"This is sodium?"
Kana peered into the bottle. Silvery metal immersed in oil.
"Don't touch it," Rei warned. "It oxidizes in air."
"Really, that much?"
"I'll show you," Toma took out a small piece with tweezers.
Dropped it on the water surface.
Violent reaction. With a sizzle, sodium raced around on the water.
"Wow!" Kana was amazed.
"Exothermic reaction," Rei explained. "Hydrogen gas is being produced."
A flame briefly glowed yellow.
"It burned?"
"Sodium's flame test. Characteristic yellow."
Kana was excited. "Why is it so violent?"
Rei opened his notebook. "Sodium is an alkali metal. Only 1 valence electron."
"One?"
"Electron configuration is 2, 8, 1. Easy to give up that last one."
Toma supplemented. "Low ionization energy."
"Ionization energy?"
"Energy needed to remove an electron," Rei defined.
"Sodium easily loses electrons."
Kana understood. "So it reacts easily?"
"Yes. It wants to give up electrons and become Na+ ion."
Toma showed the periodic table. "Elements in the same group have similar properties."
"Lithium, sodium, potassium... all alkali metals."
Rei continued. "Going down, reactivity increases."
"Potassium is more violent?"
"More dangerous than sodium. Can spontaneously ignite."
Kana became cautious. "Elements are so different."
"The periodic table is a personality chart of elements," Toma said.
Rei drew a diagram. "When valence electrons reach 8, it's stable."
"Octet rule."
"Sodium gives up 1 to return to a shell of 8."
Kana calculated. "From 2, 8, 1, removing 1 gives 2, 8."
"Same electron configuration as the noble gas neon."
Toma supplemented. "That's why it's stable. Energetically favorable."
Kana asked. "What about chlorine?"
Rei answered. "Opposite. Valence shell has 7. Wants one more."
"High electron affinity."
"Sodium gives electrons, chlorine receives them."
Toma concluded. "That's sodium chloride, table salt."
Kana was moved. "They're compatible."
"Ionic bond," Rei used the technical term. "Bound by electrostatic attraction."
Toma proposed another experiment. "Want to see other alkali metals?"
"Lithium?"
"Gentler than sodium," Rei answered. "Slightly higher ionization energy."
Put a small piece in water. Bubbles formed, but not as violently as sodium.
"It's true," Kana compared.
"Potassium?"
"Too dangerous, pass," Toma declined.
Rei explained. "Cesium and rubidium react explosively with water."
"Going down the periodic table, valence electrons get farther away."
"Attraction from the nucleus weakens."
Kana summarized. "So they easily give up electrons."
"Accurate understanding," Rei acknowledged.
Toma added. "Conversely, going up, they're smaller. Stronger attraction."
"Helium and neon don't give up electrons."
"That's why they don't react. Called noble gases."
Kana gazed at the periodic table. "Each element's personality is different."
"But there's regularity," Rei pointed out. "Periodic law."
"Same columns are similar. Same rows have similar sizes."
Toma was impressed. "Mendeleev made an amazing discovery."
"A map of chemistry," Kana murmured.
Rei began cleaning up. "Let's put sodium back in oil."
"Dangerous to expose to air."
Kana finally asked. "Do our bodies have sodium too?"
"Lots," Toma answered. "Essential for nerve transmission, muscle contraction."
"But in ionic form. Not metallic sodium."
Rei supplemented. "Na+ ion. Stable form that lost an electron."
Kana was relieved. "Good. It would be trouble if it got excited inside the body."
The three laughed.
Outside the window, the sun was setting. Each element has its own unique personality.