Atomic Mass

Individual atoms are extremely small—a carbon atom weighs less than 2 × 10−23 g—making grams impractical for measurement. Scientists use atomic mass units (amu), Daltons (Da), or unified atomic mass units (u) instead. One amu equals 1/12 the mass of a carbon-12 atom, or 1.6605 × 10−24 g, providing a more convenient scale for atomic measurements.

The mass number is the sum of protons and neutrons in an atom. Since each proton and neutron contributes approximately one amu to atomic mass, and electrons contribute far less, the atomic mass of a single atom is approximately equal to its mass number. For example, nitrogen-14 has 7 protons and 7 neutrons, giving it a mass number of 14 and an atomic mass near 14 amu.

Most elements exist naturally as mixtures of isotopes with different neutron counts. The periodic table lists a weighted average mass calculated by multiplying each isotope's mass by its fractional abundance, then summing these values. For example, chlorine's average atomic mass of 35.45 amu reflects that 75.78% of chlorine atoms are chlorine-35 and 24.22% are chlorine-37, making the average closer to 35 amu.

A mass spectrum plots the relative abundance of isotopes versus their mass-to-charge ratios. The height of each peak is proportional to the fraction of ions with that mass-to-charge ratio, showing which isotopes are present and how abundant each one is. This data allows scientists to determine both the masses and natural abundances of isotopes needed to calculate average atomic mass.

Average atomic mass is calculated by multiplying each isotope's mass by its fractional abundance on Earth, then summing these products. For boron, 19.9% of boron-10 (10.0129 amu) plus 80.1% of boron-11 (11.0093 amu) yields the average. This weighted average reflects the actual composition of naturally occurring samples and appears on the periodic table.

No single chlorine atom weighs 35.45 amu because individual atoms are either chlorine-35 or chlorine-37. The value 35.45 amu is a weighted average of all chlorine atoms in nature. Since naturally occurring chlorine contains more chlorine-35 atoms than chlorine-37 atoms, the average mass is closer to 35 amu than to 37 amu.

A mass spectrometer vaporizes a sample and exposes it to a high-energy electron beam, creating charged ions by removing electrons. These cations pass through a variable magnetic field that deflects each ion's path based on its mass and charge. The detector then records the ions, generating a mass spectrum that reveals which isotopes are present and their relative abundances.