Differences Between Compounds, Molecules & Mixtures

 Mainly elements fall into 3 categories, Molecules, Mixtures and Compounds.

Molecules refer to elements bonded via chemical bonds, a good example of this is Oxygen, which is bonded together so they are classified as a molecule, but these molecules can also be made of multiple different elements, such as water which is H20, others being Hydrogen, Chlorine and Carbon-dioxide. Molecules need 2 or more elements in order to be considered a molecule.

A Compound is made out of two elements held together by chemical bonds, for example, water is also considered a compound because it contains hydrogen and oxygen.
Along with carbon dioxide because it is made out of oxygen and hydrogen. But water, chlorine and hydrogen are not considered compounds because they only have one unique element.
Another thing about compounds is that they are found in the same proportions, so you wouldn't see a water compound with one more hydrogen. Water will always have two hydrogen atoms and one oxygen atom. And it is consistent and never changing.
Since it is consistent water will always be H2O. Now the smaller 2 is written in a "subscript" which signifies that the H in the water molecule is that there are two hydrogen atoms, and carbon dioxide is always going to be CO2, the 2 being that there are two oxygen atoms. 

Another element is H2SO4, which always has 2 Hydrogen (H2), one Sulfur (S), and four Oxygen (O4).
But for some elements, Calcium for example: Ca(OH)2, the calcium is made out of one Calcium atom (Ca), and two of OH or (OH)2. 
These atoms are actually very small as some atoms contain up to a billion atoms, like table salt or Sodium Chloride which has the elements NaCl.  
But unlike other atoms, sodium chloride has as large structure compared to other atoms, sodium chloride has an Ionic bond, and the NaCl or table salt is a 1:1 ratio, so that if there's a Na atom there is always a Cl atom.

Mixtures are substances that are not bonded together. So if we combine sodium chloride, oxygen, individual helium atoms, and carbon dioxide this would be considered a mixture,