Question

As we move along the periodic table from left to right the atomic size decreases. However, noble gases have the largest size because of (1) covalent radii (2) ionic radii (3) Van der Wall's radii (4) stable octet

Short answer

(3) Van der Wall's radii

Step-by-step solution

- Understand atomic size trend

As we move from left to right across a period in the periodic table, the atomic size generally decreases due to an increase in the nuclear charge, which pulls the electrons closer to the nucleus.

- Examine noble gases

Noble gases are known to have relatively large atomic sizes compared to other elements in their respective periods despite the general trend.

- Identify reason for larger size in noble gases

Noble gases do not form covalent or ionic bonds easily because they have a stable octet. Thus, the atomic size that is often referred to is the Van der Waals radius, which is larger since it measures the size of the atom including the space where electron clouds might repel each other.

- Conclusion

Given that noble gases are measured using Van der Waals radii, which are inherently larger, the correct answer is: Van der Wall's radii.

Key concepts

atomic size

Atomic size, also called atomic radius, is the distance from the center of an atom’s nucleus to its outermost electron. As we move across the periodic table from left to right, atomic size decreases. This happens because more protons are added to the nucleus, increasing the nuclear charge. The stronger nuclear charge pulls the electrons closer, reducing the atomic size. However, when moving down a group, atomic size increases due to the addition of electron shells.

Key Points to Remember:

• Left to Right: Atomic size decreases.
• Top to Bottom: Atomic size increases.
• Stronger nuclear charge reduces atomic size.
• More electron shells increase atomic size.

noble gases

Noble gases are a group of elements found in Group 18 of the periodic table. They include helium, neon, argon, krypton, xenon, and radon. These elements are unique because they have a complete outer shell of electrons, making them chemically stable. This stable electron arrangement is often called a 'stable octet', which means having eight electrons in the outer shell (except for helium which has two).

Due to their stable octet, noble gases don't form covalent or ionic bonds easily. This stability also means that they have relatively low reactivity.

Key Characteristics:

• Stable octet.
• Low reactivity.
• Colorless, odorless gases at room temperature.
• Found in Group 18 of the periodic table.

Van der Waals radii

The Van der Waals radius measures the size of an atom that is not bonded to another atom. It's the distance where the electron clouds of two non-bonded atoms start to repel each other. For noble gases, the Van der Waals radius is often used because they don't form covalent bonds due to their stability. This measurement is larger than the covalent radius, which measures bonded atoms.

For instance, in noble gases like neon or argon, the Van der Waals radius is bigger, explaining why their atomic size seems large compared to other elements in the same period.

Important Notes:

• Used for atoms not bonded to each other.
• Larger than covalent radii.
• Commonly used for noble gases.
• Explains larger atomic size of noble gases.

nuclear charge

Nuclear charge is the total charge of all the protons in an atom's nucleus. It has a direct effect on an atom's size because it influences how strongly the electrons are pulled towards the nucleus.

As we move from left to right across a period in the periodic table, nuclear charge increases because more protons are added to the nucleus. This stronger attraction pulls the outer electrons closer, reducing the atomic size. Conversely, moving down a group adds more electron shells, increasing the distance despite the increased nuclear charge.

Highlights:

• Total charge of protons in the nucleus.
• Stronger nuclear charge pulls electrons closer.
• Increases from left to right in a period.
• Affects atomic size significantly.