f.4 CHEM

Ammonium sulphide : covalent compound
Hydogen bromide : covalent compound
Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds. In short, attraction-to-repulsion stability that forms between atoms when they share electrons is known as covalent bonding.
Covalent bonding includes many kinds of interactions, including σ-bonding, π-bonding, metal-metal bonding, agostic interactions, and three-center two-electron bonds.[1][2] The term covalent bond dates from 1939.[3] The prefix co- means jointly, associated in action, partnered to a lesser degree, etc.; thus a "co-valent bond", essentially, means that the atoms share "valence", such as is discussed in valence bond theory. In the molecule H2, the hydrogen atoms share the two electrons via covalent bonding. Covalency is greatest between atoms of similar electronegativities. Thus, covalent bonding does not necessarily require the two atoms be of the same elements, only that they be of comparable electronegativity. Because covalent bonding entails sharing of electrons, it is necessarily delocalized. Furthermore, in contrast to electrostatic interactions ("ionic bonds") the strength of covalent bond depends on the angular relation between atoms in polyatomic

圖片參考：http://upload.wikimedia.org/wikipedia/en/f/fd/CovalentBond.png


An ionic bond (or electrovalent bond) is a type of chemical bond that can often form between metal and non-metal ions (or polyatomic ions such as ammonium) through electrostatic attraction.
The metal donates one or more electrons, forming a positively charged ion or cation with a stable electron configuration. These electrons then enter the non metal, causing it to form a negatively charged ion or anion which also has a stable electron configuration. The electrostatic attraction between the oppositely charged ions causes them to come together and form a bond.
For example, common table salt is made of sodium chloride. In the ionic bonding, the sodium (Na) loses one electron, forming a cation, and the chlorine (Cl) gains an electron to form an anion. These ions are then attracted to each other in a 1:1 ratio to form sodium chloride (NaCl).

圖片參考：http://upload.wikimedia.org/wikipedia/en/7/75/Ionic_bonding_animation.gif

In chemistry, an ionic compound is a chemical compound in which ions are held together in a lattice structure by ionic bonds. Usually, the positively charged portion consists of metal cations and the negatively charged portion is a halogen or polyatomic ion. Ions in ionic compounds are held together by the electrostatic force between oppositely charged bodies. Ionic compounds have a high melting and boiling point, and they have a high hardness and are very brittle.
Ions can be single atoms, as in common table salt sodium chloride, or more complex groups such as calcium carbonate. But to be considered an ion, they must carry a positive or negative charge. Thus, in an ionic bond, one 'bonder' must have a positive charge and the other a negative one. By sticking to each other, they resolve, or partially resolve, their separate charge imbalances. Positive to positive and negative to negative ionic bonds do not occur. (For a real world analogy, experiment with a pair of bar magnets.)
Chemical compounds are rarely strictly ionic or strictly covalent. Except for the most electronegative/electropositive pairs such as cesium fluoride, ionic compounds usually exhibit a degree of covalency. Similarly, covalent compounds often exhibit charge separations. See also HSAB theory.