What does the number after an atom's name signify?

Answer 1

Many elements have what are called isotopes. An isotope is a variant of the element and has the same atomic number but a different atomic weight. The number after the name signifies a specific isotope. For example, carbon 14 means that the isotope of carbon that we are talking about has an atomic weight of 14. It has the same atoimic number (6) as any other isotope of carbon, but it has more neutrons in the atom than other isotopes. These others may have 12, or 13.

Uranium is another well-known element with more than one isotope. Uranium 238 is known for its use in atomic poer.

Answer 2

It probably means the number of nucleons (and therefore the approximate mass). If you're talking about, for example, the "235" in "uranium-235", then that's what it is.

If you're talking about the "2" in H2O, though, it means the number of H atoms in the molecule.

Those are the two most likely cases I can think of. If you were thinking of something else, you'd need to ask a more specific question (an example similar to the ones given above wouldn't hurt).

Answer 3

The number after an atom's name typically indicates the Atomic Mass or atomic number of that specific atom. It can also represent the isotopic mass if the atom is an isotope.