Order the following elements from least to greatest ionization energy.
f.s, k, ca
f k< ca s< f< ca k< ca < f

K ∠ Ca ∠ S ∠ F

Explanation:

Trend along period:

As we move from left to right across the periodic table the number of valance electrons in an atom increase. The atomic size tend to decrease in same period of periodic table because the electrons are added with in the same shell. When the electron are added, at the same time protons are also added in the nucleus. The positive charge is going to increase and this charge is greater in effect than the charge of electrons. This effect lead to the greater nuclear attraction. The electrons are pull towards the nucleus and valance shell get closer to the nucleus. As a result of this greater nuclear attraction atomic radius decreases and ionization energy increases because it is very difficult to remove the electron from atom and more energy is required.

trend along group:

As we move down the group atomic radii increased with increase of atomic number. The addition of electron in next level cause the atomic radii to increased. The hold of nucleus on valance shell become weaker because of shielding of electrons thus size of atom increased and ionization energy decrease.

Fluorine is present on right side of periodic table so its ionization energy will be high.

F = 1681 kj/mol

S = 999.6 kj/mol

Ca = 589.8 kj/mol

K = 418.8 kj/mol

while potassium is present most left of the periodic table its ionization energy is less.

The given element can be arranged in following order:

K ∠ Ca ∠ S ∠ F