"Is it true that diamonds can't be broken?"
Kana lifted the model.
Rei answered. "Not completely impossible. But very hard."
"Why?"
"Because carbon-carbon covalent bonds are strong."
Toma supplemented. "Diamond is a giant network of covalent bonds."
Kana thought. "How strong are covalent bonds?"
"Measured by bond energy," Rei opened a notebook. "C-C bond is about 350 kJ/mol."
"kJ/mol?"
"Kilojoules per mole. The energy needed to break that bond."
Milia gave an example. "H-H bond is about 430 kJ/mol."
"Hydrogen molecules are that strong?"
"Small but strong. Two electrons bind two nuclei together."
Kana drew in her notebook. "Electron pair?"
"Yes. Covalent bonding is sharing of electron pairs," Rei explained.
"When electrons are between two atoms, they're attracted to both nuclei. That creates the bond."
Toma drew a diagram. "Atomic orbitals overlap to form molecular orbitals."
"Molecular orbitals?"
"Regions where electrons can exist. Different from atomic orbitals."
Rei supplemented. "Bonding and antibonding orbitals. Electrons enter the lower energy one."
Kana was confused. "Difficult..."
"Simply put," Toma helped. "When electrons are between atoms, it's stable."
"That's the essence of covalent bonding."
Milia asked. "Then what about double bonds?"
"C=C. Two pairs of electrons," Rei answered.
"Twice as strong?"
"No. About 610 kJ/mol. Not twice."
"Why?"
"Sigma and pi bonds. Pi bonds are weaker than sigma bonds."
Kana thought. "Triple bonds?"
"C≡C. About 830 kJ/mol."
Toma organized. "Single bond < double bond < triple bond. But not proportional."
"When bonds increase, the bond length?"
"Gets shorter," Rei answered. "More electrons, atoms are pulled closer."
Kana looked at the model. "Can these bonds really not break?"
"They can. If you provide energy."
"How?"
"Heat, light, catalysts... various methods."
Toma proposed an experiment. "Let's electrolyze water."
They assembled the apparatus. Applied current. Bubbles appeared.
"Hydrogen and oxygen!" Kana was excited.
"We broke the O-H bonds in H2O," Rei explained.
"But it takes considerable energy?"
"Yes. O-H bond is about 460 kJ/mol."
Milia supplemented. "That's why water is stable. Doesn't decompose easily."
Kana was impressed. "Covalent bonds are really strong."
"Strength that supports life," Toma said.
Rei continued. "DNA's double helix is also a combination of covalent and hydrogen bonds."
"Hydrogen bonds are weak?"
"About 20 kJ/mol. Much weaker than covalent bonds."
"But important?"
"They provide flexibility. Because they're weak, they can open and close."
Kana understood. "Both strength and weakness are necessary."
Milia nodded. "Right tool for the job. Strong bonds for skeleton, weak bonds for function."
Toma looked at the diamond. "But diamond is all strong bonds."
"That's why it's hard to break. But no flexibility."
Rei said quietly. "Perfect strength is also brittleness."
Kana pondered. "I want to believe in covalent bonds. But not blindly."
"A good balance," Rei acknowledged.
"Chemistry is harmony of strength and weakness."
The three stared at the model. Invisible electron pairs connect the world. The strength of covalent bonds is worth believing in. But that's not all.