why do sodium and neon have vastly different chemical and physical properties despite having similar atomic masses?

Just because they have same atomic masses doesn't make them similar. They have different chemical properties because they are made of different things. And physical properties because they form different bonds with different atoms in the process.

On a basic level, the atomic mass tells you the number of protons an element has. It's elements in the same groups or periods that have similar properties i. e elements in the same group or period as Na will have similar properties; same for Ne

Sodium:

group 1: 1 electron on outermost shell alkali metal, highly reactive because it can easily lose the electron on it's outer shell one electron on outermost shell. Period 3 (3 orbitals/shells)

Neon:

Group 8: 8 electrons on outermost shells; all paired up noble gas, inert/unreactive because it's hard to get rid of all 8 electrons on outer shell compared to the 1 e Na has period 2 (2 orbitals/shells)