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Atomic no. of potassium (K) is 19.
Its electronic configuration is - 2,8,8,1
Valence electrons is the total number of electrons in the last shell of the atom.
Thus, potassium has 1 electron in its valence shell.
Also, it lies in the 1st group of the periodic table.all elements in the 1st grp have same no. of valence electrons which is 1.

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1 5 1

the number of atoms present in outer most shell are the valence electrons of that atom

the atomic no of potassium is 19 and its EG is 2,8,8,1 so  1 is the valence electron of potassium as it is present in its outermost shell