A Chemical Compound is formed from two or more chemical elements acting as a single substance (stability). Compounds may be broken down into different chemical elements without dispersing into single atomic elements. Table Salt (NaCl) is a chemical compound formed from Sodium and Chlorine.
A Chemical Element is any substance containing only one atomic number and can be used together to form Compounds. Oxygen is a chemical element, as are hydrogen and nitrogen.
Salt is considered a chemical compound because it is a stable structure formed of two chemical elements. Without a chemical reaction, the salt will remain as it is, without breaking down or attaching other atoms or molecules to itself.
The Earth's Atmosphere containing a number of different chemical elements, including about 78% inert Nitrogen, 20% oxygen, 1% argon, and the remainder other gasses. Since the elements in the atmosphere can be separated without breaking up their atomic structure, as well as separated without any sort of chemical reaction, they are considered a mixture instead of a compound. The oxygen in the atmosphere does not bind to any other atom, and so remains a chemical element.