Thursday

Raven spreads a short table across the Bench. Five elements, each with a symbol and a number.

"Hydrogen is 1. Carbon is 6. Oxygen is 8," she reads. "What do you notice about the numbers?"

"They go up," Rocket says. "But what do they count?"

Nova hovers over the hydrogen row. "Think about what is in the center of every atom of that element," she says.

"Protons!" Rocket says. "Hydrogen has one. Oxygen has eight. The number is the proton count."

Raven points to the last column. "And look. Some are gases at room conditions, and some are solids."

"Sodium and chlorine make salt, and salt is a solid," Rocket says. "But chlorine by itself is a gas? Wild."

The National Institutes of Health keeps a chemistry database that lists every element. Here are five elements from its periodic table.

The atomic number of an element is the number of protons in the center of each of its atoms. Every hydrogen atom has one proton.

The number of protons is what makes each element unique compared with all the other elements.

The last column gives the state the database lists for each element under standard conditions.

Sodium is a solid and chlorine is a gas. Combined in equal parts, they make table salt, a white solid you can hold.

Hydrogen and oxygen are both gases. Two hydrogen atoms and one oxygen atom make water, a liquid at room temperature.

The compound has its own properties, different from the elements that make it. The recipe decides what you get.

A chemistry text hosted by the University of California, Davis lists the density of some gases. The values are at 20 degrees Celsius, in grams per liter.

Excellent table reading. Tomorrow you will find these molecules and crystals in everyday life and review the week.