One of the isotopes of sulfur is represented as sulfur-35 or s-35. what does the number 35 signify

mass number

Explanation:

Here 35 is the mass number of sulphur. Mass number is equal to the number of protons and number of neutrons. The atomic number of S is 16. Atomic number is equal the  number of protons which is equal to the number of electrons.  

The electron configuration of S is 1s²2s²2p⁶3s²3p⁴

Number of neutrons in S is the difference between mass number and atomic number, which is  

Number of neutrons = mass number - atomic number = 35-16 = 19