Atomic Structure: FAQ
„ Why do successive ionisation energies for a given element always increase with the
number of electrons removed? Electrons are being pulled away from an ion that has an
increasing positive charge. Therefore more energy is required each time, to overcome the
attraction between the outgoing electron and the remaining positive ion.
„ Why is there is a very large increase in ionisation energy whenever an electron is removed
from a new shell? This is because the electron is now closer to the nucleus and screened by
fewer electrons. Its greater attraction towards the nucleus means that more energy is needed for
its removal.
„ Why does atomic radius decreases across a period of the s- and p-block elements? There
is an increase in nuclear charge, but no change in the number of inner screening electrons. This
means that the attraction between the nucleus and the outer electrons increases.
„ Why does atomic radius increases down a group of the Periodic Table? The increase in
nuclear charge is balanced by an equal increase in the number of inner screening electrons. The
radius increases because there is an increasing number of shells occupied by electrons.
„ Why does ionisation energy decreases down a group of the Periodic Table? The increase in
atomic radius means that the distance between the nucleus and the outer electrons decreases,
reducing the strength of attraction between them. Therefore less energy is required for electron
removal.
„ Why does 1st ionisation energy shows a general increase across a period of the s- and pblock elements? There is an increase in nuclear charge, but no change in the number of inner
screening electrons. This means that the attraction between the nucleus and the outer electron
increases and more energy is required for electron removal.
„ Why does a group 3 element have a lower 1st ionisation energy than the preceding group 2
element? The electron is being removed from a p-subshell in the case of the group 3 element but
an s-subshell in the case of the group 2 element. The p-subshell is of higher energy than the ssubshell and so the electron needs less energy for its removal, even though the nuclear charge is
greater.
„ Why does a group 6 element have a lower 1st ionisation energy than the preceding group 5
element? The electron being removed is a paired p-electron in the case of the group 6 element,
but an unpaired p-electron in the case of the group 5 element. Even though the nuclear charge is
greater for the group 6 element, the electron requires less energy for its removal because of
repulsion between the electron pair.
„ Why do highly electronegative elements such as oxygen have an exothermic 1st electron
affinity? As they have small atoms, the outer shell is close to the nucleus. The electrons in it are
not well shielded and are strongly attracted. This attraction is so strong that the atom can accept
an extra electron into the outer shell, with the release of energy.
„ Why is the 2nd electron affinity of oxygen endothermic? Energy is required to overcome the
mutual repulsion between the in-coming electron and the negatively charged ion.
„ Why is an Mg2+ ion smaller than an Mg atom? In forming the ion the complete outer shell is
lost, and the proton / electron ratio in the ion is greater than in the atom.
„ Why is an O2- ion bigger than an O atom? The ion has more electrons in the outer shell,
increasing the repulsion between them. Also the proton / electron ratio in the ion is less than in
the atom.
„ Why is there a decrease in ionic radius along the series O2-, F -, Na+, Mg2+, Al3+? All these
ions have the same number and arrangement of electrons, but the nuclear charge increases
along the series. This results in an increasing attraction towards the outer electrons.