Metal + nonmetal = covalent bond?

[pivoxa15]

In textbooks they normally emphasis the fact that when metals and nonmetals bond, the nonmetal takes away an electron from the metal resulting in ionic bonding betwen the two.

But metal and nonmetal can still result in covalent bonding when the electronegativities between the two are similar for example BiH3 could exist with covalent bonding and polarity? Also PbH4, TiH5, SnH4 even HgH6? How and where does it stop?

 

[Gokul43201]

The terminology is used very loosely, when talking about ionic or covalent bonds.

The more correct terminology only refers to the ionic character of a covalent bond. So the quantity describing the charge separation in the bond is called the percent ionic character (or the polarity). A purely covalent bond (e.g., H2, Cl2, N2, etc.) is said to have 0% ionic character and is also called a non-polar covalent bond. At the other end, is the (idealized) realization of an ionic bond, which has 100% ionic character - for instance, LiF comes pretty close to this limit. All other bonds lie somewhere in between, and are only labeled as covalent, ionic or polar covalent according to the labeler's tastes. They are all covalent bonds with some ionic character (i.e., polar covalent bonds).

http://www.chemistry.mcmaster.ca/esam/Chapter_7/problems.html

 

[ravenprp]

While it is true that in most cases, a bond between a metal and nonmetal results in ionic bonding, there are some exceptions where covalent bonding can occur. This is typically seen when the electronegativities of the two elements are similar, as you mentioned in the example of BiH3. In this case, the electrons are shared between the two atoms rather than being completely transferred from one to the other.

The examples you provided (PbH4, TiH5, SnH4, and HgH6) all involve elements with similar electronegativities, allowing for covalent bonding to occur. However, it is important to note that the strength of the bond and the extent of polarity will vary depending on the specific elements involved. For example, PbH4 may have a stronger covalent bond than SnH4 due to the larger difference in electronegativity between lead and hydrogen compared to tin and hydrogen.

It is difficult to determine where the line between ionic and covalent bonding stops, as it depends on the specific elements and their electronegativities. In general, the larger the difference in electronegativity, the more likely it is for an ionic bond to form. But as you have pointed out, there are exceptions to this rule. Ultimately, the type of bond formed between a metal and nonmetal will depend on the specific elements and their properties.