Strange things about BeF2

BeF2 is mainly covalent in nature. The central Be atom is linked with the two F atoms by very polar Be-F bonds.

Consider the case if BeF2 is ionic. Be^2+ carries 2 positive charges, and is very small in size as it has onlyone filled electronic shell. Therefore,the charge density of Be^2+ is very high, leading to very high polarizing power(i.e., the power of cation drawing back electron clouds from anion). Be^2+ draws back electron clouds from F^- toa great extent, and thus the bond between Be and F is a very polar covalent bond.

2012-03-19 02:57:44 補充：
LiF is ionic.
Although Li^+ ion has only one electronic shell, it carries one positive charge only. Besides, the size of Li^+ is greater than that of Be^2+. Therefore, the charge density and polarizing power of Li^+ is much lower than that of Be^2+, and thus LiF is ionic while BeF2 is covalent.

2012-03-19 03:05:04 補充：
AlF3 is covalent (molecular) in nature.
The charge density and polarizing power of Al^3+are greater, because Al^3+ carries 3 postive charges and its size is rather small.

2012-03-20 23:44:18 補充：
BF3 is covalent (molecular). Both B and F are classified as non-metals.

For easy understanding
just take a look to the EN difference.

2012-03-20 19:03:21 補充：
There are sth in between ionic and covalent
It is not true that we only divide compounds by ionic and covalent
still sth are mainly ionic ionic/covalent ....