Question

How do nuclear charge and shielding affect the periodic trend and group trend of ionization energy?

Answer 1

Hint: Nuclear charge $Z$ is inversely proportional to atomic size but it is directly proportional to the ionization enthalpy. Ionization enthalpy is the energy by which an outermost electron can be pulled. It is a matter of change which is present in atomic radii of different atoms which are periodically arranged in groups and periods.

Complete step-by-step answer:
In the periodic table, there are groups and periods and we know that groups are the arrangement of atoms in a column while periods are arrangements of atoms in vertical fashion. Now, the basic building of our modern periodic table is the atomic number term. But there are various other properties which show a trend on moving down the group or across the period.

Let's first see firstly what properties they are. There is an atomic size of atom, nuclear charge $Z$ these are connected to each other and then with other properties too. As nuclear charge increases, it means the attraction of nuclei to its outermost electrons which is called its nuclear charge $Z$, poor shielding takes place so when nuclear charge increases due to the attraction size of that atom decreases.

An atom which has smaller size and higher nuclear attraction there will be less chances that the electron comes out easily by applying low energy, so that’s why the energy required to take the outermost electron from the orbital of a smaller nuclei is very difficult. More energy is required for it.

Let’s see the trend in a group, we know the basic thing that on moving down the group, size increases and nuclear charge $Z$ decreases on the outermost electron, therefore the ionisation energy decreases as we move down the group. The trend in a group is just opposite to that of in a period, in a period as we move left to right the nuclear charge increases from lithium $Li$ to neon $Ne$. Due to this increase atomic size automatically decreases and the energy required to take an electron from the most shell becomes difficult and a need for higher ionization energy comes. So, on moving in a period ionization energy increases.

Note: The trends in a group is just opposite to that of period. There are some exceptions in groups regarding the nuclear size and thus also of ionization energy, because of the presence of some extra electrons of d-orbitals. The extra $10$ electrons gives poor shielding by which nuclear charge becomes less powerful than the repulsion of electrons, overall in some cases the opposite trend can be seen.