They would have to be atoms of the same element, and the same isotope of that element.
Atomic Mass
All atoms in a pure sample of one element have the same mass. The atoms in a different element have a different mass. Different atoms is what makes different elements.
isotope
Atoms of the same element have different isotopes, differing only in number of neutrons (so differing in mass number, atomic mass).
Atoms with the same atomic number are all atoms of the same element. However, if the atoms have different molecular weights, they are isotopes of the same element.
isotopes
Yes; isotopes of the same element have the same atomic number but differ in their atomic masses.
Yes. Their atomic mass is different, among other things.
Milk is a solution of many different atoms. Atomic mass only refers to the mass of a single type of atom. Milk may have a "molecular mass" but you need to determine all of the different atoms in milk and then add their masses together.
If they have the same atomic number but different mass, then they have a different number of neutrons, and they are called ISOTOPES.
Atoms with the same atomic number (number of protons), but different mass numbers (i.e. different mass) are called isotopes. The difference in mass is due to different numbers of neutrons. For more info: http://en.wikipedia.org/wiki/Isotope
Atomic number is the number of protons in the nuclei of the atoms of an element. Atomic number is unique to each element. Atomic Mass is the combined mass of the protons, neutrons, and electrons in an atom of an element.