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$\ce{HCl}$ is a molecular compound because there is a covalent bond between $\ce{H+}$(a proton) and $\ce{Cl-}$ (a chloride ion)

$\ce{NH4NO3}$ is an ionic compound because there is an ionic bond between $\ce{NH4+}$ (an ammonium ion) and $\ce{NO3-}$ (a nitrate ion) even though all the atoms comprising the molecule are non-metal.

Then, which of the following is true?

$\ce{H3C-COOH}$ is an ionic compound because there is an ionic bond between $\ce{C2H3O-}$ (an acetate ion) and $\ce{H+}$ (a proton)? OR

$\ce{H3C-COOH}$ is a molecular compound because there is a covalent bond between $\ce{C2H3O2-}$ (an acetate ion) and $\ce{H+}$ (a proton)?

How do you tell if a bond between the two ions are molecular or ionic? (regardless of whether they are polyatomic or monoatomic ions)

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    $\begingroup$ A bond between 2 ions is always ionic, otherwise there would be no ions. HCl(g) has covalent bond, as there are no ions. OTOH, HCl(aq) means separated ions H+(aq) ( or H3O+(aq)) + Cl-(aq). $\endgroup$
    – Poutnik
    Apr 12 at 9:00
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    $\begingroup$ what ...? i don't get it. I heard someone said acetic acid is an ionic compound, and i thought it was molecular, we had a debate, the other guy said it dissociates in water to give 2 ions and so the bond would be ionic, and i gave him the case of HCl, and polyatomic ions. but couldn't be sure of the answer; if I had a place to ask this question, i probably have asked it to my teacher; which obviously isn't the case; $\endgroup$
    – briannjs
    Apr 12 at 9:17
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    $\begingroup$ There is a big difference between what compounds do in solution and how they behave as pure compounds. Some compounds are ionic as pure compounds (like sodium chloride and ammonium nitrate; some are only ionic in solution like HCl or strong organic acids). $\endgroup$
    – matt_black
    Apr 12 at 9:49
  • $\begingroup$ What's the point of us giving you the answer, if you already know it, but you can't be sure? Well, if this helps: you were right to mention HCl, which is covalent in pure form and ionic in a solution; so is CH3COOH. Generally speaking, no pure compound with H+ is ionic. $\endgroup$ Apr 12 at 9:53
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In principle, when studying a structure like HCl, CH3COOH, or any other compound, the first thing to do is drawing the symbol of the atoms in a reasonable geometry on a sheet of paper. Then consider the outer electron layers : put enough points around each symbol to get a reasonable Lewis structure. For example, one point near $\ce{H}$ and six points around $\ce{O}$. Then try to join atoms with their neighbors to make covalencies (or doublets). When all atoms are joined to neighbors, count the number of electrons around each atom, taking into account that the two electrons included in a covalence between A and B are at the periphery of both atoms A and B. Check all atoms. If an atom respects the octet law, don't modify it : its structure is covalent. If an atom A does not respect octet law, try to remove one electron from its outer layer, and send this electron to the B atom, which becomes negatively charged. The covalency disappears from the region between A and B, and the bond AB becomes ionic with a positive charge on the A atom, which becomes $\ce{A+}$, and a negative charge on $\ce{B}$ which becomes $\ce{B-}$.

Using this method, you will see that $\ce{HCl}$ and $\ce{CH3COOH}$ are both made of covalencies, because all their atoms are respecting the octet rule. You will also see that $\ce{NaCl}$ may first be drawn in a covalent structure. But then the sodium atom does not fit the octet rule. It will if an electron is removed from the sodium atom, to be given to the chlorine atom, producing then $\ce{Na+}$ and $\ce{Cl-}$ ions.

Hydroxides contains two sorts of bonds. Hydroxides like $\ce{NaOH}$ are made of covalent bonds between $\ce{O}$ and $\ce{H}$ atoms, and of ionic bonds between $\ce{Na^+}$ and $\ce{O-}$.

Oxygenated acids are a special case. They may get two sorts of bonds between $\ce{A}$ and $\ce{B}$. For example sulfuric acid $\ce{H2SO4}$ can be drawn with this method to give first a covalent structure with two bonds $\ce{H-O}$, two bonds $\ce{O-S}$, and two double bonds $\ce{S=O}$. This is OK for all atoms except the central Sulfur atom, which is surrounded by six doublets or six covalences. At this point, some chemists are thinking that it is a case of hyper valence. Some think that the octet rule may be obtained by removing the second bond from each double bond from the sulfur atom, and sending it to the outer Oxygen atom. This makes a central sulfur atom charged $2+$ (respecting the octet rule) and two oxygen atoms charged $1-$. Both theories can be discussed.

The only problem with this reasoning is the fact that it does not work properly for atoms of the third column. For example, $\ce{BF3}$ should be ionic making $\ce{B^{3+}}$ and $\ce{F-}$ ions. Indeed $\ce{BF3}$ remains covalent, and it is hard to justify. Maybe, on may admit that Boron atom prefers to stay covalent rather than to loose three electrons. Three is probably too much.

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TL;DR The correct answer is: $\ce{H3C−COOH}$ is a molecular compound because there is a covalent bond between $\ce{C2H3O2−}$ (an acetate ion) and $\ce{H+}$ (a proton).

Compounds consisting of non-metals are usually considered molecular (or covalent compound). The given compound $\ce{H3C−COOH}$ is acetic acid (or ethanoic acid) and consists of only non-metals, i.e, Carbon, Hydrogen and Oxygen. There is a difference of less than 2.0 in electronegativity on Pauling scale between the Carbon and Hydrogen. This difference results in an electrons being shared between the non-metals. The shared pairs or bonding pairs is covalent bonding.

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The Carbon–hydrogen bond and the Carbon–carbon bond are covalent bond as the carbon shares its outer valence electrons with up to four hydrogens/carbons, as shown in the Lewis structure. Infact, all the carboxylic acids having $\ce{R−COOH}$ are covalent.

In general, covalent compounds do not conduct electricity when dissolved in water. Though acetic acid is covalent it dissociates into acetate ions and hydrogen ions. Acetate ions are responsible for conducting electricity Thus, acetic acid is molecular (or covalent) as a pure liquid (and doesn't conduct) BUT, when it is dissolved in water, they become Ionic Solutions (some of the molecules break apart and form ions).

Please note that bonding is a scale, with ionic on one end and covalent on the other. Bonding has both ionic and covalent character. It really just depends where the bonding electrons are located - between the atoms (covalent), or on the negatively charged ion (ionic).

I also found these videos which may be helpful: Is CH3COOH (Acetic acid (Ethanoic acid)) Ionic or Covalent/Molecular? and CH3COOH Lewis Structure (Acetic acid) by Wayne Breslyn. Learn more about covalent bonds at Khan Academy and Wikipedia.

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