Why does hydrogen have an ionic bond with flourine but only a covalent with carbon??

Question

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Answer 1

Actually H and F DO form covalent bonds! The bond HF is just very polar.

Answer 2

Carbon doesn't ionize (well, for all practical purposes).  That makes ionic bonds pretty hard to make.  Flourine ionizes the easiest of any element.  That make ionic bonds extremely easy to make.

Answer 3

Electronegativity.

(How are there five answers and nobody has used that word yet?)

There are different ways to measure and define that term, and different ways to rationalize the effect, but qualitatively it's this: the ability of an element to attract electrons to itself.

If you have two elements of equal electronegativity, they share electrons equally, and you'll have a perfectly covalent bond.  

If you have two elements of slightly different electronegativity, the electron cloud associated with the bond will be polarized toward the element with the higher EN value, so one atom bears a partial negative charge and the other a partial positive.  That leads to a small polarity across the bond, but not so much that electrons are not still being shared -- they're just shared somewhat unequally, resulting in a polar covalent bond.

If you have two elements of dramatically different electronegativity, you completely transfer electrons from one atom to another, form a cation and an anion, and form an ionic bond.

How much of an EN difference gives you each type of bonding?  Obviously, there's a continuum of polarities, but one commonly used formula is that percent ionic character = 1 – e^(0.25*delEN^2), where delEN is the electronegativity difference.  This gives you 50 percent ionic at an electronegativity difference of 1.7.  So let's look some some electronegativity values on any decent periodic table.

(Dozens online, I use webelements.com)

Pauling values are usually listed as H: 2.2, C: 2.5, F: 4.0.  So the HF bond has a difference of 1.8, while that of the C-F is 1.5.  That means the C-F bond is 43 percent  ionic: highly polar, but still principally electron sharing.  That of the H-F is 56 percent ionic.  

Now, most chemists wouldn't call HF an ionic compound.  It's a gaseous molecule at room temperature, it doesn't contain discrete H+ and F- ions, the way e.g. NaF does (EN difference of 3.1, 91 percent ionic).  It's highly polarized, and the ionic attraction contributes as much or more to the bond strength as the covalent overlap, but it's not a completely ionic bond -- just mostly so.

Answer 4

All chemical bonds are a sharing of electrons between two atoms.    The difference between ionic and covalent is the amount of time sharing.

Ionic bond describes a relationship where one atom dominates and hogs the shared electron the majority of the time.    For hydrogen - fluorine bond, the fluorine atom is greedy, steals the lone hydrogen electron and holds it the vast majority of the time, like 99.9999%     The fluorine atom is greedy because without the extra electron, it is missing one electron from a perfect 8 set on it's outer shell.     It demands to have a perfect set of 8 and will steal any available electron from it's neighbors to complete the set.

The Carbon atom is a different character.    It has only 4 of 8 electrons in it's outer shell.    Carbon is indifferent about gaining or losing an electron with a neighboring atom, especially hydrogen.    In a carbon-hydrogen bond, the two are happy neighbors and will share the hydrogen's electron at near 50 / 50 time split.    The electron happily wonders back and forth between the hydrogen atom and the carbon atom.     The bond is covalent.

Answer 5

Fluorine is THE most electronegative element, so it wants the electrons all to itself instead of just sharing.

Carbon is less demanding and is happy just sharing with the H.

I can try explaining in a different way if you need, but it may not work as well

Answer 6

Fluorine loves electrons much much much more than carbon. Fluorine loves electrons so much, it'll try to steal from its neighbors.