The accepted model of an atom consists of a central positive nucleus surrounded by a cloud of electrons and the electrons are associated with particular energy levels, Figure 1.

Atom, with diameter of 10⁵ times that of its nucleus, is almost empty space. It is the electrons that give an atom its size and it is the nucleus which gives its mass (i.e about 2000 times that of an electron). What goes on in the nucleus of an atom has almost nothing to do with atom's Chemistry and vice versa.

Key Terms

• Nucleons
• Strong force
• Energy level