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

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)

Element and compound, and or heterogeneous and homogeneous