"Beautiful..."
Kana held a salt crystal up to the light.
Milia nodded. "Perfect structure."
"Why is it square?"
Rei answered. "Ionic bond arrangement. Na⁺ and Cl⁻ align regularly."
"Ionic bonds?"
"Electrical attraction," Milia explained.
Kana wrote in her notebook. "Plus and minus?"
"Yes. Sodium loses electrons to become Na⁺. Chlorine accepts electrons to become Cl⁻."
Rei supplemented. "Electrostatic attraction binds them strongly."
"How strong?"
"Bond energy about 800 kJ/mol. Close to covalent bonds," Milia answered.
Kana was surprised. "That much?"
"But there's no directionality."
Rei drew a diagram. "Electric field is spherically symmetric. Attracts from all directions."
"So the entire crystal is like one giant molecule."
Kana understood. "This is all connected?"
"Yes. Ionic crystal."
Milia showed another crystal. "Potassium chloride, calcium chloride... various kinds."
"All similar shapes?"
"Changes with coordination number," Rei explained.
"Coordination number?"
"How many opposite-charge ions surround one ion."
Milia added. "NaCl has coordination number 6. Each Na⁺ is surrounded by 6 Cl⁻."
Kana gazed at the crystal. "So regular..."
"Most stable arrangement."
Rei continued. "Balance of Coulomb force and Pauli repulsion."
"Pauli repulsion?"
"When electron clouds overlap, they repel."
Milia explained. "Can't be too close or too far. There's an optimal distance."
Kana asked. "Does it dissolve in water?"
"Yes. Because hydration energy is large," Rei answered.
"Hydration?"
"Water molecules surround ions."
Milia drew a diagram. "Around Na⁺, water's oxygen side faces. Around Cl⁻, hydrogen side faces."
"Because of polarity," Kana understood.
"When hydration energy exceeds lattice energy, it dissolves."
Rei showed calculations. "Heat of solution is the difference."
"Endothermic or exothermic?"
"If lattice energy is large, endothermic. If hydration energy is large, exothermic."
Kana put salt in a test tube. "It got a bit warm?"
"NaCl is slightly endothermic," Milia said.
"But CaCl₂ is strongly exothermic."
Rei added. "That's why it's used as snow-melting agent."
Kana admired. "Practical."
"Ions are also important in living organisms."
Milia began explaining. "Na⁺, K⁺, Ca²⁺, Cl⁻... all are working."
"What do they do?"
"Nerve transmission, muscle contraction, osmotic pressure regulation," Rei answered.
"Nerves?"
"Action potential. Na⁺ flows into cells, causing depolarization."
Milia continued. "K⁺ flows out, causing repolarization."
"Pumps maintain concentration gradient."
Kana showed interest. "Pumps?"
"Na⁺/K⁺-ATPase. Uses ATP to transport ions."
Rei drew a diagram. "Pumps out 3 Na⁺ and brings in 2 K⁺."
"Why?"
"To create concentration gradient. That's the basis of electrical signals."
Milia added. "Calcium ions are also signaling molecules."
"Muscle contraction, neurotransmitter release, gene expression..."
Kana was surprised. "One ion does so much?"
"Concentration is strictly controlled."
Rei showed numbers. "Intracellular Ca²⁺ is normally 100 nM. But jumps to 1 μM during signaling."
"Ten times?"
"Thousand times," Milia corrected.
"That becomes a powerful signal to cells."
Kana looked at the crystal again. "Ions from here might become life someday?"
"They circulate," Rei said quietly.
"Ocean salt, body salt, crystal salt... all connected."
Milia smiled. "Ions quietly attract and keep working."
Kana murmured. "Invisible forces move the world."
"Electrostatic attraction. One of the universe's fundamental forces."
Rei closed the notebook. "Small ions play large roles."
The three gazed at the crystal. Inside the transparent salt, countless ions were aligned.
Kana murmured. "Thank you, ionic bonds."
Milia and Rei laughed.
"Kana thanks every kind of molecule."
On the lab bench, the crystal quietly reflected light.