"The water molecule model looks kind of distorted."
Kana tilted the model. The laboratory at night. A quiet time with just the three of them.
Rei answered. "Because it's not symmetrical. H and O have different electronegativities."
"Electronegativity?"
"The power to attract electrons. It differs by atom."
Milia pointed at the periodic table. "Larger toward the upper right. Fluorine is the largest."
"And oxygen?"
"Larger than hydrogen. So the shared electrons in the covalent bond shift toward oxygen."
Kana drew in her notebook. "The line connecting H and O is closer to O?"
"The electron cloud is. Electrons gather around O," Rei explained.
"And then?"
"O becomes slightly negative, H slightly positive. Partial charges are born."
Milia supplemented. "Represented as δ- and δ+. Not complete ions, but polarized."
Kana understood. "So water is a polar molecule?"
"Yes. It has a dipole moment."
"Dipole moment?"
Rei drew a vector. "An arrow from positive to negative charge. It has magnitude and direction."
"In water's case, from the midpoint of H toward O."
Kana rotated the model. "This polarization determines water's properties?"
"Mostly. Hydrogen bonding, high boiling point, solvent capability... all related."
Milia brought a beaker of water. "This can be bent by static electricity."
"What?" Kana was surprised.
She rubbed a clear rod. Brought it close to the water stream. The water bent.
"Amazing!"
"Because it's a polar molecule. It responds to electric fields," Rei explained.
"The positive side is attracted."
Kana thought. "Then are all molecules polar?"
"No. Symmetrical molecules are nonpolar."
Milia gave an example. "CO2. Because O and O are symmetrical, the dipole moments cancel out."
"Cancel out?"
"Two vectors in opposite directions. Total zero."
Rei drew a diagram. O=C=O. The O on both ends pull C's electrons equally.
"Balanced," Kana was satisfied.
"But what about CH4?"
"Also symmetrical. Four H in tetrahedral arrangement."
Milia assembled the model. "Looks the same from any direction. No polarity."
Kana asked. "When there's polarity, what happens?"
"Intermolecular forces become stronger," Rei answered.
"Intermolecular forces?"
"Attraction between molecules. Van der Waals forces, dipole-dipole interactions, hydrogen bonding..."
"Hydrogen bonding too?"
Milia nodded. "An especially strong dipole-dipole interaction. Between H and N, O, or F."
"That's why water has a high boiling point."
"Yes. Despite being a small molecule, it's liquid until 100 degrees."
Kana pondered. "Just from electron density polarization, it changes that much?"
"In the molecular world, slight polarization has major impact," Rei emphasized.
Milia proposed an experiment. "Let's mix oil and water."
She poured both. They separated.
"They don't mix," Kana observed.
"Oil is nonpolar, water is polar. Like dissolves like."
"Like dissolves like," Milia said in English.
Rei supplemented. "Polar solutes in polar solvents, nonpolar solutes in nonpolar solvents."
Kana summarized in her notebook. "Electron density polarization determines solubility."
"Biological membranes too," Milia continued. "Lipid bilayer. Hydrophobic inside, hydrophilic outside."
"The cell boundary?"
"Yes. The arrangement of polar and nonpolar divides life."
Rei said quietly. "Electron polarization divides the world."
Kana looked out the window. Stars in the night sky.
"The night electron density becomes polarized."
Milia smiled. "Poetic."
"But true. Even now, electrons are moving inside molecules."
The three fell silent. Invisible polarization shapes chemistry, life itself. The night of electron density quietly deepens.