Electron configuration

For some atoms it is easier to combine with other atoms, because they can easily lose or gain electrons, but why are the electrons in some atoms more stable than in others?

We can compare the stability of the electrons in an atom's orbit with the strength of a wooden wheel:

	Primitive wheels were very heavy, but not very strong.
	Later, wheels became lighter and stronger (and easier to make and repair), thanks to radia.
	The more radia a wheel had, the lighter and stronger it was.
	In an atom, as in this wheel, there is usually more than one layer of electrons (only hydrogen and helium have a single layer).
	We can imagine electrons as nails holding together the radia to the wheel's axis and circumference: The wheel will be stronger if the nails are evenly spaced, that is, regularly distributed. The atom's electrons will be more stable if the outer layer has all the available spaces filled with electrons.

Remember that layers do not fill with atoms consecutively, but following the Aufbau principle (also known as "Moeller's rubric"):


Study the electron configuration of these atoms to understand it better:

Hydrogen → 1 electron → 1s1		Hydrogen has only one electron, so its outer layer is incomplete. It is therefore not very stable, and that is why it can combine with other atoms so easily.
Oxygen → 8 electrons → 1s2 2s2 2p4		Oxygen, another very reactive atom, can easily accept two electrons to complete its outer layer and become more stable.

How do atoms combine to form molecules?

Salt (sodium chlorine) → ionic bonding

Sodium releases an electron and becomes a positive ion; fluorine accepts it and becomes a negative ion. Both ions attract each other and form an ionic bond.

Water (H2O) → covalent bonding

Oxygen shares two electrons with two hydrogen atoms (in the picture, the hydrogen electrons are blue). This way, both elements have a complete layer of electrons.

(Metallic bonding is very different)

Why do some atoms not react with others?

Helium → 2 electrons → 1s2		The outer layer of Helium, in contrast to that of hydrogen, is complete, so it cannot easily combine with other atoms.
Ne → 10 electrons → 1s2 2s2 2p6		The same happens with other noble gases, like neon (10 electrons) or argon (18 electrons) and in general, with atoms which have 8 electrons in their outer layer.
Carbon → 6 electrons → 1s2 2s2 2p2		Carbon is very versatile because it can just as easily accept as lose up to 4 electrons to complete its outer layer. When it combines with four hydrogen atoms, it accepts four electrons from them, and it becomes methane. When it combines with two oxygen atoms, it gives them four electrons, and it becomes carbon dioxide.
Silicon → 14 electrons → 1s2 2s2 2p6 3s2 3p2		Silicon has similar properties to carbon: it can accept or lose up to 4 electrons to have 8 in its outer layer —8 electrons in the outer layer mean a very stable atom. When it combines with two oxygen atoms, it gives them four electrons, and it becomes silicon dioxide, or silica, which forms quartz rocks, a very strong and erosion-resistant material which is used to make glass.