CHLORINE Cl      Z = 17

This element was discovered by Scheele in Uppsala, Sweden, in 1774. The name comes from the Greek chloros (pale green).

Ionization energies

ClI 13.0 eV, ClII 23.8 eV, ClIII 39.7 eV.

Absorption lines of ClI

Artru et al. (1989) report some ClI lines in the ultraviolet spectra of B 7 and B9V stars.

A weak line at 8376(2) in the solar spectrum has been attributed to ClI and some other weak lines have been observed in sunspot spectra. The identification should be confirmed. Nevertheless, the presence of Cl is certain because of the identification of vibration-rotation bands of HCl in sunspots (Hall and Noyes 1972).

Phillips and Keenan (1990) have reported the detection in an X-ray flare of ClXVI.

Absorption lines of ClII

Table 1: Equivalent widths of ClII

  3860(25)   4794(1)

Group V Ia V

B 0 0.014    
B 2     0.003
B 5   0.016  

Weak lines of ClII are seen in early B-type stars.

Weak Lines of ClII are seen in early B-type stars.

Behavior in non-normal stars

ClII is strong in some early Bp stars (Bidelman 1966, Jaschek and Lopez García 1967, Cohen et al. 1969), with W(4794) = 0.031. Sadakane (1992) calls attention to one star with exceptionally strong ClII lines, with W(4253) = 0.082, which leads to an overabundance of about 4.0 dex. This star also has highly enhanced Co lines as well as other abundance anomalies.

Cl is also strong in some HB stars (Faber 1991), where W(4794) = 0.035.


There exist 11 isotopes and isomers, including two stable ones, Cl35 and Cl37. In the solar system they have respectively 76% and 24% abundances. One short-lived isotope, Cl36, has a half life of 3 × 105 years.

The isotopes Cl35 and Cl37 have been observed in the circumstellar envelope of the C-type star IRC + 10216 through the presence of the Cl isotopes in NaCl and AlCl molecules. The ratio Cl35 / Cl37 is compatible with the terrestrial value (Cernicharo and Guelin 1987). A similar procedure could be applied to the isotopes of Cl in HCl.


Cl35 is produced by explosive nucleosynthesis and Cl37 by this process or by either carbon burning or the s process.

Published in "The Behavior of Chemical Elements in Stars", Carlos Jaschek and Mercedes Jaschek, 1995, Cambridge University Press.