chapter 7 dmitri mendeleev arranged periodic table according to atomic masses chemical periodicity-...
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Chapter 7Dmitri 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 e-
Ti
4 valence e- and 18 core e-
Ge
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 radius- twice 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 e- closer 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 charge only (anions larger than cations)
isoelectronic series- group of ions all containing the same # of e-
ex: 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 e- Ca= 2 val e- S= 6 val e-
Na, b/c 2nd will be a core e-
2) Which has the greater I3, O or Mg?
O= 6 val e- Mg= 2 val e-
Mg 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 e- from 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: Li(1s22s1)
Li+ (1s2) + e-
Fe([Ar]3d64s2)Fe2+ ([Ar]3d6) + 2e-
-if Fe3+ would be ([Ar]3d5) + 3e-
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
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