Periodic Table Trends

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Trend (in the Periodic Table)

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A pattern observed for a certain property when looking from top to bottom within a group or from left to right within a period.

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Core reason underlying most periodic trends

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It generally comes down to the size of the atoms, specifically the distance between the nucleus and the valence electrons.

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Atomic Radius

Back

The size of an atom, which is physically the radius of the atom. It is the distance from the nucleus to the outermost electron in the electron cloud.

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Trend of AR: Top to Bottom (Down a Group)

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The atomic radius increases. This happens because the increasing number of electrons requires adding more electron shells to accommodate them, making the atom larger.

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Trend of AR: Left to Right (Across a Period)

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The atomic radius actually decreases. This is because as you add protons and electrons, the attraction of the electrons to the nucleus becomes greater, causing the entire electron cloud to move in closer.

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Example of AR (Period 2: Lithium to Neon)

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Lithium is the largest atom in period two, and neon is the smallest atom in that group.

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Smallest atom overall

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Helium.

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Largest atom overall

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Francium (or cesium).

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Ionization Energy

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The amount of energy needed to remove an electron from an atom. It can be thought of as the "cost" of taking an electron.

Front

What does a low ionization energy indicate?

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It means it is very easy to remove an electron from that atom.