"What's the weight of this molecule?"
Milia showed Kana a structural formula.
"Should I calculate it?"
"Add the atomic mass of each atom. Carbon is 12, hydrogen is 1, oxygen is 16."
Kana calculated. "For C₆H₁₂O₆... 180?"
"Correct. Glucose has a molecular mass of 180," Rei confirmed.
"But what does this number mean?" Milia asked.
Kana thought. "Weight...?"
"What unit?"
"Grams?"
"Not quite. Precisely, atomic mass units. Or grams per mole."
Rei explained. "A mole is a unit of quantity. 6.02×10²³ particles."
"Avogadro's number," Kana remembered.
"Right. So molecular mass 180 means 180 grams contains 6.02×10²³ glucose molecules."
Milia brought a reagent bottle. "This is 180 grams of glucose. One mole."
"Looks like ordinary powder," Kana observed.
"But astronomical numbers of molecules are packed inside."
Rei showed another compound. "Ethanol has molecular mass 46. Lighter than glucose."
"When lighter, what's different?"
"Diffusion speed differs. Lighter molecules move faster," Milia answered.
"Why?"
"At the same temperature, kinetic energy is the same. ½mv². When mass m is small, velocity v becomes large."
Kana wrote in her notebook. "Lighter means faster."
Rei continued. "That's why fragrance molecules are light. They diffuse quickly and reach the nose."
"What about heavy molecules?"
"Move slowly. DNA and proteins are huge and heavy."
Milia drew a diagram. "Hemoglobin has molecular mass 64,500. About 360 times glucose."
"That much!" Kana was surprised.
"Macromolecules have complex functions. Carry oxygen, catalyze, store information."
Rei added. "When molecular mass is large, three-dimensional structure becomes complex."
"So function becomes richer?"
"Exactly. But synthesis and decomposition also slow down."
Kana asked. "How do cells control molecular mass?"
"Determined by gene length," Milia answered. "Long genes produce large proteins."
"Adjusted as needed?"
"Yes. Some enzymes are small, some structural proteins are huge."
Rei thought. "Molecular mass is part of a molecule's identity."
"Identity?"
"In mass spectrometry, molecules are identified. Measure molecular mass and know what substance it is."
Milia showed experimental data. "This peak is molecular mass 180. Confirmed to be glucose."
Kana understood. "A molecular fingerprint?"
"Good metaphor. Mass doesn't lie."
Rei gave another example. "Isotope differences also show. Carbon-12 and carbon-13."
"Slight differences?"
"Just 1. But mass spectrometers can distinguish them."
Milia murmured. "Molecular mass tells a molecule's story."
"Story?" Kana showed interest.
"What atoms it's made of. What structure it has. What properties it possesses."
Rei looked outside. "Molecules in the air also have different molecular masses."
"Nitrogen 28, oxygen 32, carbon dioxide 44."
"So due to weight differences, composition changes with altitude."
Kana picked up the reagent bottle. "This weight contains molecular individuality."
"And that individuality makes the world," Milia said quietly.
Rei nodded. "Molecular mass isn't just a number. It's the weight of a molecule's life."
The three quietly felt the meaning of invisible mass.