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So, I know that the trend in the periodic table for the atomic radius is that it increases down a group and decreases from left to right across a period. I was wondering what the explanation for this is, that is what is the reason for this trend?
Atomic radius increases across a period because the additional electrons being added are all in the same shell while at the same time we are adding protons to the nucleus. As we add more protons to the nucleus, the effective nuclear charge pulls the electron closer towards the nucleus. However, each group as we go down on the periodic table has electrons on a new shell, and each subsequent shell is further away from the nucleus. Therefore the atomic radius increases down a group.
The reason why atomic radius increases down a group can further be explained with electron shielding: as each new shell is added to an atom (due to other shells being filled w/ electrons), shells closer to the nucleus will "shield" electrons in higher energy shells from the effective nuclear charge, allowing these electrons to stray slightly farther away from the nucleus's pull.
The atomic radius decreases across the period because the effective nuclear charge increases. For every electron added, a proton is added as well. However, because the electrons are added to the same shell, they are more spread out as they fill out the probability field. This means there is less electron shielding, and therefore a higher effective nuclear charge. In other words, the positive attraction becomes stronger.
The radius increases down a group since the additional shells are further from the nucleus. It decreases down a period because as electrons start filling the same shell the increasing difference in charge causes electrons to be pulled closer to the nucleus.
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