Chemistry from Hydrogen to Tellurium - BORON

Boron

The 5^th element in the periodic table is Boron and has the chemical symbol B.

By now you will probably have noticed that each element we discuss is slightly bigger that the last. We started with Hydrogen, Atomic Number 1 and 1 electron, through Helium with 2 electrons, Lithium with 3, Beryllium with 4 and now Boron with 5.

Don’t know if you remember but Hydrogen and Helium were both gases and extremely light.

Lithium and Beryllium are both metals and solid. When describing something as a solid, liquid or a gas, scientists would always add some information about the temperature. This is because if you heat a solid you can turn it into a liquid and if you keep heating, the liquid will become a gas. This happens at different temperatures for different materials.

You will all be familiar with water – in Ireland it is a liquid, but if you lived in the North Pole you would be more used to seeing water as a solid! So the correct way to describe Lithium and Beryllium is say they are solids at room temperature. Room temperature is not a very precise term. It is the temperature that a room is at when people are comfortable in it. This is usually between 20°C and 25°C.

So what about Boron – well firstly, like Beryllium it doesn’t exist on its own. Scientists can separate it from other molecules but it is never found in nature as a clump of Boron atoms.

Almost all the elements in the periodic table can be grouped or classified as either metals or non-metals. Hydrogen and Helium are non-metals, Lithium and Beryllium metals but Boron is different. It is classified as a Metalloid. This means that it is in “no man’s land” – chemically it is somewhere in between. So sometimes it behaves like a metal and other times like a non-metal.

So far when I have drawn atoms I have always shown the nucleus in the middle with the electrons orbiting in circles around the nucleus – a bit like Saturn and its rings. Boron’s fifth electron can’t fit into the outer circular orbital as it already containing 2 electrons so is now full. The next empty orbital isn’t circular like the previous ones but it is the same shape as the number 8.

Boron Molecule

Although elemental boron is not found, boron is present in many chemical compounds. It has 3 electrons available for bonding and it usually forms the sharing, covalent type of bonds. An example of such a compound would be BF₃ – Boron triFluoride. Fluoride needs one extra electron to fill its outer electron shell so three fluoride atoms link up with one boron to make a stable compound.

It is borosilicate glass which gives pyrex its ability to be put into hot ovens without the glass shattering. Boron compounds are used in bullet proof vests and boron is also used in the computer industry to change the properties of semiconductors. Boron is found in borax, Na₂B₄O₇, which in the past was used as a cleaner.

Borax Molecule

Today and among children in particular, borax’s claim to fame is that it is the cross linking agent used to transform PVA glue into Slime.

Check out the following cool activity using the boron containing compound, Borax:

You will need: Borax, water, PVA glue, 2 beakers, weighing scales, spatula or spoon

Step 1 Dissolve 5g borax in 100mls warm water.

Step 2 To 50g PVA glue add 150mls water. Mix well. (The quantity of water required depends on how thick the PVA is so you might need to adjust the amount of water needed).

Step 3 While stirring the PVA solution gradually add small amounts – say 1 teaspoon at a time, of the borax solution you prepared in step 1. Continue stirring and adding borax solution until the PVA turns to a rubbery slime.

Step 4 Take the slime out of the container and have fun playing with it.

From Hydrogen to Tellurium - LITHIUM

Lithium

Lithium is the third element on the periodic table. It is not as famous as either hydrogen or helium so you might not have heard of it before. Being the third element, it has 3 electrons, 2 in the inner circle and one on its own in an outer orbit.

Lithium is quite happy to give away this outer electron and because of this scientists say that it is reactive. It gives away its outer electron to elements such as Hydrogen which need one additional electron to fill their outer shell of electrons. When a Lithium atom gives an electron to a hydrogen atom a new compound called Lithium Hydride is formed. There is no sharing of electrons involved. Scientists call this type of bonding ionic bonding.

I think of Lithium as being like a family with 3 children (electrons). The younger 2 are happy to stay near their parents (the nucleus). The older child or electron likes to go on sleepovers and often stays away for long periods of time.

The bonds in this type of compound are different to those discussed when we were talking about hydrogen joining with another hydrogen atom to make H2. In H2 both hydrogen atoms share the 2 electrons. Scientists call this a covalent bond.

The covalent bonds of Hydrogen remind me of 2 families with one child each living next door to each other. The children play together, moving from house to house, garden to garden. It is almost as if both families have 2 children.

Other elements which need one extra electron are Fluorine and Chlorine and these also form ionic bonds with lithium. We will be talking about these elements during weeks 9 and 17 respectively.

So what is Lithium like? Well, it is a metal and a solid, but unlike most metals it is very light - it is so light it can float on water. However if you put it on to water it will start to react - it will dart around and bubbles of hydrogen will be formed. As well as being very light it is soft and can be cut with strong scissors.

And its uses - the most common use for Lithium is in batteries.