Transcript Chapter 7
Chapter 7 Dmitri Mendeleev arranged periodic table according to atomic masses chemical periodicity- predicting a property based on an element’s position within the periodic table -Henry Moseley later arranged according to increasing atomic # -elements in the same group have similar # of valence e- and similar chemical properties valence orbitals- occupied orbitals which hold the electrons involved in bonding (outermost levels) -in transition metals the d e- are included in the valence electrons even though they are not in the outermost energy level core electrons- all other e- besides the valence e- *Identify the valence and core e- for potassium, titanium and germanium K 1 valence e- and 18 core eTi 4 valence e- and 18 core eGe 4 valence e- and 28 core e- Periodic Trends 1) Effective Nuclear Charge (Zeff) -the net positive charge of the nucleus experienced by an e- in a many-electron atom *Coulomb’s Law the strength of the interaction between two electrical charges depends on the magnitude of the charges and on the distance between them *force inc as Zeff inc and dec as distance inc Zeff = Z – S Z = # of p+ S= screening constant (usually close to # of core e-) *Zeff < actual nuclear charge *Zeff includes the effect of the other e- in the atom -core e- partially screen valence e- from the attraction of the nucleus **Zeff increases from left to right across a period -core e- stays the same, but p+ inc -added valence e- screen one another ineffectively ** Zeff going down a group changes far less than it does across a period ex- Li = 3-2 = 1+ Na = 11-10 = 1+ -inc slightly as you move down a group b/c the more diffuse core e- cloud is less able to screen the valence e- from the nuclear charge 2) Atomic Radii/Size nonbonding atomic radius/van der Waals radiustwice the radius of two nuclei bonding atomic radius/covalent radius- half the distance between two nuclei page 254 figure 7.5 -increases as you move down a group *due to inc. in n (prin. quantum #) of outer e*as you inc. n outer e- are further from nucleus -decreases as you move across a period (left to right *due to the inc. in Zeff which draws valence ecloser to the nucleus, causing smaller radius -length of the atomic radius of elements will be given -page 255 figure 7.6 *transition metals do not show a regular decrease moving across -given in angstroms 1Å = 10-10m Problems: 1) Natural gas used in heating and cooking is odorless. Because natural gas leaks pose the danger of explosion or suffocation, various smelly substances are added to detect a leak. One example is mercaptan, CH3SH. Predict the lengths of C—S, C—H, and S—H bonds. C—S = 0.77 + 1.02 = 1.79 Å C—H = 0.77 + 0.37 = 1.14 Å S—H = 1.02 + 0.37 = 1.39 Å 2) Predict which is longer: a) P—Br bond in PBr3 b) As—Cℓ bond in AsCℓ3 a) P—Br = 1.06 + 1.14 = 2.20 Å b) As—Cℓ = 1.19 + 0.99 = 2.18 Å **P—Br 3) Arrange (as much as possible) the atoms P, S, As, and Se in order of increasing atomic size. S P Se As 4) Arrange Na, Be, and Mg in order of inc. atomic radius. Be Mg Na 3) Ionic Radius/Size -cations are smaller than their parent atoms *e- lost less e- repulsions -anions are larger than their parent atoms *e- gained more e- repulsions -for ions with the same charge, ionic radius inc. as you move down a group -dec. as you move across a period for ions with same # of e- only isoelectronic series- group of ions all containing the same # of eex: Na+, O2-, Mg2+, F-, Aℓ3+ -all have 10 e-ionic radius dec. with inc. atomic # b/c e- are more strongly attracted to nucleus **Put above in order of dec. ionic radius (from above) O2- F- Na1+ Mg2+ Aℓ3+ Problems: 1) Arrange Mg2+, Ca2+ and Ca in order of decreasing radius. Ca Ca2+ Mg2+ *atoms larger than their cations, inc. as you move down 2) Which of the following atoms and ions is largest: S2- , S, O2-? S2*anions larger than their atoms, inc. as you move down 3) Arrange the ions K+, Cℓ-, Ca2+, and S2- in order of decreasing size. S2- Cℓ- K+ Ca2+ 4) In the isoelectronic series Rb+, Sr2+, Y3+, which ion is largest? Rb+ 4) Ionization Energy -minimum energy needed to remove an electron from the ground state of the gaseous atom or ion (kJ/mol) first ionization energy (I1)- energy needed to remove the first e- from a neutral atom second ionization energy (I2)- energy needed to remove the second e- *as each e- is removed the ion. energy increases *greatly increases when an inner e- is removed -due to the greater Zeff on inner e-page 259 Table 7.2 Problems: 1) Of the following elements, which has the greatest I2? Na Ca S Na= 1 val eCa= 2 val eS= 6 val eNa, b/c 2nd will be a core e2) Which has the greater I3, O or Mg? O= 6 val eMg= 2 val eMg b/c 3rd will be core e- Trends in first ionization energy -inc. as you move across a period -dec. as you move down a group -tran. metals generally inc. left to right *alkali metals have the lowest *noble gases have the highest *smaller atoms have higher I1 -the energy depends on Zeff and distance of efrom nucleus -inc. Zeff or dec. the distance from the nucleus inc. attraction between e- and the nucleus and makes it harder to remove e- Problems: 1) Arrange the atoms Ne, Na, P, Ar and K in order of increasing first ion. energy. K Na P Ar Ne 2) Which has the lowest first ion. energy, B, Aℓ, C, or Si? Aℓ E- configs of ions *e- lost from highest occupied levels first Ex: lithium Li(1s22s1) *loses from 2s and becomes (1s2) Li Li+ + eEx: Fe2+ Fe([Ar]3d64s2) *loses from 4s and becomes ([Ar]3d6) Fe Fe1+ + 1eFe1+ Fe2+ +1e*if loses 3e- the third e- is lost from 3d *transition metals lose e- from outermost first then d 5) Electron Affinity -energy change that occurs when an electron is added to a gaseous atom -in kJ/mol -measures the attraction of the atom for the added e-when e- is added energy is released, so negative values *halogens have most negative e- affinities -due to only needing 1e- to achieve octet *Group 2A has + energy b/c added e- goes to p orbital which is higher in energy and unoccupied *Noble gases also + b/c added e- goes to next higher s -no definite trends to follow