Number the elements sodium , magnesium, and potassium in the predicted order of ionic radius from the largest 1 to the smallest 3

1 potassium

2 sodium  

3 Magnesium

The ionic radius is the measure of an atom's ion in a crystal lattice. It is half the distance between two ions that are barely touching each other.  

The ionic radius may be larger or smaller than the atomic radius (radius of a neutral atom of an element), depending on the electric charge of the ion. Cations are typically smaller than neutral atoms because an electron is removed and the remaining electrons are more tightly drawn in toward the nucleus. An anion has an additional electron, which increases the size of the electron cloud and may make the ionic radius larger than the atomic radius.

•As you move from top to bottom down an element group (column) ionic radius increases. This is because a new electron shell is added as you move down the periodic table. This increases the overall size of the atom.

•As you move from left to right across an element period (row) the ionic radius decreases. Even though the size of the atomic nucleus increases with larger atomic numbers moving across a period, the ionic and atomic radius decreases. This is because the effective positive force of the nucleus also increases, drawing in the electrons more tightly. The trend is particularly obvious with the metals, which form cations. These atoms lose their outermost electron, sometimes resulting in the loss of an entire electron shell. The ionic radius of transition metals in a period does not, however, change very much from one atom to the next near the beginning of a series.

Values for ionic radius are difficult to obtain and tend to depend on the method used to measure the size of the ion. A typical value for an ionic radius would be from 30 pm (0.3 Å) to 200 pm (2 Å). Ionic radius may be measured by using x-ray crystallography or similar techniques.

Potassium 1.38 Å

Sodium 1.02 Å

Magnesium 0.72 Å

if you mean hydrogen and oxygen then water is the product