Atom
The actual mass of an atom at rest is often expressed using the unified atomic mass unit (u), also called dalton (Da). This unit is defined as a twelfth of the mass of a free neutral atom of carbon-12, which is approximately . Hydrogen-1 (the lightest isotope of hydrogen which is also the nuclide with the lowest mass) has an atomic weight of 1.007825 u. The value of this number is called the atomic mass. A given atom has an atomic mass approximately equal (within 1%) to its mass number times the atomic mass unit (for example the mass of a nitrogen-14 is roughly 14 u). However, this number will not be exactly an integer except in the case of carbon-12 (see below). The heaviest stable atom is lead-208, with a mass of .
Atomic mass unit
The unified atomic mass unit and the dalton are different names for the same unit of measure. As with other unit names such as watt and newton, "dalton" is not capitalized in English, but its symbol, Da, is capitalized. With the introduction of the name "dalton", there has been a gradual change towards using that name in preference to the name, "unified atomic mass unit":