Noble gases have larger atomic size than the preceeding halogens because

Answer

To answer the question of why noble gases have larger atomic sizes than the preceding halogens, we can break down the explanation into a series of logical steps:

### Step-by-Step Solution:

1. **Understanding Atomic Size**: Atomic size refers to the distance from the nucleus to the outermost shell of electrons. It is influenced by the number of electron shells and the effective nuclear charge.

2. **Position in the Periodic Table**: Noble gases are located in Group 18 of the periodic table, while halogens are in Group 17. As we move from left to right across a period, the atomic number increases.

3. **Nuclear Charge**: As we move across a period from left to right, the nuclear charge (the total charge of the nucleus due to protons) increases. This is because each successive element has one more proton in its nucleus.

4. **Number of Electron Shells**: The number of electron shells remains constant for elements in the same period. For noble gases and halogens in the same period, they have the same number of electron shells.

5. **Effective Nuclear Charge**: Although the nuclear charge increases, the increase in the number of protons also leads to an increase in the effective nuclear charge experienced by the outermost electrons. However, since noble gases have a complete outer shell, they experience a greater repulsion among their electrons compared to halogens.

6. **Comparison of Atomic Sizes**: Due to the complete outer shell in noble gases, the electrons are held less tightly compared to halogens, which have one less electron in their outer shell. This results in a larger atomic radius for noble gases compared to halogens.

7. **Conclusion**: Therefore, noble gases have larger atomic sizes than the preceding halogens due to their higher effective nuclear charge and the complete outer electron shell, which leads to less effective attraction of the outermost electrons to the nucleus.

### Final Answer:
Noble gases have larger atomic sizes than the preceding halogens because, as we move across a period, the nuclear charge increases while the number of electron shells remains the same. The complete outer shell of noble gases leads to a larger atomic radius compared to halogens, which have one less electron in their outer shell.

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