Chemistry: What´s First Ionisation Energy?

First Ionisation energy is the measure of energy needed to remove a mole of electrons from a mole of gaseous atoms to form a mole of positively charged gaseous ions, also known as a cation. The general trend is that as the atomic number increases between elements Lithium to Neon the level of the first ionisation energy increases, as there is a stronger nuclear attraction. [Braid, K. et al. (2015)]. The stronger nuclear attraction along the period is caused by the number of protons increasing. When the nucleus has a higher number of protons it has a more positive charge, and so the attraction of electrons to the nucleus is stronger. A stronger nuclear attraction means that more ionisation energy is needed to remove the first electron, leading to the general tendency of increased first ionisation energies along a period, as shown in the graph. There is a decrease in the first ionisation energies at Boron and Oxygen due to a lessened nuclear attraction, overcoming the increase of nuclear charge from an addition of a proton. The first ionisations energies of Boron and Oxygen being slightly lower, despite an increase in nuclear charge, provide evidence that the second
[Older, J. and Smith, M. (2015)]. As the outer electron for both Nitrogen and Oxygen is in the same sub-orbital, 2p, there is no difference in the amount of shielding occurring. Electron. With Oxygen in the 2p sub-orbital the electron which is removed is paired with another electron that is moving in the opposite direction, in a shared 2p sub-shell. Electron repulsion occurs in the shared 2p sub-shell as electrons have a negative charge. Electron repulsion therefore lowers the energy requirement to remove the first electron, so the first ionisation energy of Oxygen is lower than that of nitrogen. This decrease in first ionisation energy is small due to Nitrogen and Oxygen both being in the same principle