Periodic Trends

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Among the elements Na, Mg, Al, and Si, which has the highest first ionization energy?

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**First Ionization Energy ($IE_1$)** is the energy required to remove the outermost electron from an isolated gaseous atom.
- Across a period (from left to right), the **atomic radius decreases** and the **effective nuclear charge ($Z_{eff}$) increases**.
- This makes it progressively harder to remove an electron.
- For Period 3 elements: $Na < Al < Mg < Si$.
- (Note: $Mg$ has higher $IE_1$ than $Al$ because $Mg$ has a stable $3s^2$ filled subshell).
- However, among the four, **Silicon ($Si$)** has the highest nuclear charge and smallest size, resulting in the highest $IE_1$.

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