Differentiate an ionic from a covalent compounds

[Godwin Kessy]

I have come to a situation that I can hardly differentiate an ionic from a covalent compounds!
Before this confusion I had firstly known that whenever I find a compound with constituents of atoms with a very large difference in electronegativity, then i just conclude that its ionic, or when I find it in form of crystal, If it can dissolve in water the =n it is an ionic bond

But During my thoroughly studying of bonding I found most of metals forming covalent bond and just from there I couldn't straight way tell, unless I know the compound very well and have seen it probably!

Take an example of AlF₆, Fe₂O₃
Are these ionic or covalent compounds?

 

[Borek]

Welcome to the chemistry

Not every compound can be easily classified. In the case of every bond there is some "ionicity" and some "covalenticity" - if one character prevails, we can classify the bound as either covalent or ionic, but sometimes it is not possible.

 

[mazinse]

can the F and Al dissociate in water? same with Fe and O. If they can then it's ionic.

 

[Borek]

If they can then it's ionic.

Following this logic gaseous HCl is ionic as well. Rest assured it is not.

 

[mazinse]

Following this logic gaseous HCl is ionic as well. Rest assured it is not.

--
chemical calculators - buffer calculator, concentration calculator
www.titrations.info - all about titration methods

that rule for metal and nonmetal is no longer used in biochemistry.

 

[Borek]

No idea what you are referring to. There is one general chemistry that is base of both chemistry and biochemistry. Sometimes definitions used in different areas of science differ, but reality is always one - HCl doesn't behave differently in chemistry or biochemistry labs.

 

[alxm]

There's no black-and-white of covalent/ionic, unless you're practicing 19th century chemistry.

It's a sliding scale. Basically the amount of 'ionic' versus 'covalent' character can be quantified by the difference in the respective electronegativities of the atoms involved. For the definitive explanation of the concept, see Chapter 3 of Pauling's "The Nature of the Chemical Bond".

He brings up AlF₃ (which I think you mean?) Quoting the man:

[O]f the fluorides of the second-row elements:
Melting point: NaF 995C MgF₂ 1263 AlF₃ 1257 SiF₄ -90 PF₅ -94 SF₆ -51
those of the high melting points have been described as salts, and the others as covalent compounds; and the great change from aluminum to silicon fluoride has been interpreted as showing the bonds change sharply from the extreme ionic type to the extreme covalent type. I consider the bonds in aluminum fluoride to be only slightly different in character from those in silicon flouride, and I attribute the abrupt change in properties to the change of the atomic arrangement.

(He then explains that the first three are octahedral; each metal atom coordinates to a number of fluorines from another metal atom. But in SiF₄, each silicon atom coordinates only to its own four fluorines, and the whole thing is held together by weak van der Waals forces between the SiF₄ molecules.)


It's good to be able to tell if a bond is more ionic or covalent in character, but a strict distinction between covalent and ionic isn't possible, and therefore not very useful.