Chlorine is element number 17, with the symbol Cl. Joined with sodium, it becomes table salt (sodium chloride). Chlorine gas on its own is yellow-green.
Atomic number 1735.45
ClChlorine塩素 (enso)
Symbol
Cl
Atomic weight
35.45
Place in the table
Period 3 · Group 17
Atomic weight from the Chemical Society of Japan, “Table of Standard Atomic Weights with Four Significant Figures (2026)”.
Named for its color
The symbol Cl comes from chloros, Greek for yellow-green, because chlorine gas is yellow-green. It was discovered in 1810.
Chlorine gas has a sharp smell and is harmful. In Japanese junior high school science, good ventilation is considered important in experiments that give off gases like chlorine.
Protecting tap water
Japan’s tap water is disinfected with chlorine. A rule made in 1957 requires at least 0.1 mg/L of chlorine to remain in the water at the faucet.
The formula for table salt is NaCl: one sodium (Na) and one chlorine (Cl) joined together.
Think about it
Q1Chlorine gas is harmful, yet we eat salt every day. Both contain chlorine, so why are they so different?
Q2Why is a little chlorine left in tap water on purpose?
Q3Are there other elements named after their color?
There is no single right answer. Talking it over with someone at home is a good way to learn.
Did you know? — In Japanese ninth-grade science, one example experiment is passing an electric current through a solution of copper chloride, a compound of copper and chlorine.