History of the Universe

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Molecules

Life as we know it could never have arisen from material held together with ionic bonds. They are much too strong. No, life requires that atoms can easily join together and separate again, constantly forming new arrangements.

Fortunately there is a totally different way of joining atoms together. This uses a far weaker force (called a covalent bond) to hold the atoms together. These weak forces are easily made and broken.

However the atoms coming out of an exploding star were far too hot and moving far too quickly for these weak forces to hold them together. They were a little like excited children, running around, bumping into each other and bouncing away.

Imagine yourself as a teacher, trying to make these children hold hands. What could you do? What you need is some sort of stable base, something like a carpet where the children can sit down and be brought gently together. Luckily exactly this kind of platform existed in the shape of the newly formed stardust!

But what force could be used to make the free atoms stick to a grain of stardust?

van der Waals Force

We have already seen that atoms are surrounded by an electron cloud which carries a negative electric charge. The density of this cloud varies randomly with time, so that it is thicker sometimes on one side of the atom, sometimes on the other. When two atoms meet these changing electric fields can attract each other, producing a very weak force between the atoms. This weak force is called the van der Waals force.

Adsorption

At first the stardust was very hot and the van der Waals force was too weak to hold free atoms onto its surface. But the stardust quickly cooled and the van der Waals force made free atoms stick in a process called adsorption (from the verb “to adsorb” which means “to collect on a surface”).

Atom adsorbed onto stardust grain

Covalent Bond

After they had been adsorbed by stardust, atoms were still free to move across the grain’s surface. Recall that the electrons around the outside of an atom are confined to a thin shell. When two adsorbed atoms met, what happened depended upon how many electrons they contained in their shells.

Let’s first consider the simplest case. The simplest of all atoms is hydrogen. When two hydrogen atoms meet on a grain of stardust, their electron clouds overlap. Where the clouds overlap they are thicker, and their negative electric charge is stronger.

Two hydrogen atoms electron shells overlap and create an inward force

There is a proton at the centre of each hydrogen atom. Both protons feel a force of attraction towards this thick negative electron cloud, and so the two atoms are pulled together. However the nuclei can not approach too closely because each of their positive charges repels the other. The result is that the two hydrogen atoms are held together at a very specific distance. From now on they will form a single unit.

A group of atoms joined in this way is known as a molecule. This joining up of atoms is very lucky. Without it there would be no planets, no life, no people.

The force which holds a molecule together is stronger than the van der Waals force but not so strong as the ionic bond. It is called a covalent bond, and is the most important bond in this story.

Valence Number

Once the two hydrogen atoms were tied together, their two electrons were able to circulate around both the atoms. So both electron shells now contained two electrons! This had a very special consequence.

We have already seen that a helium atom contains two electrons in its shell. The result was that the helium shell was unable to absorb any more electrons, and so behaved as if it was hard. Well now the same thing happened with the two hydrogen atoms. They both acquired a hard shell, and would not allow any more electrons to enter their space.

The consequence is that one hydrogen can only form one covalent bond. This is called the valence or valence number of the atom.

Hydrogen Molecule

Two hydrogen atoms tied together with a covalent bond are called a hydrogen molecule. It is given the symbol H2, meaning there are two hydrogen atoms. From the outside we see the electron shells, like this:

Electron shell view of H2 molecule

When we think about larger molecules, we will sometimes be more interested in the bonds between their atoms than in their external appearance. So we will sometimes ignore the electron shells and show a ball and stick diagram to show their bonds, like this.

Ball and stick view of H2 molecule

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History of the Universe eBook
History of the Universe eBook
Only $2.99

Written by Wyken Seagrave
Copyright © 2017 Penny Press Ltd