Here's why:

* Reactivity is determined by the arrangement of electrons, particularly valence electrons. These are the electrons in the outermost shell of an atom and are responsible for chemical bonding.

* Atomic mass is primarily determined by the number of protons and neutrons in an atom's nucleus. While the number of protons defines the element, the number of neutrons can vary (creating isotopes), but this doesn't directly influence reactivity.

Factors that actually influence reactivity:

* Electronegativity: This measures an atom's tendency to attract electrons in a bond. Highly electronegative elements are more likely to gain electrons and form anions.

* Ionization energy: This is the energy required to remove an electron from an atom. Elements with low ionization energies easily lose electrons and form cations.

* Electron affinity: This is the change in energy when an electron is added to a neutral atom. Elements with high electron affinity readily gain electrons.

* Number of valence electrons: Elements with a nearly full or empty outer shell are more reactive than those with a half-filled shell.

Example:

* Lithium (Li) has a low atomic mass (6.941 amu) but is highly reactive because it has only one valence electron, which it readily loses to form a positive ion.

* Gold (Au) has a high atomic mass (196.967 amu) but is relatively unreactive due to its full outer shell of electrons.

In summary, while atomic mass provides information about the nucleus of an atom, it does not directly determine its chemical reactivity. Reactivity is primarily dictated by the arrangement and behavior of an atom's electrons.