Compounds are pure substances that are combinations of two or more elements. The most basic unit of a compound is the molecule, which is a combination of two or more atoms of different elements. Different elements have different properties, and hence, form different kinds of compounds.
Elements have a property called electronegativity, which is the tendency of an atom of an element in a compound to attract electrons towards its nucleus. A more electronegative atom will attract electrons more. Hence, depending on the electronegativity difference, some compounds may be polar, while others not. For instance, `H_2` is non-polar since there are identical elements. Also, the `C-O` bond is more polar than the `C-N` bond due to higher electronegativity difference.
At some point, the electronegativity difference is so large that ions are simply formed.
A molecular compound is formed by sharing of electrons. Depending on the electronegativity, molecular compounds are either polar or non-polar. These kinds of bonds are called covalent bonds. Due to the presence of an actual bond, bonds in a molecular compound are directional - towards the nuclei of the atoms. Meanwhile, when the electronegativity difference is so large that ions are formed, we get ionic bonds in ionic compounds. Ionic bonds are not actual bonds, but are coulombic interactions between two full negative and full positive charges. Unlike the directional bonds in molecular/covalent compounts, ionic bonds are non-directional, and hence instead of forming discrete molecules, they form crystal lattices. In solution, if dissolved, molecular compounds will exist as individual molecules. On the other hand, ionic compounds will not be molecules, but rather solvated and separated ions.
A molecular compound is formed by sharing electrons to achieve an octet. They are usually between two nonmetals.
An ionic compound is formed by transferring electrons. They are usually between a metal and nonmetal. They are made of cations and anions.
The biggest thing to look at when trying to figure if something is a molecular compound or an ionic compound is the type of bonding. How are the atoms/elements bonding? Are they transferring electrons or are they sharing electrons?
In a molecular compound, the atoms share electrons to form a compound. The atoms share electrons because they both want to achieve a full octet (8 electrons).
In an ionic compound, the atoms transfer electrons to form a compound. Electrons are transferred because of the difference in electronegativity. One thing you need to understand about ionic compounds is that they are made of cations and anions. Anions are much more electronegative than cations. Thus, anions will be receiving the electrons while the cations give them away. In a sense, because the anions are so electronegative, they "take away" electron(s) from the cations, resulting in the transfer of electrons.
Ionic compounds are between cation and anions. They are usually between a metal and a nonmetal or a polyatomic ion. Ionic compounds are bonded with the cation "giving away" its valence electrons and the anion "receiving" them for a full octet.
Molecular compounds are usually between two or more nonmetals. The nonmetals simply share their valence electrons for a full octet.
When the difference in electronegativity of two atoms is greater than 1.7, the bond is ionic. When it is less than 1.7 but greater than 0.4, the bond is polar covalent. When the difference is less than 0.4, the bond is nonpolar covalent.
One difference between molecular compounds and ionic compounds is that molecular compounds are formed by the sharing of electrons, and ionic compounds are formed by the transfer of electrons. Molecular compounds result from covalent bonding and ionic compounds result from ionic bonding. Another difference is that a molecular bond is formed between two non-metals, and an ionic bond is formed between a metal and a non-metal.