CAS Registry number: periodicity

All chemicals have a CAS (Chemical Abstracts Service) registry number. Each and every chemical has a number which is usually unique.

This is given in each case in square brackets. If you have access to CAS on-line, or some other search services, you can search for the element using the CAS registry number. The registry number is a "smart number". The last digit is a check-digit to which helps to ensure that any given CAS number is authentic.

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Notes

The way to check whether the CAS number is authentic is as follows. Take the second digit from the right and multiply it by 1. Take the third digit from the right and multiply it by 2. Repeat this process for the fourth (multiply by 3), fifth (multiply by 4), sixth digits (multiply by 5) and so on for as many digits as there are. Now add all the numbers resulting from the multiplications together. Take the unit digit from this addition. It should match the last digit of the CAS registry number. If not, the CAS number is invalid.

As an example, consider the registry number for hydrogen: [1333-74-0]. The calculation is (1*4) + (2*7) + (3*3) + (4*3) + (5*3) + (6*1) = 4 + 14 + 9 + 12 + 15 + 6 = 60. The last digit of the CAS registry number matches the units digit in the number 60, so the number is legal.

Literature sources

1. Any volume of Chemical Abstracts, published by the American Chemical Society.
2. CAS documentation
3. CAS check-digit documentation.
4. Dictionary of Inoganic Compounds, Ed. J.C. Macintyre, Chapman and Hall, UK, 1992.

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