Understanding the Relationship Between Atomic Size and Nonmetallic Character

The question asks how the nonmetallic character changes as the atomic size increases. To answer this, we need to understand what nonmetallic character and atomic size represent in chemistry and how they relate to each other based on periodic trends.

What is Nonmetallic Character?

Nonmetallic character refers to the tendency of an atom to gain electrons to form a negative ion (anion). Nonmetals are typically found on the right side of the periodic table (excluding group 18, noble gases, which are generally unreactive). High nonmetallic character is associated with high electronegativity and high electron gain enthalpy (more negative values).

What is Atomic Size?

Atomic size, often measured as atomic radius, is the distance from the nucleus of an atom to its outermost electron shell. Atomic size is influenced by the nuclear charge and the shielding effect of inner electrons.

How Atomic Size Impacts Nonmetallic Character

The ability of an atom to attract and gain an electron depends on the strength of the attraction felt by an incoming electron towards the nucleus. This attraction is significantly affected by the atomic size.

• When the atomic size is small, the valence electrons are closer to the nucleus. The positive charge of the nucleus exerts a strong pull on these electrons.
• An incoming electron will also experience a strong attraction from the nucleus because the outer shell is close to the nucleus, and there is less shielding from inner electrons. This strong attraction makes it easier for the atom to gain an electron, thus exhibiting high nonmetallic character.

Conversely, when the atomic size is large:

• The valence electrons are farther away from the nucleus.
• An incoming electron attempting to join the valence shell will be at a greater distance from the nucleus.
• The attraction from the positive nucleus is weaker over this larger distance.
• There is also increased shielding effect from the greater number of inner electron shells, which further reduces the effective nuclear charge felt by the incoming electron.

Because the attraction for an incoming electron is weaker when the atomic size is large, the atom has a reduced tendency to gain electrons. This means its nonmetallic character is lower.

Periodic Trends and Nonmetallic Character with Atomic Size

This relationship is clearly observed in periodic trends:

• Down a Group: As you move down a group in the periodic table, the number of electron shells increases. This leads to an increase in atomic size. Correspondingly, the nonmetallic character generally decreases down a group (e.g., from Fluorine to Iodine in Group 17). The larger atomic size makes it harder to attract an electron.
• Across a Period: As you move across a period from left to right, the nuclear charge increases, but the number of main electron shells remains the same. This results in a decrease in atomic size. Simultaneously, the nonmetallic character increases across a period (e.g., from Sodium to Chlorine in Period 3). The smaller atomic size and higher effective nuclear charge make it easier to attract an electron.

Therefore, there is an inverse relationship: as atomic size increases, the nonmetallic character decreases.

Conclusion

Based on the fundamental principles of electron attraction by the nucleus and how it is affected by distance and shielding, an increase in atomic size leads to a decreased ability to attract and gain electrons. This directly translates to a decrease in nonmetallic character.