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How is molecular weight calculated from atomic weights?

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representations of molecular structure — molecule Several methods of representing a molecule's structure. In Lewis structures, element symbols represent atoms, and dots represent electrons surrounding them. A pair of shared electrons (covalent bond) may also be shown as a single dash. The ball-and-stick model better illustrates the spatial arrangement of the atoms. For aromatic compounds, the Kekulé structure is common, in which each bond is represented by a dash, carbon atoms are implied where two or more lines meet, and hydrogen atoms are usually omitted. Bond-line formulas, similar to the Kekulé structure, are often used for complex nonaromatic organic compounds. Sugars are often drawn as Fischer projections, in which the carbon “backbone” is drawn as a straight vertical line, with carbon atoms implied where horizontal lines intersect the vertical one.

Molecular weight is calculated by summing the atomic weights of all the atoms present in a substance's chemical formula.12

This calculation is based on the standard scale where the atomic weight of carbon-12 is defined as 12. The resulting value is expressed in atomic mass units (amu) or daltons (Da). For example, a molecule of water ( $H_{2} O$ ) consists of two hydrogen atoms (each approximately 1.008 amu) and one oxygen atom (approximately 15.999 amu), resulting in a total molecular weight of 18.015 amu.13

For substances that do not exist as individual molecules, such as ionic compounds like sodium chloride, this same summation process is used to determine the formula weight. While molecular weight is a theoretical calculation based on atomic composition, experimental techniques like mass spectrometry can also be used to determine the mass of a molecule by analyzing its ionized fragments.14