Periodic Trends In Properties Of Elements

The reactivity of metals generally increases as you go down a group in the periodic table and decreases as you move from left to right across a period. The metals listed are Potassium (K), Sodium (Na), and Calcium (Ca). - Potassium (K) and Sodium (Na) are in Group 1 (alkali metals).

Potassium is below Sodium in the group, so it is more reactive than Sodium. - Calcium (Ca) is in Group 2 (alkaline earth metals). Potassium and Sodium are more reactive than Calcium.

To put it another way, since reactivity generally decreases from left to right across the periodic table, it is more useful to look in a group which in this case would be the other choices of alkaline metal- in this instance a higher ranking in group 1 metals, to alkaline earth metals.

The reactivity of elements becomes more difficult with the increased shell count within the metallic groups- however the two other listed metals can at least be compared. Therefore potassium is more reactive due to a further atomic count.

Reactivity: K > Na Calcium is in period 4, potassium is in period 4 and sodium is in period 3. Sodium reactivity is far below the range of calcium making a comparision far more difficult.

Considering their positions in the periodic table, the correct order of increasing reactivity is: Ca < Na < K (Calcium < Sodium < Potassium).

Therefore, the reactivity increases in the order Calcium, Sodium, Potassium (Ca, Na, K) which corresponds with choice (3).