Anomaly in nuclear charge/atomic weight order

[Loren Booda]

In his book The Ascent of Science, Brian L. Silver states "The order of increasing nuclear charge was, with one exception, identical with the order of increasing atomic weight."

What is that exception, and why?

 

[mathman]

The exception referred to is probably Argon (18) and Potassium (19), each of which has several isotopes. It just happens the most common isotope for Argon has 40 nucleons, while for Potassium has 39.