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# Atomic Structure and Isotopes An atom has: * Protons (+) * Electrons (-) * Neutrons (0) The nucleus contains protons and neutrons. Electrons orbit the nucleus. **Isotopes** Isotopes of an element have the same number of protons (Z), electrons (e⁻) and atomic number (Z) but different num...

# Atomic Structure and Isotopes An atom has: * Protons (+) * Electrons (-) * Neutrons (0) The nucleus contains protons and neutrons. Electrons orbit the nucleus. **Isotopes** Isotopes of an element have the same number of protons (Z), electrons (e⁻) and atomic number (Z) but different numbers of neutrons (N) and mass number (A). * Z = atomic number * A = mass number * A = Z + N **Calculating Average Atomic Mass** The average atomic mass (Avg A) of an element is calculated by taking the weighted average of the atomic masses of its isotopes. * Avg. A = ( %1 * mass1) + ( %2 * mass2) +... **Example: Carbon** * Carbon-12: 98.99% 12 amu * Carbon-13: 1.11% 13.003 amu Avg Atomic Mass of Carbon = (0.9899 * 12) + (0.0111 * 13.003) = 12.011 amu