The correct answer is IE decreases down the group.
On moving down the group, the size of the element increases due to the increase in the number of shells. The element with smallest size has less the attraction of the nucleus on the valance electron. It needs more energy to remove an electron from its valance shell. Hence, the IE decreases down the group.
Why does ionization energy increase down the group but decreases going across a period?
Because there are more protons with time, the ionization energy rises. As a result, there will be more attraction because the nuclear charge has increased.
Even if there is stronger attraction, one should be aware that the shielding effect and distance from the nucleus remain largely constant. The same primary quantum shell contains all of the valence electrons, which explains this.
Therefore, while distance from the nucleus and the shielding effect stay fairly constant, an increase in nuclear charge causes an increase in attraction and increases the energy required to remove an electron.
The ionization energy drops with each group. This is because the outside electrons acting as a shield or screen for the nucleus make the attraction between them weaker and make it easier for them to be withdrawn.
To learn more about ionization energy refer the link:
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