# When talking about atomic mass, how is $E=mc^2$ factored in? [duplicate]

When talking about atomic mass in the periodic table of elements, is this number the mass of the element at rest?

If I understand correctly, the (relativistic) mass of an element will increase as the speed of that element increases, until the (relativistic) mass reaches infinity at the speed of light, right?

Now, the mass of an object is, as Einstein has shown, equivalent to the total energy content of that object in rest up to a conversion factor of $c^2$. The reason why $c^2$ appears here is because we use incompatible units for lengths and time intervals and then the formulas will have to compensate for our bad habits. The meaning of $E = m c^2$ is made visible much better in natural units where $c = 1$; obviously $E = m$ conveys far more clearly that mass and rest energy are one and the same thing (so, it's not that energy can be converted to mass or vice versa).
"Mass" nowadays almost always refer to rest mass $m_o$. One might talk about the inertia $m_o\gamma=m_o/\sqrt{1-v^2/c^2}$ depending on speed, but not the intrinsic property mass.