Ionization Energy Trend

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First ionization energy order (highest to lowest):

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1. **General Trend**: Ionization Energy (IE) generally increases from left to right across a period.
2. **Noble Gases**: **Neon ($Ne$)** has a completely filled octet, making it extremely stable. It has the highest IE in its period.
3. **Halogens**: Fluorine ($F$) is next with a very high effective nuclear charge.
4. **The Exception (N vs O)**:
   - Nitrogen has a configuration of $1s^2 2s^2 2p^3$ (**half-filled $p$-subshell**). This is inherently stable due to symmetry and exchange energy.
   - Oxygen is $1s^2 2s^2 2p^4$. Removing the $4^{th}$ electron relieves electron-electron repulsion in the $p_x$ orbital.
5. **Result**: $N$ has a higher first IE than $O$. 
6. **Final Order**: **$Ne > F > N > O$**.

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Ionization Energy Trend

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