Periodic Trends

Question 1

What is a ligand

Answer 1

An ion or molecule with a functional group that binds to a central metal atom to form a coordination complex

Question 2

Explain ions and moelcules as ligands

Answer 2

Ions as ligands:

Cl- ; F-; Br-; CN-; OH-; COO-; C2O42-

Molecules :
mono - tri -

Question 3

What is a complex?

Answer 3

• Composed of a transition element and ligands

e.g.[Fe(CN)6]3−, where CN− is the ligand

• More atoms (ligands) bond to the central atom compared to the demand of ionic charge of the metal e.g. Fe in this example is Fe3+ and each ligand is CN−
• Have a specific geometry due to the hybridization of the central element e.g. d2sp3 (octahedral)
• Ligands alone are responsible for bonding to the central metal by way of
  donating 2 electrons to form the bond

Question 4

What is Electronegativity

Answer 4

An empirical measure of the tendency of an atom in a molecule to attract electrons

Question 5

What is Electron affinity

Answer 5

The energy change for the process of adding an electron to a neutral atom in the gaseous state to form a negative ion
(enthalpy of electron attachment).

Question 6

Explain what Mulliken did with regards to electronegativities and elecrton affinities

Answer 6

Electronegativities are determined in part by the tendency of an atom to gain additional electron density and by its tendency to retain the electron density it already has

*****Limitation– electron attachment enthalpies not available for all
elements.

Question 7

Explain Pauling with regards to electronegativities and elecrton affinities

Answer 7

• If two atoms A and B had the same χ, the strength of the A-B bond would be equal to the geometric mean of the A-A and B-B bond energies.
• For a majority of A-B bonds the energy exceeds the geometric average, because atoms have different χ, and because of an ionic contribution in addition to the covalent contribution.
• Pauling proposed that the “excess” bond energies could be used to determine electronegativity differences

Question 8

What are partly responsible for the periodic trends

Answer 8

1. The different penetrations of the atomic orbitals can be judged from evaluation
   of the size of the orbitals and the orientation of the orbitals
   - size + incr in electrons thus more orbitals
2. Because of the different penetrations and orientations, orbitals are used in the
   sequence as discussed earlier
3. Because of the different penetrations and orientations, the valence electrons of
   the atoms experience different effective nuclear charges
   
   • more rings around nucleus = electrons are further away from nucleus thus is easier to lose e- and to react with valence shell
4. Properties such as first ionization enthalpy also follow trends that reflect the
   different electron configurations
   - incr along period due to more e- & decr down grp thus shielding incr as more e- added

Question 9

Explain Ionization enthalpy

Answer 9

It is the minimum energy needed to remove the highest energy electron (outermost) from the neutral atom in the gaseous state

Question 10

What are the Three major trends for ionization enthalpy

Answer 10

1. Maxima occur at the noble gases and the minima at the alkali metals
2. Ionization enthalpies increase across any row of the periodic table
3. The increase is not smooth – there are two well defined jogs in each series

Question 11

Explain Maxima occur at the noble gases and the minima at the alkali metals

Answer 11

• The closed-shell configuration of the noble gases are very stable and resist disruption, either to form bonds or to become ionized.
• There is an electron outside the preceding noble gas configuration in the alkali metal atoms. This electron is well shielded from the attraction of the nucleus and is therefore easy to remove.

** less energy required to remove e-…e- outside preceeding NG are well shielded from attraction thus easy to remove

Question 12

Explain Ionization enthalpies increase across any row of the periodic table

Answer 12

• The effective nuclear charge grows across a row because of the cumulative effects of imperfect shielding by orbitals of the same principal quantum number.
• As the effective nuclear charge increases, so does the energy necessary to ionize the atom

*** atomic radius = more e-

Question 13

Explain The increase is not smooth – there are two well defined jogs in each series

Answer 13

• In each case the ionization enthalpy drops from s2 to s2p1 and again from s2p3 to s2p4
• The p electrons are less penetrating than s electrons. The p electrons are more shielded and easier to remove
• The 2p shell is half-full at N and each further 2p electron added enters an orbital already singly occupied. These electrons are partly repelled by the electron already present and are thus less tightly bound

** Z grows across row bc cumulative effects of imperfect shielding by orbitals of same principal quantum nummber ( poor shielding )