<P> The ionization potential is the minimum amount of energy required to remove one electron from each atom in a mole of atoms in the gaseous state . The first ionization energy is the energy required to remove the first electron, and generally the nth ionization energy is the energy required to remove the atom's nth electron, after the (n − 1) electrons before it has been removed . Trend-wise, ionization energy tends to increase while one progresses across a period because the greater number of protons (higher nuclear charge) attract the orbiting electrons more strongly, thereby increasing the energy required to remove one of the electrons . Ionization energy and ionization potentials are completely different . The potential is an intensive property and it is measured by "volt"; whereas the energy is an extensive property expressed by "eV" or "kJ / mole". </P> <P> As one progresses down a group on the periodic table, the ionization energy will likely decrease since the valence electrons are farther away from the nucleus and experience a weaker attraction to the nucleus's positive charge . There will be an increase of ionization energy from left to right of a given period and a decrease from top to bottom . As a rule, it requires far less energy to remove an outer - shell electron than an inner - shell electron . As a result, the ionization energies for a given element will increase steadily within a given shell, and when starting on the next shell down will show a drastic jump in ionization energy . Simply put, the lower the principal quantum number, the higher the ionization energy for the electrons within that shell . The exceptions are the elements in the boron and oxygen family, which require slightly less energy than the general trend . </P> <P> The electron affinity of an atom can be described either as the energy released by an atom when an electron is added to it, conversely as the energy required to detach an electron from a singly charged anion . The sign of the electron affinity can be quite confusing, as atoms that become more stable with the addition of an electron (and so are considered to have a higher electron affinity) show a decrease in potential energy; i.e. the energy gained by the atom appears to be negative . For atoms that become less stable upon gaining an electron, potential energy increases, which implies that the atom gains energy . In such a case, the atom's electron affinity value is positive . Consequently, atoms with a more negative electron affinity value are considered to have a higher electron affinity (they are more receptive to gaining electrons), and vice versa . However, in the reverse scenario where electron affinity is defined as the energy required to detach an electron from an anion, the energy value obtained will be of the same magnitude but have the opposite sign . This is because those atoms with a high electron affinity are less inclined to give up an electron, and so take more energy to remove the electron from the atom . In this case, the atom with the more positive energy value has the higher electron affinity . As one progresses from left to right across a period, the electron affinity will increase . </P> <P> Although it may seem that Fluorine should have the greatest electron affinity, the small size of fluorine generates enough repulsion that Chlorine has the greatest electron affinity . </P>

Explain the increase in reactivity of elements further down the group