My OperaTransition Metals (Part II)
Sunday, February 28, 2010 3:38:03 AM
You have listened to the song, now , do you want to know more about these transition metals? Why are they so important such that they occupy half of the periodic table?[/color]
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Note: The first ionization energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions.[br]
1. A transition element (or d-block element) is one which forms some compounds in which there is an incomplete sub-shell of d electrons.[/B][/color]
2. The tendency of transition elements to have variable oxidation states:[/B][/Font][/COLOR]
o Transition metals have variable oxidation states is due to their electronic configuration and their tendency to lose electrons to be stable.
o Group 1 metals have only 1 valence electron which possesses the highest energy and lowest stability, so this electron is usually donated/given away, so group 1 metals have an oxidation state of +1.
o Transition metals have different valence electrons: for 1st row of transition metals, electrons are usually filled in 4s orbital of the atom first with 2e-, then 3d orbital.
o Since the electrons of 4s orbital have highest energy, they will get excited first (less stable) and be removed from the atoms. After that, if the atom has not gained stability, it continues losing electrons from 3d orbital. There can be maximum of 10 electrons in the 3d orbital and they have same level of energy, so when 1 electron is removed from this 3d orbital, it’s not hard to remove another one. Electrons will be continued removing from the atoms until the atom becomes stable. It explains why transition elements have variable oxidation state, which depends on the electronic configuration of the stable formed ions. Usually, atoms tend to stop losing electrons when its 3d orbital is half-filled. [/COLOR]
Link this to their usage as catalysts:[/B][/Font][/COLOR]
In reactions requiring catalysts, the catalyst has to be in the same physical state as the reactants.
- The catalytic activity of transition metals depends on their ability to exist in variable oxidation states. The transition metal can have different oxidation states when reacting with different element, so that they can donate electrons and reduce other elements, or receive electrons and oxidize other elements, depending on which reaction they act as catalysts for.
- For example, Fe3+ (aq) is a catalyst in the oxidation of I- (aq) by S2O82- (aq). This reaction requires catalyst because both I- and S2O82- are anions so they will repel each other and cause high activation energy (energy is needed for the reaction to occur), resulting in a longer reaction time.
-The catalysed pathway involves the following steps with low activation energy:
-Both steps involve the interaction of oppositely charged ions (Fe3+ and I- in step 1; Fe2+ and S2O82- in step 2), which attract one another strongly and hence, enhances the rate of reactions.
3.Predict from a given electronic configuration, the likely variable oxidation states of a transition metals.[/U][/B][/color]
In order to predict the likely variable oxidation state of a transition metal, we have to base it on the electronic configuration of its formed ion.
For example,
The most common oxidation states of Fe are +2 and +3. It’s because the two electrons at the 4s-sub-shell can be removed easily due to the highest energy level of the outermost shell Fe2+. Oxidation state of Fe can likely be +3 than that of +4, +5, and +6 and so on. This is because the stability of half filled sub-shell of 3d5, so only 1 electron is removed from 3d orbital to gain stability.
Fe2+: [Ar] 3d6
Fe3+: [Ar] 3d5
For example
The 2 electrons at the 4s orbital can be removed easily because they possess the highest energy level, so oxidation state of Mn can be +2 when Mn ion has stable electronic configuration of half-filled 3s orbital.
Mn2+: [Ar] 3d5
Exceptions: The electronic configuration of Cu: [Ar] 3d10 4s1
Cu+ (aq) -> Cu2+ (aq) + Cu (s)
Theoretically, Cu+ is supposed to be more stable than Cu2+ due to the stability of the filled sub-shell 3d10, however, it is opposite in reality. There is an acceptable hypothesis that the original electronic configuration of Cu is [Ar] 3d9 4s2. There is one electron moving from the high energy level of 4s-sub-shell to 3d-sub-shell to fully fill up the 3d-sub-shell and make it more stable. Therefore, Cu2+ will be more stable[/COLOR]
4.The electronic configuration of first row transition elements and of its ions.[/B][/Font][/color]
Note: Notice that the pattern of filling isn't constant: it is broken at both chromium and copper, when the 4s orbital is only half-filled by 1 electron.
5.Transition metal ions are coloured due to d-d transition.[/B][/Font]
Transition metal complexes are often coloured due to the presence of incompletely filled d-orbitals in the metal ion.
o A substance will appear coloured if it absorbs light from some portion of the visible spectrum.
o Ions with no d orbital electrons are colourless, eg, Sc3+, Ti2+.
o Ions with d10 electron configurations are colourless, d-d transitions are impossible because the d orbital are all filled, eg, Zn2+
o Ions with d orbital electrons appear coloured because energy from visible light is absorbed and used to promote the unpaired electrons of d orbital to a higher energy d sublevel (referred to as d-d transitions). [/COLOR]
