WJ Question 3

A nitrogen atom with 5 bonds (-NH3- ) in a chain structure would bother modern chemists, but was reasonable in the 19th century. Considering NH4Cl, chemists of that era concluded that nitrogen can have a valence of 5. Thinking about modern Lewis structures, how many bonds does nitrogen usually form? Today we know about both covalent and ionic bonds. What is the bonding like in NH4Cl?

Werner identified two types of valences in his structures. For instance, in Co(NH3)6Cl3, the secondary valence is satisfied by the ammonia groups, which are held to the cobalt and do not form ions when the compound dissolves in water. The chlorines, which satisfy the primary valence, are easily removed and form insoluble silver compounds if silver nitrate is added. Think about ionic and covalent bonding . How would you explain these two types of valences using modern bonding theory?

Many compounds are hydrated. Their formulas are written in a way that is a throwback to the days of dualism --CoCl2·6 H2O, for instance. How would you explain the structure of this hydrate using Werner's coordination theory?

Covalently bonded compounds are held together by electrons shared between atoms. We describe covalent compounds with Lewis structures, in which chemical bonds (shared pairs of electrons) are represented by lines.

H2O
NH3

Ions in an ionic crystal are held together by the electrostatic attraction of the charges. In NaCl, for instance, there is no chemical bond holding one sodium ion to one chloide ion. Instead the ions are stacked together in a regular arrangement called a lattice.

Na+
Cl-

Each Na+ ion (yellow) is surrounded by six Cl- ions (blue). Each Cl- is surrounded by six Na+ ions.

Ionic compounds also can contain polyatomic ions, such as NH4+, CO32-, SO42-, OH-, and NO3-. Polyatomic ions stack as "balls" in the lattice similaly to ions of the elements.

Polyatomic ions stacked in an ionic crystal lattice can be represented by one "ball" just as ions of a single atoms can. For instance, the carbonate ions, CO32-, is a flat trigonal ion held together by covalent bonds between the C and O atoms.

Ca2+
CO32-