Brownian Motion Revisited

This is an updated post  – I’ve deleted the original one.

Nothing’s new under the sun…we are told. In about 60 BCE, the poet Lucretius wrote…

“Observe what happens when sunbeams are admitted into a building and shed light on its shadowy places. You will see a multitude of tiny particles mingling in a multitude of ways… their dancing is an actual indication of underlying movements of matter that are hidden from our sight… It originates with the atoms which move of themselves.”

Robert Brown, a doctor and a botanist, was extremely ticked off. No matter how hard he tried to twiddle the focus knob on his fancy new microscope – they weren’t very good in 1827 –  the pollen grains he was trying to look at wouldn’t STAY IN FOCUS! They seemed to have a mind of their own, jiggling about as if being pushed around by even tinier little particles. At first, he thought the pollen grains were alive. In fact, they were being pushed around by air molecules. Because they were so small, the colliding air molecules could actually move the pollen grain.

Imagine it like this.. a really big balloon, 10m in diameter in a football stadium. The balloon is so large that it sits on top of lots of fans in the crowd at the same time. Because they are excited, these fans hit the balloon at different times and in different directions with the motions being completely random. In the end, the balloon is pushed in random directions, so it should just be jiggled about. Consider now the force exerted at a certain time. We might have 20 supporters pushing to the right, and 21 other supporters pushing left, where each supporter is pushing with the same force. In this case, the forces exerted from the left side and the right side are unbalanced in favour of the left side so the balloon will move slightly to the left. A bit later on, the number of pushers and their direction might randomly change so causing random motion of the balloon. If we imagine an observer in a helicopter looking down at this situation from far above, he can’t see the supporters, all he sees is the big balloon jiggling randomly about.

It was dear old Uncle Albert who finally came up with the best solution to this problem – here he is again, riding a bike

Brownian motion is the most important piece of evidence for the random movement of gas (and liquid) molecules

This little Java applet shows how it works

Here’s a picture of Dr Brown. A face for radio, perhaps….brnrobt

Covalent Bonding

Covalent bonds are formed when electrons are shared. You might wonder why chlorine only ‘comes as a molecule’ of chlorine,  Cl2.

Both atoms have one space to fill so they both have the same desire to collect an electron. We say that they have the same ‘electronegativity’.  Ionic bonds are formed when electronegativities are very different.  Covalent bonds are formed by atoms having similar electronegativities. The only way round this is if each chlorine shares one each of its outermost unpaired electrons.

If we imagine the electrons like to be paired, (examiners like you to draw them as pairs, incidentally) there’s three pairs in each outermost shell of a chlorine atom and one lone electron left over, which can be shared with another lone electron from another chlorine atom, making a covalently bonded molecule.

This Word document is a very comprehensive handout and you need to learn the major points thoroughly.

Print it off , add it to your notes. Here’s a dot-cross diagram showing the structure of methane. Four hydrogen atoms each share their one electron with a carbon atom having four spaces

Try drawing dot-cross diagrams to show the following covalent bonds, hydrogen, fluorine, carbon dioxide, carbon tetrafluoride.

Here are the solutions… Please learn them

It’s easy to name covalent compounds. This link is really easy to follow and there’s some useful stuff on properties also.

There’s also a section in the document on giant covalent structures,  in particular diamond, graphite and silicon dioxide. ‘Giant’ here dosn’t mean’big’ – it just means lots and lots of the same repeating structure – having  high melting and boiling  points.

Ionic Bonding (Updated)

Metals donate (give away) electrons, non-metals receive or accept them. The objective for both is to finish up with full shells, so elements with less electrons than spaces in the outermost shell  will tend to give them away, and elements with less spaces than electrons will tend to receive them.

It’s all here….. ionic-bonding is a Word document that walks you through all the basic ideas.

Here is the ionic bonding in calcium chloride. Ca is in Group 2 and has 2 outermost electrons to give away to 2 chlorine atoms, in Group 7 with 1 space each.

In exams, when you’re asked to draw diagrams like this, you must show where the electrons go with arrows like in the diagram. Ionic compounds form giant (lattice-like)  structures which conduct electricity when either molten or dissolved because the ions are charged and are mobile. They have high mp’s and bp’s because a lot of energy is required to break a lot of bonds.