Would you expect each atom to have the same mass?

No. I would expect each atom of any particular compound to have similar but not the same mass. This is because, although each atom will have the same number of protons and electrons, they can have different numbers of neutrons. The mass of an atom is almost entirely dependent on the number of protons and neutrons.

Tin, for example, has atomic number 50 so that it has 50 protons and 50 electrons. But is has 10 stable isotopes:

• Sn 112 - with 62 neutrons
  
• Sn 114 - with 64 neutrons
  
• Sn 115 - with 65 neutrons
  
• Sn 116 - with 66 neutrons
  
• Sn 117 - with 67 neutrons
  
• Sn 118 - with 68 neutrons
  
• Sn 119 - with 69 neutrons
  
• Sn 120 - with 70 neutrons
  
• Sn 122 - with 72 neutrons
  
• Sn 124 - with 74 neutrons

In addition, there are 29 known non-stable isotopes, including Sn 126 which has a half-life of approx a quarter of a million years.

Six of the stable isotopes have an abundance of 5% or more so it is quite likely that atoms of tin have different masses.