We consider atoms to be formed from three fundamental particles, we can determine the relative atomic mass using a mass spectrometer.
Amount of Substance
Chemists use this concept to count atoms.
Using this concept we can calculate reacting quantities, in a given reaction.
Bonding within molecules can be described as covalent, ionic or metallic.
Often a bond is a hybrid between ionic and covalent.
Trends within the periodic table can frequently be explained by concepts of nuclear charge and shielding.