23.7+Transition+Metals


 * 23.7 Transition Metals**



Transition metals occupy the d block of the periodic table. Some include chromium, iron, nickel, and copper.


 * Physical Properties**

The properties of transition metals are vary similarly across each series. As the nuclear charges increases, there is a decrease in the radius from left to right in each series. However, metallic bonding strength increases until the middle of the series, then decreases as antibonding orbitals are filled. Therefore, there is a decrease of radius again before the radius increases.

The added electrons on the nuclear charge of transition metals cause incomplete screening, which is important in the third transition-metal series. Beyond the group 3B elements, the second and third transition-series elements have basically the same radii. This originated in the lanthanide elements (elements with atomic number 57-70), in which the 4f orbitals that are filled in the lanthanide elements cause a contraction in size due to the increase in effectiveness of nuclear charge. This is called the lanthanide contraction. It causes the radii to contract just so that it offsets the increase expected when going from the second to third series. This is why the second and third-series transition metals in each group have somewhat the same radii and why they have many similarities in their chemical properties.




 * Electron Configurations and Oxidation States**

Many chemical and physical properties of transition metals are due to the unique characteristics of d orbitals. The d orbitals are smaller than corresponding valence s and p orbitals. Thus, occasionally they act like core electrons and other times they behave like valence electrons.

The oxidation of transition metals causes them to lose outer s electrons before losing electrons in the d subshell. Most ions of transition metals have partially occupied d subshells, which give them unique characteristics. Transition metals usually have more than one oxidation state and many compounds are colored. Also, they exhibit magnetic properties as well.



The 2+ oxidation state is caused by the loss of the two outer 4s electrons. This state is found in almost all of these first-series metals except for Sc, where 3+ is more stable. The losses of 3d electrons cause oxidation states above 2+. Notice how the oxidation states increase as the series goes left to right, peaks in the middle, and starts decreasing again. This is caused by the attraction of d orbital electrons to the nucleus. Maximum oxidation states are usually found only when they are combined with the most electronegative elements (i.e O, F, and sometimes Cl).


 * Magnetism**

Measuring magnetic properties provides information about chemical bonding and many uses are made from magnetic properties in modern technology. An electron has a "spin" that gives it a magnetic moment, which means it will behave like a miniature magnet. A diamagnetic solid is when the electrons are all paired up, effectively cancelling spin as the up-spin and down-spins cancel each other out. They are generally nonmagnetic, but the motions of the electrons in a magnetic field cause it to be weakly repelled by the magnet. Paramagnetism refers to when an atom or ion has one or more unpaired electrons. In this type of solid, the electrons on nearby atoms or ions are not affected by unpaired electrons. Therefore, the magnetic moments on individual atoms or ions are random. However, the magnetic moments become aligned almost parallel to one another when placed in a magnetic field, which produces a net attractive interaction with the magnet.

A type of magnetism is called ferromagnetism, which is exhibited by iron. It is a stronger form - much stronger - form of magnetism than paramagnetism. Ferromagnetism occurs when unpaired electrons of atoms or ions in a solid are affected by the orientations of other electrons. The lowest energy (most stable) arrangement occurs when spins of electrons align in the same direction with adjacent atoms or ions. In a magnetic field, the electrons align parallel to the magnetic field. When the magnetic field is removed, a magnetic moment is maintained in the solid due to the interaction between electrons - this is now a permanent magnet. Only iron, cobalt, and nickel are the transition metals that display ferromagnetism.Alloys also have ferromagnetism, which is sometimes stronger than the ferromagnetism of pure metals.



Another type of magnetism is called antiferromagnetism, where unpaired electrons' spin on a given atom are aligned in the opposite direction as the spins on neighboring atoms. In antiferromagnetism, the spins cancel each other out. They are found among metals (like Cr), alloys (like FeMn), and transition metal oxides (like MnO). There is another type of magnetism called ferrimagnetism, where is contains characteristics belonging to both ferromagnets and antiferromagnets. There are similar to antiferromagnets in which unpaired electrons are aligned so they are facing opposite directs compared to their neighbors. However, the net magnetic moments of the electrons do not fully cancel out. This occurs because the magnetic centers contain different numbers of unpaired electrons, the number of magnetic sites aligned in one number is larger than the other direction, or both. The magnetic moments are not cancelled, so most of the properties of ferrimagnetic materials are like ferromagnets.

All magnetically ordered materials become paramagnetic when they are heated above a critical temperature. The thermal energy is so great that it overcomes forces responsible for orienting the spins with respect to their neighbors. For ferromagnets and ferrimagnets, this temperature is called the Curie temperature, TC and for antiferromagnets it is the Néel temperature, TN. For Fe, the Curie temperature is 770 degrees Celsius, 1115 degrees Celsius for Cobalt, and 354 degrees Celsius for Nickel.


 * References**

Brown, Theodore L, et al. //Chemistry: The Central Science//. New Jersey: Pearson Education, 2009. Print.

"Classes of Magnetic Materials." //Institute for Rock Magnetism.// University of Minnesota. Web. 23 Mar. 2011. 