Why does electron energy increase across a period?

Electron energy increases across a period due to the increasing nuclear charge without sufficient increase in shielding.

In more detail, as you move across a period in the periodic table from left to right, the number of protons in the nucleus (the atomic number) increases. This means that the positive charge of the nucleus, also known as the nuclear charge, increases. At the same time, the number of energy levels, or shells, remains the same. This results in a stronger attraction between the nucleus and the electrons in the same energy level, which increases the energy of the electrons.

The shielding effect, which is the reduction in effective nuclear charge due to the repulsion of electrons in inner shells, does not increase significantly as you move across a period. This is because the additional electrons are added to the same energy level, not to inner levels. Therefore, the increase in nuclear charge is not offset by an increase in shielding, leading to a higher effective nuclear charge. This stronger attraction between the nucleus and the electrons increases the energy of the electrons.

Furthermore, the increase in electron energy across a period is also related to the concept of ionisation energy. Ionisation energy is the energy required to remove an electron from an atom. As the nuclear charge increases across a period, the attraction between the nucleus and the electrons increases, making it harder to remove an electron. Therefore, the ionisation energy increases, which is another indication of the increase in electron energy.

In summary, the increase in electron energy across a period is due to the increasing nuclear charge, which strengthens the attraction between the nucleus and the electrons, and the lack of a significant increase in shielding. This results in a higher effective nuclear charge and a higher ionisation energy, both of which indicate an increase in electron energy.