When considering how atomic radii change as electrons are added to a shell, it’s important to focus on what happens across a period in the periodic table (from left to right), where electrons are added to the same principal energy level (shell).
As electrons are added to a shell, the number of protons in the nucleus also increases (since atomic number increases across a period). While you might expect that adding more electrons would make the atom larger, the effect is actually the opposite.
Here’s why:
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Each additional electron enters the same shell, so there is no increase in the distance from the nucleus due to new shells.
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At the same time, the nuclear charge increases with each new proton, pulling all the electrons more strongly toward the nucleus.
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The increased attraction between the higher positive charge in the nucleus and the negatively charged electrons causes the electrons to be drawn closer to the nucleus.
As a result:
The atomic radius decreases as electrons are added to a shell across a period. The greater effective nuclear charge pulls the electron cloud tighter, making the atom smaller despite the increased number of electrons.
In summary:
When electrons are added to the same shell, atomic radii decrease across a period due to the increasing positive charge of the nucleus, which pulls electrons closer.
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