Potassium has an atomic number of 19. Write its electron arrangement…

The valence electrons are those in the outermost shell. With $1$ to $4$ of them, the valency is that number; with $5$ to $7$, it is $8$ minus the number. A full outer shell gives a valency of $0$. Atoms lose, gain or share electrons to reach a full outer shell, like a noble gas.