Chap 27A - Transition Metals

Define transition element

Transition element (Def.): d block element whose atom has an incomplete d subshell (3d1 - 3d9) OR can give rise to cations with an incomplete d subshell

Transition elements 1st set

• Zn is NOT a transition element (has a completely filled 3d subshell)

• 1st set of transition elements: Scandium to copper

Describe Melting point trend for transition elements (general trend + reason)

Melting point

General trend: Transition elements have high melting points (above 1000 °C), higher than s block metals Reason:  s block metals only use up to 2 of the valence electrons in the s subshell for formation of metallic bonds. Transition elements are able to form stronger metallic bonds as both 4s and 3d electrons can be delocalised as they are close in energies More delocalised electrons result in stronger electrostatic attraction between the larger mobile electron cloud and the nuclei -> stronger metallic bonds

Density between transition elements and s block (general trend + reason)

1. Between transition elements and s block 

• General trend: Transition elements have high densities, denser than that of s block

Reason: 

• Transition elements have greater relative atomic mass than s block elements of the same period.

• Transition elements have smaller atomic radii than s block elements due to higher nuclear charge (more protons) and poor shielding by the d electrons of the transition elements -> have higher effective nuclear charge than s block elements -> smaller atomic volume

• Transition elements have greater relative atomic mass to atomic radius ratio

Density between transition elements across same period (general trend + reason)

1. Between transition elements across same period 

• General trend: Gradual increase in density

• Reason: Relatively invariant atomic radius coupled with increasing relative atomic mass across the period

Atomic radius/1st IE/electronegativity transition elements trend across the period (general trend + reason)

Atomic radius/1st IE/electronegativity

General trend: Relatively invariant across the period Reason:  Across the period for transition elements, nuclear charge increases due to increasing number of protons Additional electrons enter the penultimate 3d subshell, increasing the shielding effect between the nucleus and outermost 4s electrons slightly. Increase in nuclear charge is partially nullified by the increase in shielding effect leading to only a slight increase in effective nuclear charge across the period

3rd and higher IE transition elements trend across the period + EXCEPTION

3rd and higher IE

General trend: Increase across the period NOTE: 3rd IE of Fe is lower than expected as removal of one electron from Fe2+ [Ar]3d6, forms Fe3+, [Ar]3d5, which has a stable half-filled 3d subshell -> electron removed comes from a paired 3d orbital -> inter-electronic repulsion -> requiring less energy

State chemical properites of TS

1. Show variable oxidation states

2. Form stable complexes

3. Form coloured compounds and ions

4. Transition elements and their compounds often show catalytic activity

Describe TS oxidation state trend

General trend: Variable oxidation state (Except Sc only has +3 OS) 

Reason: Due to the close similarity in energy of 3d and 4s electrons, transition elements can make use of different numbers of electrons of both subshells in bond formation when they form compounds

Describe s block elements oxidation state trend

• For s block elements, only one oxidation state -> valence electron(s) are used in bond formation

• Once the valence s electrons are removed, the further removal of inner shell p electrons would require too much energy as the difference in energy level between the electrons in the s and inner p subshells is too large

Maximum OS is…

• Maximum oxidation state of a transition element = number of 4s + unpaired 3d electrons

• Paired 3d electrons are unreactive and not used for bonding

OS for Mn and TS carbonyls

1. Manganese (Mn) has the greatest number of unpaired 3d electrons, ONLY first row transition element with +7 OS 

2. Transition elements in the elemental state and transition metal carbonyls (Eg. Ni(CO)4) have 0 OS 

1st row TS OS trends

1. All first row transition elements except Sc have +2 OS = loss of the two 4s electrons.

2. All first row transition elements have +3 OS (not important for Ni) 

3. Relative stability of transition elements in each OS is dependent on Eo