This lessening of the pull of the nucleus by inner electrons is known as screening or shielding. Hydrogen has an electronic structure of 1s 1. It is a very small atom, and the single electron is close to the nucleus and therefore strongly attracted. There are no electrons screening it from the nucleus and so the ionization energy is high kJ mol Lithium's first ionization energy drops to kJ mol -1 whereas hydrogen's is kJ mol Talking through the next 17 atoms one at a time would take ages.
We can do it much more neatly by explaining the main trends in these periods, and then accounting for the exceptions to these trends. The first thing to realize is that the patterns in the two periods are identical - the difference being that the ionization energies in period 3 are all lower than those in period 2.
The general trend is for ionization energies to increase across a period. In the whole of period 2, the outer electrons are in 2-level orbitals - 2s or 2p. These are all the same sort of distances from the nucleus, and are screened by the same 1s 2 electrons. The major difference is the increasing number of protons in the nucleus as you go from lithium to neon. That causes greater attraction between the nucleus and the electrons and so increases the ionization energies. In fact the increasing nuclear charge also drags the outer electrons in closer to the nucleus.
That increases ionization energies still more as you go across the period. In period 3, the trend is exactly the same. This time, all the electrons being removed are in the third level and are screened by the 1s 2 2s 2 2p 6 electrons. They all have the same sort of environment, but there is an increasing nuclear charge. The explanation lies with the structures of boron and aluminum. The outer electron is removed more easily from these atoms than the general trend in their period would suggest.
You might expect the boron value to be more than the beryllium value because of the extra proton. Offsetting that is the fact that boron's outer electron is in a 2p orbital rather than a 2s. This has two effects. The explanation for the drop between magnesium and aluminum is the same, except that everything is happening at the 3-level rather than the 2-level.
The 3p electron in aluminum is slightly more distant from the nucleus than the 3s, and partially screened by the 3s 2 electrons as well as the inner electrons. Both of these factors offset the effect of the extra proton. Once again, you might expect the ionization energy of the group 6 element to be higher than that of group 5 because of the extra proton. What is offsetting it this time? The screening is identical from the 1s 2 and, to some extent, from the 2s 2 electrons , and the electron is being removed from an identical orbital.
The difference is that in the oxygen case the electron being removed is one of the 2p x 2 pair. The repulsion between the two electrons in the same orbital means that the electron is easier to remove than it would otherwise be. The drop in ionization energy at sulfur is accounted for in the same way. As you go down a group in the Periodic Table ionization energies generally fall. You have already seen evidence of this in the fact that the ionization energies in period 3 are all less than those in period 2.
Taking Group 1 as a typical example:. Why is the sodium value less than that of lithium? Active 4 years, 1 month ago. Viewed times. Improve this question.
Does it include, in particular, the photoelectric effect? It's how many lab plasma devices work. Add a comment. Active Oldest Votes. Improve this answer. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password.
Post as a guest Name. Email Required, but never shown. What is Plasma Gas? The phases of matter are solid, liquid, gas and plasma. Since plasma and gas are closely related and plasma is rarely referred to when talking about the phases of matter, it is easy to understand the confusion. Gas turns into plasma when heat or energy is added to it. The atoms that make up the gas start to lose their electrons and become positively charged ions.
The lost electrons are then able to float freely. This process is called ionization.
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