Science

The core of any chemical bonding as well as chemical and physical properties lie within the nucleas of the atoms consisting in the compound.
The nucleas contains protons and nutrons and other elementary particles. The protons in the nucleas determines the chemical properties of an element.
The number protons and nutrons together determines the mass of an atom, thus determines the properties like electron affinity etc. These properties are very much inportant in forming chemical bond in a compound.
Thus atomic number(no of protons in the nucleas) and mass number(number of protons and nutrons together) are two basic parameter determining the properties of a compound.