Lets take a different example. The reaction between Sodium (Chemical Symbol: Na) and Chlorine (Chemical Symbol: Cl).

Sodium (Na) has 11 protons, thus 11 electrons (we know from the Atomic Number). The electrons will be arranged in shells: 2 in the first shell, 8 in the second shell, and 1 in the outer shell. We write this electron arrangement in this format: 2,8,1.

Chlorine (Cl) has 17 electrons: arranged 2,8,7.

Chlorine (2,8,7) needs one electron to fill its outer third shell to the ideal of 8, Sodium (2,8,1) would be better to lose the electron in its outer third shell, and allow the full second shell of 8 underneath to become its outer shell.

Sodium needs to lose an electron, and chlorine needs to gain one - these two atoms are a very compatible pair. When a reaction takes place between them, there is no sharing of electrons, sodium gives away one electron to chlorine.

However, this introduces something new. Sodium now has 11 protons, but only 10 electrons (because it has given one away). Chlorine has 17 protons, but now has 18 electrons (as it has taken an extra one from Sodium). The effect of this is easy to calculate - it leaves the atoms with a charge - they are no longer neutral.

The Na (Sodium) with one more positive proton than negative electrons is positively charged, and the Cl (Chlorine) with one extra negative electron is negatively charged. However as a pair, they are still neutral (the sodium is "+1" and the chlorine is "-1").

Any atom that carries a charge is called an ION (pronounced "eye on"), and ions can be positive or negative. The ions in this example would be written as Na+ and Cl-. If sodium lost another electron, it would become Na2+ and so on.

Because our Na+ and Cl- ions carry opposite charges, they are attracted together as a pair. They are bonded by ionic attraction. This type of bonding, where atoms give away or accept electrons to form a pair of charged ions, is called IONIC bonding. The compound formed when Na and Cl react is written NaCl. This is Sodium Chloride, which is more commonly known as salt. The next time you sprinkle some on your dinner, you will know exactly how it is formed.

When atoms have an ideal number of electrons in their outer shell, we know that they are stable and unreactive. But the closer that they are to being ideal, the more reactive they are. Atoms that are just one electron away from ideal are very reactive. It's almost as if they know that the ideal shell is within their reach, so they are very keen to achieve it.

The sodium in our last reaction needed to lose just one electron, so we can predict that sodium is very reactive. The chlorine needed to gain just one electron, so again we can predict it is very reactive.

Because these atoms are both very reactive, and are also perfect partners (one needs to lose an electron, and one needs to find an electron), we would expect the reaction between them to be very fierce - and it most certainly is. The reaction between sodium and chlorine is so violent, that if you simply mixed the two together there would be a huge explosion.