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