Does first ionisation energy increase or decrease in moving from oxygen to fluorine across the second period? Answer with increase or decrease.

Final Answer

Increase

Explanation

As we move from oxygen to fluorine across the second period of the periodic table, the first ionization energy increases. This trend can be attributed to several factors related to atomic structure.

First, the atomic number increases from oxygen (atomic number 8) to fluorine (atomic number 9), which means that fluorine has a greater nuclear charge. This increased positive charge in the nucleus attracts the electrons more strongly, making it more difficult to remove an electron, thus increasing the ionization energy.

Second, both oxygen and fluorine are in the same period, which means they have the same principal energy level leading to similar shielding effects from the inner electron shells. However, with the additional proton in fluorine, the effective nuclear charge felt by the outermost electrons increases, further contributing to the rise in ionization energy.

Therefore, due to increased nuclear charge and corresponding increased effective nuclear attraction, the first ionization energy moves from oxygen to fluorine clearly shows a trend of increase.