Possible Clues:

• Microphysics unit: Abbr.
• H2O, for one: abbr.
• Basic unit: Abbr.
• Cluster of atoms (Abbr.)
• Grp. of atoms
• Collection of atoms: Abbr.
• H2O, e.g.
• Group of atoms: Abbr.
• 40-Down, e.g.: Abbr.
• Small matter: Abbr.