Which of the compounds, C3H8, MgCl2, Zn(NO3)2, OCl2, are expected to exist as molecules?
The answer is to do with electronegativity difference. Metals generally have low values and non metals higher values. When an element with a low value combines with an element with a high value, usually the element with the low value LOSES electrons to form a positive ion while the other element gains the electron(s) and forms a negative ion. So an ionic compounds is formed. Compoiunds of this type usually contain a metal elements joined to non-metal element - so compounds 2 and 3 in your list are ionic.
When the compound is made from only non-metal elements, electrons are not lost or gained but shared as these elements tend not to lose electrons or gain them. Instead they share electrons to make covalent bonds. If the result are small structures they will be molecular eg 1 and 4 in your list.
Sun, 2011-09-04 17:31