When magnesium and potassium are compared which atom forms a cation, which cation will be smaller in size?
Magnesium will from the cation Mg2+. This is positively charged because it has lost two electrons. The cation will be smaller than its neutral atom. this is because there are less electrons, but the proton number remains the same, therefore the positively charged nucleus pulls more strongly on the fewer electrons, meaning the atom is smaller. Compared to the potassium cation, which forms K+, the Mg2+ cation will be smaller than the K+ cation.
Looking at the periodic table Mg is in row 3 and K is in row 4, which means that K atoms have more atomic orbitals than Mg, which immediately implies that K has a larger ionic radius. Also, coupled with the fact that Mg forms 2+ ions whereas K forms 1+ ions, the attraction that the electrons have towards the nucleus in Mg2+ is much larger than that in K+, hence the ionic radius decreases.
