The modern atomic weight scale is based on

Explore an atom's interior to discover the layout of its nucleus, protons, and electrons Atomic mass is expressed as a multiple of one-twelfth the mass of the −23 gram, which is assigned an atomic mass of 12 units. In this scale, 1 −24 gram. The atomic mass unit is also called the dalton (Da), after English chemist

Radioactivity pops up fairly often in the news. For instance, you might have read about it in discussions of nuclear energy, the Fukushima reactor tragedy, or the development of nuclear weapons. It also shows up in popular culture: many superheroes’ origin stories involve radiation exposure, for instance—or, in the case of Spider-Man, a bite from a radioactive spider. But what exactly does it mean for something to be radioactive? Radioactivity is actually a property of an atom. Radioactive atoms have unstable nuclei, and they will eventually release subatomic particles to become more stable, giving off energy—radiation—in the process. Often, elements come in both radioactive and nonradioactive versions that differ in the number of neutrons they contain. These different versions of elements are called isotopes, and small quantities of radioactive isotopes often occur in nature. For instance, a small amount of carbon exists in the atmosphere as radioactive carbon-14, and the amount of carbon-14 found in fossils allows paleontologists to determine their age. Atoms of each element contain a characteristic number of protons. In fact, the number of protons determines what atom we are looking at (e.g., all atoms with six protons are carbon atoms); the number of protons in an atom is called the atomic number. In contrast, the number of neutrons for a given element can vary. Forms of the same atom that differ only in their number of neutrons are called isotopes. Together, the numbe...

Modern atomic weight scale is based on C − 1 2. The standard unit for expressing the mass of atom is amu (atomic mass unit). It is equal to 1/12 of the mass of an atom of carbon-12 equal to 1 . 6 6 0 5 × 1 0 − 1 9 g. amu is also called as avogram. Avogram is a unit of mass and weight equal to one gram divided by the Avogadro's number.