Potassium

Potassium is an alkali metal, found in group 1 in the periodic table.  With the atomic number 19 it is heavier than Sodium (Na) that sits above it in the periodic table (with atomic number 11) and lighter than Rubidium (Rb) that has the atom number 37 sitting below it. Potassium (shorthand K), when becoming an ion, loses an electron from its outer shell (4s¹), becoming  K⁺. Due to the outer electron being further from the nucleus it is more reactive than sodium and reacts faster in water to produce potassium hydroxide, however it reacts slower than Rubidium. 

It plays an important role in neurones where it is pumped into the neurone in exchange for sodium^[1].

Potassium also contributes to the physiological process of hormone secretion and action.^[2]

References

1. ↑ http://highered.mheducation.com/sites/9834092339/student_view0/chapter44/sodium-potassium_exchange_pump.html , 22/10/2015
2. ↑ Mount DB, Zandi-Nejad K. Disorders of potassium balance. In: Taal MW, Chertow GM, Marsden PA, Skorecki KL, Yu ASL, Brenner BM, eds. The kidney. 9th ed. Philadelphia: Elsevier, 2012:640-688