What is the energy required to remove an electron from an atom in the gaseous state called

The energy required to remove the the electrons from gaseous atom is called  ionization energy.

Explanation:

Ionization energy:

The amount of energy required to remove the electron from gaseous atom is called ionization energy.

Trend of ionization energy in periodic table:

Along period:

The atomic size tend to decrease in same period of periodic table with increase of atomic number 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.

Along group:

Atomic size increases with increase of atomic number from top to bottom. The nuclear attraction on valance shell became weaker and thus it becomes easy to remove an electron from valance shell and this can be done with less amount of energy. That's why  ionization energy decreases from top to bottom.