The polar head of a soap molecule would be more attracted to calcium ions than that of sodiu ions. Sodium ions has a positive charge only. It has a +1 charge. While, calcium ion carries a double positive or a +2 charge. So, the negative charged polar part of a soap molecule would be more attracted to the double positive charge than a positive charge only. This is true for a magnesium ion also since it also has a double positive charge. Soap molecules has properties for both polar and non-polar which can be found at the end of a molecule. When it is in water, it forms micellar structure where the polar end would be facing or attracted to water while the non-polar end would move away from the water.