Selenium is element number 34, with the symbol Se. Its name comes from selene, the Greek word for “Moon.”
Atomic number 3478.97
SeSeleniumセレン (seren)
Symbol
Se
Atomic weight
78.97
Place in the table
Period 4 · Group 16
Atomic weight from the Chemical Society of Japan, “Table of Standard Atomic Weights with Four Significant Figures (2026)”.
Named for the Moon
Selenium was discovered in 1817. It was named after the Moon because its vapor had a moon-like color.
Tellurium (Te), directly below selenium on the periodic table, gets its name from the Latin word for “Earth.” The Moon and the Earth sit together in Group 16.
Selenium and the body
In the body, selenium works as part of proteins. It is a trace mineral important for the antioxidant system that protects the body from damage and for thyroid hormones.
Selenium is in Group 16, the same column as oxygen and sulfur.
Think about it
Q1What view do you think the person who saw the Moon in selenium’s vapor had in mind?
Q2Look for other elements named after heavenly bodies.
Q3What do you think “damage” to the body from oxidation means?
There is no single right answer. Talking it over with someone at home is a good way to learn.
Did you know? — Elements named after heavenly bodies include selenium (Moon), tellurium (Earth) and helium (Sun).