The combination of two or more atoms to form bound units called molecules involves only the electrons - in fact just the outer electrons. Electrons can be transferred from one atom to another, or can be shared between two atoms. When this happens the nuclei remain separated by a distance typically around 10-10 m. But the range of the nuclear force is much smaller than that, so the nuclei never interact significantly in molecules, or any chemical processes.

Physical Properties

When a large number of atoms are held together to form a solid or a liquid (for example, copper, mercury, or liquid nitrogen), the forces holding them together also involve only the outer electrons of the atoms. Thus physical properties, like the hardness of a solid or the fluidity of a liquid, are determined only by the electrons.

Atomic Number

In general only one property of the nucleus affects the physical or chemical properties of matter, namely, the number of protons - since this determines the number of electrons. The number of protons is symbolized by Z, and is called the atomic number. Since historically all the elements were identified by their chemical and physical properties, we can say that each element corresponds to a given atomic number. Hydrogen is Z = 1, helium is Z = 2, iron is Z = 26, etc.


  • Atomic Number
  • Only outer electrons of an atom play a role in chemistry and in forming solids and liquids.