"Why carbon?"
Kana asked as she closed her organic chemistry textbook.
"Why carbon?" Rei repeated.
"Life is carbon-based, right? But why is only carbon special?"
Milia answered quietly, "Because carbon's bonding ability is overwhelmingly diverse compared to other elements."
"Diverse?"
Rei wrote element symbols on the whiteboard. "Carbon has four valence electrons."
"So it can form four covalent bonds."
"Right. But that's not all."
Kana opened her notebook. "There are other tetravalent elements, right? Like silicon."
"Sharp," Rei acknowledged. "But silicon can't form bonds as stable as carbon."
Milia supplemented, "Especially silicon-silicon double and triple bonds are unstable."
"Meanwhile, carbon can stably form single, double, and triple bonds."
Kana took out molecular models. "Ethane, ethylene, acetylene."
"All carbon-carbon bonds, but properties are completely different," Rei pointed out.
"This diversity is the foundation of organic chemistry."
Milia showed another model. "And carbon can make long chains."
"Methane, ethane, propane, butane... can continue infinitely."
"Theoretically infinite," Rei nodded. "In fact, polymers with hundreds of carbon atoms exist."
Kana was surprised. "Infinite?"
"Carbon-carbon bonds are strong and stable. So even long chains don't break easily."
"Furthermore," Milia continued, "it can make ring structures."
"Benzene, cyclohexane, fullerene..."
Rei took out a complex model. "Three-dimensional structures are possible too. Diamond, graphite, graphene."
"Same carbon, but completely different properties," Kana was impressed.
"Allotropes," Milia explained. "Just atomic arrangement determines properties."
Kana pondered. "But why did life choose carbon?"
"Rather than chose," Rei answered carefully, "carbon was most suitable, so it was naturally selected."
"Suitable?"
"Balance of stability and reactivity," Milia said.
"Carbon compounds are stable at room temperature. But they react under appropriate conditions."
Rei supplemented, "If too stable, they don't react. If too unstable, they break immediately."
"Carbon is just right."
Kana wrote in her notebook. "Exquisite balance."
"And," Milia continued, "carbon can bond with hydrogen, oxygen, nitrogen, sulfur, and phosphorus."
"Compatible with all elements necessary for life."
Rei showed biological molecule models. "Glucose, amino acids, fatty acids, nucleic acids..."
"All have carbon skeletons."
Kana was moved. "Carbon creates life's diversity."
"With four bonding hands, it makes infinite combinations," Milia smiled.
"Like eternal Lego blocks," Kana murmured.
Rei laughed. "Good analogy. Carbon is molecular Lego."
"But," Kana thought, "there might be other life forms in the universe, right?"
"Interesting question," Milia became serious. "Silicon-based life has been imagined."
"But hasn't been found," Rei said. "At least, it wouldn't be as efficient as carbon."
Kana reviewed her notes. "Carbon structures have eternal potential."
"Right. That's why organic chemistry expands infinitely."
Milia looked out the window. "Our bodies are also made of carbon structures."
"Carbon atom arrangement determines our individuality," Rei said quietly.
Kana looked at her hands. "These hands are made of carbon."
"DNA, proteins, lipids... all are carbon artwork."
"Eternal structures," Kana murmured.
"As long as life continues, carbon structures continue," Milia smiled.
Stars were visible through the lab window. Carbon floats there too. An element born from cosmic dust, nurturing life.
"Next, shall we learn about the carbon cycle?" Rei suggested.
"There's more!" Kana was surprised.
"Carbon's story doesn't end," Milia said gently.