Concept:Ionization energy is the energy required to remove an electron from a gaseous atom in its ground state.
Explanation:Moving across a period from left to right, the number of protons in the nucleus increases.
This raises the effective nuclear charge experienced by the outermost electrons.
The attraction between the nucleus and the outer electrons becomes stronger.
As a result, the atomic radius decreases because the electrons are pulled closer to the nucleus.
The electrons are held more firmly, so they become more difficult to remove.
More energy is therefore required to detach an outermost electron.
Thus, ionization energy generally shows an increasing trend across a period.
Answer:The correct answer is that ionization energy increases from left to right across a period.