the gas state i.e., for the process: X(g) + e X (g) (where X = symbol for general element, usually nonmetal) always neg. or near 0 (maybe slightly positive) i.e., energy released for the process atom still attracts e even if neutral somewhat related to Z eff of orbital that the next e would go into EA... Pet. Fig EA s in kj/mol Examples EA for F has large neg. value (attracts e strongly) since Z eff ( & IE) for Ne is high EA for Ne is near 0 (does not attract e much) since Z eff ( & IE) for Na is low rule of thumb: if IE of next element is high then EA of current element has large neg. value General Summary for EA s Going L R in P.T. starts near 0, more neg., back to near 0 for N.G.'s other more subtle effects come into play; consult Pet. Fig for numerical values

6 Def. substance is diamagnetic: paramagnetic: Magnetic Properties if repelled by magnetic field if attracted by magnetic field (ferromagnetic: strongly attracted...) single, spinning e s set up magnetic field and are attracted by external magnetic field effect is cancelled if e s are paired in an orbital, resulting in repulsion species where all e s are paired = diamagnetic species with unpaired e s = paramagnetic Note: applies to neutral atoms & ions e.g., O para, O 2 dia Ti para, Ti 2+ para, Ti 4+ dia Chemical Properties e s in outer ( valence ) shell are mostly responsible for chem. reactivity atoms w/ similar e config n in valence shell (having similar Z eff, IE, EA) should behave similarly elements in same group (esp. 1, 2, 16, 17, 18) react similarly Examples: Group 1 elements (alkali metals) lose e s easily, (low IE) group 17 elements (halogens) gain e s easily, (large neg. EA) group 18 elements (noble gases) generally don t react (high IE, EA near 0) 7-22 chemical properties... Ex. 1 all group 1 elements have s e s as valence e s low Z eff, low IE, e s easy to remove in Cs more easily than in Rb,..., Li b/c e s more distant from nucleus all alkali metals react w/ water, Cs most strongly (more details, see demo) Ex. 2 trends in atomic and ionic radii important for structure/density of metals and ionic compounds (see later) 7-23 Demo on Chem. Properties reactivity of alkali metals (group 1) w/ H 2 O reaction involves removal of e from neutral atoms 2 M + 2 H 2 O 2 M OH + H 2 expect: less IE greater reactivity 7-24 all group 1 elements should react easily bottom elements faster than top ones observations: Li reacts slowly Na at moderate rate K immediately phenolphthalein turns red due to OH formation to see Rb & Cs react google brainiac alkali, for the entertainment value

7 7-25 Summary of Lesson AS VII Definition of Z eff know terms: shield, cancel, cover up, repel core (inner) e s shield General Trend: L R : Z eff T B : Z eff atomic radius follows this trend IE also, but note Be/B, N/O... kinks cations < neutral < anions isoelectronic: size as Z EA of element X relates to IE of next element: if IE of next element is high, EA of element X is large neg. magnetic properties relate to unpaired e s: all paired = dia at least one unpaired = para elements in same group ( same outer e config n ) have similar chem. properties