Meet the transition metals. This group has too many members to name individually, but copper (Cu), gold (Au), zinc (Zn) and iron (Fe) stick out as some of the better known transition metals. Of course, we also have the decidedly obscure tantalum (Ta), rhenium (Rh), bohrium (Bh) and roentgenium (Rg) in the same group. So, what does this group of elements have in common? Prior experience suggests that the answer is going to involve valence electrons. Prior experience is correct. Sort of.
Take a look at this periodic table - it’s the same one we looked at back in March. Now, look at the valence electron configurations for the transition metals (listed on the right edge of each element square). There are a lot of 2s, a lot of 1s...and palladium, which has 18 valence electrons. Let’s leave palladium aside. It’s interesting, isn’t it, that as we move across periods the number of valence electrons either remains the same or alternates between the same two numbers? This is very different from the progression where lithium has one valence electron, beryllium two, boron three, carbon four, etc. What is going on here?
Look at those configurations again. Specifically, look at the number above the valence number (second from the bottom). As you move across a period, those numbers increase more or less steadily. Ok, so that fits the pattern, but why is the outer shell configuration remaining stable? Time for another trip into valence chemistry!
First off, I recommend a quick review of how electrons fill in orbitals and, eventually, fill the valence shell. We have the s orbital (holds 2 electrons, max), the p orbital (holds 6 electrons, max), the d orbital (10 electrons, max), and finally the f orbital (14 electrons, max) (http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/eleorb.html). As we move from hydrogen to bigger and bigger elements, the 1s shell fills first, then the 2s, the 2p, the 3s, the 3p, the 4s, the 3d, the 4p, the 5s, the 4d, the 5p, the 6s....and so on. We’ll deal with the f orbitals at a later date. There is no 1p or 2d orbital, because atoms that have valence shells in the 1 or 2 range simply don’t have enough electrons for a d orbital. The pattern goes 4s, 3d, 4 p rather than 3s, 3p, 3d because the nucleus exerts a stronger pull on s orbitals than d orbitals in general, to the point that a 4 s orbital is closer to the nucleus than a 3d orbital, and thus fills in before the 3d orbital. It’s all strange and counterintuitive, and the full answer involves more quantum mechanics than I really want to get into, but just remember the general order - when there is a d orbital, it fills between the s and p orbitals of the next level.
The d orbitals are what make the transition metals different from all the other designations - all of the transition metals are marked by full (more or less) outermost orbitals, and incompletely filled inner d orbitals. It’s valence shells, with a twist. A really strange twist. So, a little more about this group. As the name would suggest, they are metals. What exactly does that mean? Well, in order to be a metal, an element has to have the properties of ductility and malleability - metals form solid substances which can be bent and deformed without being torn apart. Diamond is a solid form of carbon - compare the relative flexibilities of diamond and copper. Only one of these two is a metal... Metals also conduct heat and electricity. Naturally, these properties are connected to the weirdness of transition metal electron chemistry.
All metals, transition or otherwise, have a tendency to give up their electrons - rather forcefully, in the case of the alkali metals. When a number of metal atoms are in close proximity, the tendency is to give up elections so as to form a kind of electron cloud, with embedded positive ions (cations).
Introducing other metal elements to the mix results in alloys - ions of different elements embedded in the same cloud. Famous alloys include bronze (tin and copper), brass (copper and zinc) and steel (interestingly, an alloy between a metal, iron, and a nonmetal, carbon).
Strange, but useful.
All of the electrons and cations are attracted to each other with an equally strong force - hence, the unusual physical properties of solid metal compounds. Heat energy and added electrons move easily through the electron cloud - hence the conduction of heat and electricity. Transition metals differ slightly from the other metals in that they are a little less likely to give up electrons, and a little more likely to form regular covalent bonds. That being said, copper, one of the best conductors out there, is a transition metal. Transition metals also have very complicated ionic chemistry - sometimes, a transition metal releases the electrons from the actual valence shell (an s orbital), sometimes, those electrons are joined by an electron from the d orbital. So, you wind up with elements like iron, which can exist as an ion with a 2+ charge, or as an ion with a 3+ charge. The multiple options for ionic charge thing means that any given transition element can combine with elements in a lot of different ways - the possibilities for iron alone are dizzying.
It goes without saying that quite a lot of human existence is predicated on the transition metals. While elements like seaborgium and meitnerium are essentially theoretical and seldom found outside a particle collider, and iron and copper are common enough to not even rate an explanation, the transition metals contain some hidden gems. Vanadium, while not often used on its own, is alloyed with steel to create a strong, corrosion resistant metal used in car, machine and airplane parts. Molybdenum is used as a catalyst in refining petroleum, and is a trace nutrient for plants to boot. Iridium is an element with some deeply philosophical uses. An iridium alloy makes up the standard meter bar - if weights and measures begin to seem abstract, know that each base unit of the metric system has a definition, and that once upon a time, the meter definition was given a solid form. The bar used to define the meter was an iridium alloy. Iridium is also used in geologic dating. Asteroids contain a greater concentration of iridium than the crust of the Earth - areas rich in iridium are almost definitely sites or asteroid impact. A thin, worldwide layer of iridium is thought to mark the impact of a massive asteroid. Not coincidentally, this lines up with the extinction of the dinosaurs. From the subatomic quirks of electron behavior to the mass extinction of some of the largest animals to ever walk the Earth, the transition metals tie it all together.
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