IUPAC Element Symbol Convention

The International Union of Pure and Applied Chemistry (IUPAC) sets the standards for chemical nomenclature and notation, including the writing of element symbols.

Element Symbol Writing Rules

• Element symbols typically consist of one or two letters derived from the element's name (often English, but sometimes Latin or Greek).
• Single-letter symbols: Always written in uppercase. Example: Oxygen is O, Hydrogen is H.
• Two-letter symbols: The standard convention requires the first letter to be uppercase and the second letter to be lowercase. Example: Helium is He, Sodium is Na (from Natrium), Chlorine is Cl.
• Symbols are never written using only numeric codes.
• Symbols should not be written with both letters in uppercase when two letters are used.

Therefore, the correct IUPAC rule for writing element symbols, particularly when they consist of two letters, is that the first letter is capitalized and the second letter is lowercase.