Answer: The sum of an atom's protons, neutrons, and electrons, with protons equal to 1 AMU each, and electrons are 0 AMU each.
Explanation: By convention, atomic mass of an element is calculated by adding the masses of it's protons, neutons, and electrons. By definition, protons and neutrons are assigned values of 1 "atomic mass unit," or AMU, each, and electons are so small that they are assigned a value of 0 AMU each. Thus, a carbon 12 isotope atom (6 protons, 6 neutrons, and 6 electrons) would have a total of (6+6+0 =) 12 AMU. Carbon-13 has an additional neutron, so it's mass is (6+7+0)= 13 AMU. Molecules are the sum of each of it's individual atoms.
What is an AMU in actual mass? The definition states that 1 AMU is 1/12 the mass of a sinlge Carbon 12 isotope. By defining it in this manner, independent laboratories can calibrate their own instruments with a universal standard. Yes, even the Klingons can calibrate in their own galaxy, far, far away. For those physicists who know the value of even small amounts of mass, 1 AMU is equal to 1.66054x10^-24 grams. Before you scoff, just be reminded that E=mc^2, so while m, mass, may be small, c^2 (speed of light squared) is very large.