# CAS Registry number

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

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

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## Notes

The way to check whether the CAS registry numberr 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 numberr. If not, the CAS registry numberr 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 numberr 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 check-digit documentation.
3. Dictionary of Inoganic Compounds, Ed. J.C. Macintyre, Chapman and Hall, UK, 1992.
Explore the element of your choice through this periodic table.
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