The atomic number is the number of protons in the nucleus. An atom is the smallest particle of an element that maintains the chemical identity of that element. The number above the element symbol on the periodic chart is the atomic number. Some elements exist in pure form as polyatomic molecules. All atoms of a given element have identical chemical properties, which differ from those of other elements.
Compounds form when masses of different elements combine in small whole-number ratios.
The relative numbers and kinds of atoms are constant in a given compound. Under normal conditions nitrogen exists as a diatomic molecule. An ionic solid always consists of cations and anions in equal numbers. FeSO 4 20. A molecule is the smallest part of a compound that can have a stable independent existence.