In the iron case, the extra ionization energy is compensated more or less by the extra lattice enthalpy or hydration enthalpy evolved when the 3+ compound is made. However, there is much less increase when you take the third electron from iron than from calcium. There is an increase in ionization energy as you take more electrons off an atom because you have the same number of protons attracting fewer electrons. The figures for the first three ionization energies (in kJ mol -1) for iron compared with those of calcium are: metal There is not a huge jump in the amount of energy you need to remove the third electron compared with the first and second. The 4s orbital and the 3d orbitals have very similar energies. Here are the changes in the electronic structure of iron to make the 2+ or the 3+ ion. Thinking about a typical transition metal (iron) It definitely is not energetically sensible to make CaCl 3! Although there will be a gain in lattice enthalpy, it is not anything like enough to compensate for the extra ionization energy, and the overall process is very endothermic. There is a large jump in ionization energy between the second and third electron removed. That is much closer to the nucleus and therefore much more difficult to remove. What about CaCl 3? This time you have to remove yet another electron from calcium. Because the formation of CaCl 2 releases much more energy than making CaCl, then CaCl 2 is more stable - and so forms instead. There is much more attraction between chloride ions and Ca 2 + ions than there is if you only have a 1+ ion. By making a Ca 2 + ion instead, you have to supply more ionization energy, but you get out lots more lattice energy. Why is that? If you tried to make CaCl, (containing a Ca + ion), the overall process is slightly exothermic. The formula for Calcium chloride is CaCl 2. Thinking about a typical non-transition metal (calcium) But off-setting this, the more highly charged the ion, the more energy is released either as lattice enthalpy or the hydration enthalpy of the metal ion. The more highly charged the ion, the more electrons you have to remove and the more ionization energy you will have to provide. This will either be lattice enthalpy if you are thinking about solids, or the hydration enthalpies of the ions if you are thinking about solutions. The amount of energy released when the compound forms.The amount of energy needed to ionize the metal (the sum of the various ionization energies).There are several energy terms to think about, but the key ones are: The more energy released, the more stable the compound. On the whole, the compound formed is the one in which most energy is released. When a metal forms an ionic compound, the formula of the compound produced depends on the energetics of the process. We'll look at the formation of simple ions like Fe 2 + and Fe 3 +. Copper is definitely a transition metal because the Cu 2 + ion has an incomplete d level.Įxplaining the variable oxidation states in the transition metals However, the more common Cu 2 + ion has the structure 3d 9. In the Cu + ion the electronic structure is 3d 10. The zinc ion has full d levels and does not meet the definition either.īy contrast, copper, 3d 104s 1, forms two ions. When it forms ions, it always loses the two 4s electrons to give a 2+ ion with the electronic structure 3d 10. Zinc has the electronic structure 3d 104s 2.The Sc 3 + ion has no d electrons and so does not meet the definition. When it forms ions, it always loses the 3 outer electrons and ends up with an argon structure. Scandium has the electronic structure 3d 14s 2.On the basis of this definition, scandium and zinc do not count as transition metals - even though they are members of the d block. Not all d block elements count as transition metals!Ī transition metal is one that forms one or more stable ions which have incompletely filled d orbitals. You will notice that the pattern of filling is not entirely tidy! It is broken at both chromium and copper.Transition metals The electronic structures of the d block elements shown are: Sc The first row of these is shown in the shortened form of the Periodic Table below. The elements in the Periodic Table which correspond to the d levels filling are called d block elements. At argon, the 3s and 3p levels are full, but rather than fill up the 3d levels next, the 4s level fills instead to give potassium and then calcium. You will remember that when you are building the Periodic Table and working out where to put the electrons using the Aufbau Principle, something odd happens after argon. They don't - there's a subtle difference between the two terms. The terms transition metal (or element) and d block element are sometimes used as if they mean the same thing.
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