How do scientists distinguish between an element and a compound? list two elements and two compounds.

Element and compound , and or heterogeneous and homogeneous

A compound is a substance consisting of atoms or ions of two or more different elements in definite proportions joined by chemical bonds into a molecule. The elements can not be separated by physical means. Water, for example, is a compound having two hydrogen atoms and one oxygen atom per molecule. An element is a substance consisting of atoms which all have the same number of protons - i.e the same atomic number. Elements can not be broken down using chemical methods.

Elements:
1. Magnesium (Mg)
2. Calcium (Ca)

Compounds:
1. Sodium chloride (NaCl)
2. Hydrogen chloride (HCl)