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Why Ar (argon) (at, wt. = 39.94) has bee...

Why `Ar` (argon) (at, wt. `= 39.94`) has been placed before `K` (at. Wt. `= 39.10`) in the periodic table?

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In modern perodic table, elemenys have been placed in order of their increasing atomic numbers (not atomic masses). The atomic number (i.e. `Z`) of `Ar` is `18` and that of potassium `(K)` is `19`. Thus, `Ar` has been placed before potassium.
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