The nuclear binding energy varies between nuclei.This increases the mass of nuclei with more neutrons than protons relative to the atomic mass unit scale based on 12C with equal numbers of protons and neutrons. The neutron is slightly heavier than the proton.There are two reasons for the difference between mass number and isotopic mass, known as the mass defect: For example, 63Cu (29 protons and 34 neutrons) has a mass number of 63, and an isotopic mass in its nuclear ground state is 62.91367 u. For other isotopes, the isotopic mass usually differs and is usually within 0.1 u of the mass number. One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically equivalent to 1 g/mol.įor 12C, the atomic mass is exactly 12u, since the atomic mass unit is defined from it. One atomic mass unit is equal to 1.66 x 10 -24 grams. The unit of measure for mass is the atomic mass unit (amu). Therefore this resulting atomic mass is calculated from naturally-occurring isotopes and their abundance. Note that each element may contain more isotopes. How does the atomic number determine the chemical behavior of atoms? Atomic Mass of Lead Since the number of electrons is responsible for the chemical behavior of atoms, the atomic number identifies the various chemical elements. Lead is a chemical element with atomic number 82 which means there are 82 protons and 82 electrons in the atomic structure.
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