Find a pair of elements in the periodic table with atomic numbers less than 20 that are

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Find a pair of elements in the periodic table with atomic numbers less than 20 that are an exception to the original periodic law.

Periodic Table:

2 3 4 5 6 7 3 11 Li 6.94 Na 22.99 19 1 IA K 39.10 37 55 4 87 2 IIA Be 9.01 12 Mg 24.31 20 21 38 Rb Y Sr 85.47

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