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What happens in covalent bonding?
In pairs of electrons are shared between two non-metal atoms. The two atoms share to fill outer energy shells.
Because it takes so much energy for an atom to give away electrons, these atoms choose to share them.
An example of a covalent bond between atoms is in water. Hydrogen and Oxygen are both non-metals. In order for the atoms to bond and create a water molecule they need to share electrons.
The oxygen atom will share one of it's electrons with each of the two hydrogen atoms. Each hydrogen atom will share its one electron with the oxygen atom. In doing this each atom has a full valence shell.
Very good applet are on this site: http://www.pbslearningmedia.org/resource/lsps07.sci.phys.matter.covalentbond/covalent-bonding/