1. Across a period , the ionization energy increases because the formation of a cation becomes more difficult owing to the increase in nuclear charge.
2. Ionization energy decreases down the group because the outer electrons become progressively separated from the nucleus and so are less tightly held .
The statement that best describes the trend in first ionization enery of elements on the periodic table is:
It generally decreases down a group because valence electrons are farther from the nucleus.
The first ionization energy measures how difficult is to release an electron from the outermost shell. The higher the ionization energy the more difficult it is to release an electron, the lower the ionication energy the easier to release an electron.
As the atomic number of the atom increases (which is what happens when you go down a group) the furthest the outermost shell of electrons will be (the size of the atoms increases) and so those electrons require less energy to be released, which means that the ionization energy decreases.
