How does covalent bonding differ from ionic bonding?
In a covalent bond electrons are shared between two atoms. In an ionic bond electrons are transferred from one atom to another, forming a postive and a negative ion. The ions are held together by the electrostatic attraction between the opposiste charges.
An ionic bond forms between a metal and a non-metal, because metals lose electrons and non-metals gain electrons. Each metal atom tends to lose the number of electrons that leaves it with a complete octet, or eight electrons in its outer level. The group one elements such as lithium and sodium have one valence or outer electron so by losing this electron they have a complete octet in the next lower energy level. This makes them very reactive. The group 2 elements have two valence electrons, both of which they lose easily. The non-metals achieve a complete octet by gaining electrons. For example, the group 7A elements such as fluorine and chlorine have 7 valence electrons so they react easily to gain one.
Atoms that form covalent bonds also achieve a complete octet. For example, oxygen exists as the molecule O2 with a double covalent bond because each oxygen atom has six valence electrons. When two pairs of electrons are shared between them they have a complete octet. The shared electrons are considered valence electrons for both atoms.
Covalent bonds in which the electrons are shared unevenly are called polar covalent bonds. In water, oxygen forms polar covalent bonds to the two hydrogen atoms. The electrons are more attracted to oxygen than hydrogen because oxygen is more electronegative.