Describe and explain the trend in atomic radius

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Describe and explain the trend in atomic radius as atomic number increases across a period from alkali metal to noble gas.

Final Answer:

As atomic number increases across a period from alkali metals to noble gases, the atomic radius decreases due to an increase in nuclear charge that pulls electrons closer to the nucleus. Electrons are added to the same shell, which does not effectively shield them from the nucleus’s increasing positive charge. This trend results in a stronger attraction to the nucleus and a smaller atomic size.

Explanation:

The atomic radius of an element refers to the distance from the nucleus to the outermost shell of electrons. When examining the trend in atomic radius as the atomic number increases across a period from alkali metals (Group 1) to noble gases (Group 18), there are a few key points to consider.

Increase in Protons: As we move from left to right across a period, elements gain more protons in their nucleus. This increase in positive charge affects the attraction between the nucleus and the electrons.

Addition of Electrons: While protons are being added to the nucleus, the electrons are added to the same principal energy level or shell. This means that although there are more electrons, they are not shielding the outer electrons significantly from the increased nuclear charge.

Effective Nuclear Charge: The increased positive charge from protons results in a stronger effective nuclear charge experienced by the valence electrons. This increased pull draws the electrons closer to the nucleus, leading to a decrease in atomic radius as you move across the period.

Resulting Trend: Consequently, the atomic radius decreases across a period. For example, if we look at the trend from sodium (Na) to chlorine (Cl), the atomic radius decreases because of the stronger attraction of the nucleus on the outer electrons.

Comparison with Noble Gases: At the end of the period, noble gases have the smallest atomic radii in their respective periods. This is due to the full valence shell, where the electrons are held very close to the nucleus due to the high effective nuclear charge.

Summary of the Trend: In summary, as you move from alkali metals to noble gases across a period, the atomic radius decreases due to the increase in nuclear charge, which pulls the electrons in tighter as more protons are added, while electrons are added to the same shell, leading to a stronger attraction without significant shielding.

Overall, understanding atomic radius trends helps chemists predict how elements will behave in chemical reactions and bonds.

Examples & Evidence:

For example, the atomic radius of sodium (Na) is larger than that of chlorine (Cl) because Na has fewer protons, resulting in less effective nuclear charge. Similarly, noble gases like argon (Ar) have the smallest atomic radius in their period due to complete valence shells and strong nuclear attraction.

This explanation is supported by the basic principles of atomic structure and periodic trends documented in chemistry, which confirm that atomic radius decreases due to increased effective nuclear charge across a period.

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