Atom Size

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The periodic table is arranged into rows called periods and columns known as groups. Elements in the same group have similar properties. Elements that are metals are found in the left side and middle of the periodic table. The nonmetals are found in the top right of the periodic table. Metalloids (which are somewhere in between) are found around the bold diagonal line shown in the periodic table to the right, which represents the boundary between metals and nonmetals.

Effective Nuclear Charge

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Let's begin this unit by looking again at effective nuclear charge ($Z_{eff}$). Most of the periodic trends we will consider can be related back to effective nuclear charge. $Z_{eff}$ increases as we move across the row on the periodic table because Z (atomic number) increases while the number of shielding core electrons stays the same. By our simple calculation of $Z_{eff}$ for a valence electron in Be and F, we can see this is true.

$Z_{eff} = Z -S$

For Be: $Z_{eff} = 4-2 = +2$

For F: $Z_{eff} = 9-2 = +7$

$Z_{eff}$ increases ever so slightly down a group if we look at precisely calculated values. The orbitals become less dense and more diffuse, so they are less effective at shielding.

The Trend in Atom Size

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Try it!

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