Molecule Mass and Weight

This chapter provides quick introductions about molecule mass and weight.

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What Is the Mass of an Atom

Measured Mass of Atomic Isotopes

What Is Relative Atomic Mass

What Is Mass Number of Element

What Is Mass Defect or Mass Excess

What Is Molecular Weight

What Is Exact Mass of Molecule

What Is Nominal Mass of Molecule

Takeaways:

• Atomic Mass Unit or "u" is a unit of measure for mass defined as: 12 u = The mass of 12Carbon atom.
• The mass of an atom is measured, not calculated.
• The mass defect of an atom is the mass lost when protons and neutrons fuse together to form the atom.
• The mass defect of an atom can be calculated as the difference of the measured mass and the sum of mass of all neutrons, protons and electrons in the atom.
• Atomic Isotopes are atoms that have the number of protons but a different number of neutrons in their nuclei.
• The Isotopic Abundance of an atomic isotope is the estimated percentage of the isotopes among all isotopes of the same element observed in our Solar system.
• The Relative Atomic Mass, or Atomic Weight, is the average mass of all isotopes weighted by their isotopic abundances of a given element.
• The Relative Atomic Mass is usually included in the periodic table for each element.
• The Mass Number of an atom is the integer rounded from its measured mass.
• The Mass Number of an element is the integer rounded from its relative atomic Mass.
• The Molecular Weight of a molecule is the sum of Relative Atomic Mass of all elements in a given molecule.
• The Exact Mass an element is the sum of measured mass of the most common atomic isotope of each element in a given molecule.
• The Nominal Mass of a molecule is the sum of mass numbers of all elements in a given molecule.