Why does the VIIIB group have 12 elements?

Hi,
so I got this question that you can read above (it is about the periodic table and its groups). When I was trying to find the answer I basicaly bumped only on one page that explains it. It is important to note that this system is old one and nowdays you only number these groups from 1-18. If you want to read the answer from first hand go here: 1)


So in the new system they are not in the same group, but why were they there before?

The groups are made by comparing the physical and chemical characteristics of the top electron shells. This special VIIIB group are the collumns under Fe, Co and Ni which makes it quite unusual.

Basicly as you move across the periodic table, different oxidation states are more and more stable (in chemistry most things just want to get as stable as possible).

Those elements that have similar stable oxidation states are then put together. Take for example halogens. Those are the very very reactive elements next to noble gases.

Halogens in periodic table.

All of them will have oxidation number -I (most of the time). This puts them together. Transition metals are much more difficult to grasp with their d-orbitals. As you have seen in my other post they make some problems. The oxidation numbers for them are not very clear. Iron can be in II as well as III but generally these twelve/nine elements are occuring in the similar ones. Oxidation for these elements is actually almost always 2 or 3, this is especially true for nickel.

Dragallur

Elements of Periodic table: 4) Selenium

Hi,
why Selenium? Well I was not sure what element to choose, I did not want to write about some precious metal since last two time I have covered gold and palladium. I used random generator and I got 34, which is proton number of Se.


History

Selenium is one of those elements which were find in last centuries. This usually happens because the element is not enough common for some alchemists to find it or it is always well bonded with other element (here it is sulfur) and it took some time before we were able to extract it.

Jöns Jakob Berzelius and Johan Gottlieb Gahn were chemists in Sweden and they were working close to Falun mine [1], which you can see on the next pictureThe Copper Mine in Falun, the Great Pit

Guys there wanted to know what was it what they mined so they gave sample to those two chemists. It seemed to be some kind of arsenic compound but it was smelling strange and soon they found out that it is similar to sulfur and tellurium. Tellurium was named after Earth so they named Selenium (in Greece) after Moon, this happened in 1817.

Occurrence

  • Universe: 3.0×10-6 %
  • Earth’s crust: 5.0×10-6 %
  • Ocean: 4.5×10-8 %
  • Human: 5.0×10-6 %

You see it is not very common, at least it does not seem to be. But if I compare it’s occurrence in Universe with other elements it is not so bad. Most metals have one or two magnitudes less occurrence, from what I found selenium is not produced in solar fusion which means that it is created from supernovas, which would explain it’s  relative rarity.

On the picture above you can see some Se in piece of sandstone from uranium mines.

Usually you will find it with sulfur so you need to extract it using methods as SX/EW [2].

Properties

Se is non metal element which can exist in several allotropes [3]. SeBlackRed.jpg

It is usually red powder as you can see above but if you melt it rapidly you will get this kind of grey form which is usually sold. Se makes big and complex structures.

There are five Se stable isotopes and the couple of others with half life ranging to thousands of years, than there is Se 82 which has half life of 10^20 years which is considered relatively stable.

It has extremely important property of photo-conductivity, its conductivity depends on the amount of light hitting its surface.

Selenium is toxic element.

Usage

Selenium dioxide has a great usage during manganese electrolysis because it decreases the energy needed to get manganese (it works as catalyst). China is largest consumer of this compound for such use. For ton of manganese there are two kilograms of Se used. There are 31,200,000 tonnes of manganese produced each year.

Glass gets often kind of yellow or green color because of iron or other things accidentally mixed into it so Se is used to color it into red, this is the most common usage.

Also since lead is so toxic Se is used in brass with bismuth. At low concentrations it also improves machinability of lead, which is similar usage as of vanadium which improves the strength of steel.

Because of its photo voltaic property, it is used in photo voltaic cells called CIGS (copper indium gallium selenide solar cell).

There are other trace uses like radiography.

On the pie graph you can see the production of Se. While I did not find the graph of biggest consumers of Se, on the top is clearly China, as always.

Selenium in organisms

Organisms need selenium as we found out. It is important in some cellular processes but only really small amount like tens of micro grams.

At one time people thought that it is going to be good if they get more Se through some pills. Soon it was found out that more than 200 micro grams can cause cancer and so on. More accurately in one test, doctors got their patients 17% higher chance of prostate cancer which is number two of cancers killing male humans.

There are some illnesses with the lack of selenium but they are not very common even in places where is small amount of Se in the soil like New Zealand.

The price for Se is about 60$ for pound while at some times it lowered to about 30$.

Dragallur


[1] Falun mine was extremely important mine producing 2/3 of Europe’s needs of copper. It was working for almost 1,000 years and it is now cultural heritage with museum.

[2] SX/EW (Solvent extraction and electrowinning) is method used to get some metals like nickel, cobalt, zinc and uranium.

[3] Allotrope of carbon is for example graphite and diamond, both made up of only carbon but with different shape of molecules.

Elements of Periodic table: 3) Gold

Hi,
it is probably one of the best known metals in the world and even throughout the history. Humankind knows it for a long time. It is precious metal with amazing properties from conductivity to ductility. It does not react so much which means you can actually eat it pretty easily!


Right, we are talking about gold, or aurum.
There is really lot about this element so I will start with its chemical properties and where you can find it.

Attributes

Gold (Au) is amazing because it does not react much. It does not react with oxygen so you can easily have it in open place (while silver does react with oxygen), not only that but gold does not react with the strongest an-organic acids like hydrochloric acid (HCl) so only thing in which you can dissolve it is in aqua regia. 
On the left you can see aqua regia with piece of platinum which is very resistant too.

Once in Denmark during WW2 one chemist had two Nobel medals for some people. Those medals are made of gold and since Denmark was going to be captured by Nazis he did not want them to get those medals. He took them and placed them in aqua regia.
After the world he returned and the solution was still there so he extracted it and made new medals.

On the next picture you can see thin plate of gold. Gold is so ductile that one gram can be spread on 1 square meter. This picture is from Japan Toi museum.

This is very useful since gold is not very cheap but when you use only one gram for such a large area it gets to whole new level!

There is only one stable isotope of gold which is the one we are mining and using. There were found many others but all of them decay in matter of weeks, days or even couple of microseconds (actually some radioactive isotopes are used for curing cancer).

While gold does not react and so on, it is very soft so if you want to use it in jewelry it is always mixed up with palladium (white gold) or copper.

Gold is conductive. This is great but how does that happen?
Well gold and silver and copper are in the same column which means that they have similar properties. All of them are very conductive because they have one free valence electron.
So gold for example has configuration: 4f14, 5d10,6s1
“d” and “f” orbitals are both full since “d” has limit of 10 electrons and “f” 14.
You could ask why other elements with one valence electron are not so conductive and it is because the farther away from the core of the atom the easier it is to break the electron free. Also different atoms have different ionizing energy which means that it takes different amount of energy to free their electron.

Mining

Before I get to mining of gold I will mention the places where you can find it.

In the Earth’s crust there are about 0,0075-0,01 ppm of Palladium about which I wrote earlier. There is about 190 ppm of Vanadium but for gold it is: 0.0031 ppm. Which means that there is not much of it.

Unlike Palladium, gold was detected in the ocean to be 0.011 μg Au/l.
In whole Universe the estimated value of Au is one atom per 300 billions of hydrogen atoms, which is ten times less than palladium.

This is only guess but I think that this could be caused because in supernovas it is surely much easier to create element with 33 less protons.

Lets see how it looks like for different countries. The list of the countries is taken from Wikipedia and it is measured in tons (it is for mining in the year of 2013).

  • 1. Čína China 420
  • 2. Austrálie Australia 255
  • 3. USA USA 227
  • 4. Rusko Russia 220
  • 5. Peru Peru 150
  • 6. Jihoafrická republika South Africa 145
  • 7. Kanada Canada 120
  • 8. Mexiko Mexico 100
  • 9. Uzbekistán Uzbekistan 93
  • 10. Ghana Ghana 85

It is assumed that there were altogether mined 175,000 tons of gold. If you would pile it, it would use the volume of 21 cubic meters. While you may say that this is not much, imagine it standing next to your house + gold is very heavy. If you would make the same pile from hydrogen it would be about 200 times as large.

On the map below you can see the approximate storage of country’s gold (you could easily criticize it, look at South Africa.. but I did not find better, just don’t take it to be a good source).

There are few ways to mine gold. The older one, which you may have seen in movies and it is not used anymore, is the method of panning. This is based on the effect of gravity which pulls heavier gold down to the bottom of the bowl while the rest of the sand is slowly floating away (this was used often in rivers).

The modern way is to crush the material to small pieces and then leach it in some acid, rich in chlorine ions. It is then excluded in electric current on the cathode. There was also way to get it using mercury but only in specific situations and the soil was polluted with mercury.

Now there are ecological problems all the same because of hydrometalurgy which is the process of getting metals from their ores. There are huge amounts of cyanides used which in history already happened to cause some disasters, polluting large rivers and letting them toxic for years.

Usage

For the last chapter lets get to usage of this amazing metal.

In history the first usage was only jewelry. This is obvious since people back then did not even know what electricity is and they did not have enough gold to make isolation from it but it has such a nice color so they made lot of jewelry.

You can surely find lot of rings and various necklaces, this one is ring of Egyptian pharaoh. Like I said at the start, now gold is mixed with some other metal so it is not so soft.
You can see that metal gets different colors depending on the percentage of all the various metals.

Most often the objects are not whole made up of gold but rather only the top layer which can be extremely thin, even transparent.

Even in food you can find gold like a decoration, it is tagged as E175 and it is not dangerous at all since anything in human body is, won’t react with gold.

In industry, the property of high conductivity is used in computers in processors.

In dentistry gold is used as dental alloy, because again it does not react. There is not only gold but other metals too, mercury for example (because of Hg it is important for the alloy to not react and survive the environment in mouth).
I still remember when I went with my grandmother to guy who was collecting gold with her old golden dental alloys.

Gold was used as backup for money, this was called gold standard and I am going to write a whole separate post about it. This system fell and it is not used anymore.

Gold is even used by NASA. I read that for some flight there was 43 kg of gold used. To give specific example, there is gold used in astronauts helmet. This is because of radiation in space where you are not protected by atmosphere.
Gold foil will reflect most of the ultraviolet and infrared radiation which could otherwise cause serious damage to astronauts eyes. Full explanation.

This is all from me for today, already this has been the longest post ever so I hope someone will read it.

Dragallur

Cube of gold
Map of world gold storage
Other images from wiki articles about gold and gold mining and aqua regia

 

 

 

Some elements of periodic table

Hi,
today I am writing from Brazil which is why I had this few day pause from writing. I have got and idea to write about some elements of periodic table so this post will rather be mess of some stuff I found over time, probably a bit related to astronomy.

First of all I will start with one thing that comes to my mind and that is Hydrogen and Helium. Only those two elements in astronomy count as something different than metal.

PeriodicTable.gifThe reason for this is that astronomers wanted to make some difference between the most important elements in universe and the rest and the rest was almost all metal.

Iron will be the next stop for me. Iron is very special because it is the last element which can be created through thermonuclear fusion. Actually star can fuse iron but only to isotope 55 and then the reaction would take energy to fuse any higher so it is the ending point of stars life. Higher elements can be created in supernovas.

Now let me jump right on the end of periodic table. The last known element is Ununoctium. It has proton number 118. He is also the last known noble gas right after Radon but this element with many others is synthetic which means that you can not find him in the nature and is only made in lab.
Once I asked my chemistry teacher where will be next element since Ununoctium marks the end of another period.
Sadly I had to find out by myself. The next one of course will be right under Francium.
But what is more interesting is that element with 121 proton number will have another shell filled, now it will be G shell. This element would also be the first superactinide.

To read more on this topic check out:
Island of stability
Interactive periodic table

Dragallur