US - Science - Chemistry

The ratios known as “parts per million” (ppm) and “parts per billion” (ppb) indicate how many grams of solute there are in one million or one billion grams of sample, respectively. Like percent, these are a measure of concentration. One drop for every million drops, one gram for every million grams, etc. could be considered a “part per million.”

When neutral “sodium” (Na) loses one electron from its outer shell, it forms the “sodium ion” (Na+) (i.e., positively charged sodium atom). "Sodium ion" implies elemental sodium, a monovalent inorganic cation, and “an alkali metal cation”. “Sodium ion” is a monatomic, monocation derived from sodium.