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I get that it's because of the increase number of protons, but I still don't understand the concept. When you move left to right on a periodic table, the protons increase at the same rate as the electrons. In fact, the number of electrons end up being the same as the number of protons, no matter how far you are across a period (unless it's an ion).

So shouldn't the attractive force of the nucleus and the forces of electron-electron repulsion be cancelling out due to the equal number of protons and electrons in an atom? Or is it because protons have a greater force of attraction compared to the repulsion force between electrons?

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marked as duplicate by Jan, hBy2Py, Klaus-Dieter Warzecha, paracetamol, bon Dec 18 '16 at 9:02

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    $\begingroup$ @Mithoron Certainly a duplicate, but the answers there are rather sloppy. $\endgroup$ – Karl Dec 18 '16 at 0:43
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Atomic size across a period, in fact, has everything to do with electron shielding and the effective nuclear charge (charge felt by the electrons).

Across the period, electrons are added to the same outer level, meaning that shielding by inner electrons does not change, however, the numbers of proton increases. This results in the effective nuclear charge rising significantly (as no. protons increases while added electrons are shielding inefficiently), which of course, means that the atom has a smaller size.

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Through the period, the core electron shell shrinks, because the charge of the nucleus increases. The outer electrons have no influence on the core electrons: In an electrically charged sphere, there is no electric field in the inside. (Actually the valence electrons do have a finite propability density in the core, so this is not 100% correct, but the conclusion holds.)

The core electrons shield the valence shell from the core, so the effective charge of the nucleus is always identical to the number of valence electrons, but also the repulsion from the outermost electrons of the core only kicks in at a smaller radius.

Also the electrons tend to avoid each other pair-wise (Pauli principle), so the electrostatic repulsion between them is weaker than expected. This makes for the funny sawtooth line of atomic diameters vs. group number.

And, for heavy elements, the innermost core electrons become (relativistically) heavier, because they "move" very fast, so their orbital radius shrinks even more, letting the rest of the atom shrink as well (like above).

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