CHEM103 Chapter Notes - Chapter 2.5: Unified Atomic Mass Unit, Mass Number, Carbon-12

• School: University of Alberta
• Department: Chemistry
• Course: CHEM103
• Professor: John Veinot

Document Summary

Composed of positively charged central nucleus surrounded by one or more negatively charged electrons. Electrons move rapidly within the available volume, held there by the attraction of the nucleus. Nucleus contributes 99. 97% of atom"s mass, occupied only about 1 quadrillionth of its volume, and is incredibly dense (~10^14 g/ml) Atom is neutral b/c the #of protons in the nucleus = # of electrons surrounding the nucleus. The atomic number (z) = # of protons in the nucleus. All atoms of an element have the same atomic number, and the atomic number of each element is different from that of any other element. Mass number (a) = total # of protons and neutrons in the nucleus of an atom. Atomic symbol (or element symbol) of an element is based on its. All atoms of an element have the same atomic number but not the same mass number. Isotopes have different numbers of neutrons and therefore different mass numbers.