Finding the Largest Atom Size Using Periodic Trends

The question asks us to identify the atom with the largest size among the given options: Silicon (Si), Aluminum (Al), Phosphorus (P), and Sulfur (S).

Atomic size, also known as atomic radius, is a fundamental property of an element. It generally refers to the distance from the nucleus of an atom to its outermost electron shell.

Understanding Atomic Size Trends in the Periodic Table

Atomic size exhibits clear trends across the periodic table:

• Down a group: Atomic size generally increases as you move down a group. This is because new electron shells are added, increasing the distance between the nucleus and the valence electrons, despite an increase in nuclear charge.
• Across a period: Atomic size generally decreases as you move from left to right across a period. This trend occurs because electrons are added to the same valence shell, while the number of protons in the nucleus increases. The increased positive charge of the nucleus pulls the valence electrons closer, resulting in a smaller atomic radius. This effect is related to the increase in the effective nuclear charge ($Z_{eff}$).

Analyzing the Given Elements

Let's look at the position of the given elements in the periodic table:

• Aluminum (Al) is in Group 13, Period 3.
• Silicon (Si) is in Group 14, Period 3.
• Phosphorus (P) is in Group 15, Period 3.
• Sulfur (S) is in Group 16, Period 3.

All four elements are in the same period, Period 3. They are arranged from left to right in the periodic table in the order: Al, Si, P, S.

Applying the Periodic Trend to Find the Largest Atom

Since Al, Si, P, and S are all in the same period (Period 3), we apply the trend for atomic size across a period. Atomic size decreases from left to right across a period. Therefore, the element furthest to the left in Period 3 among the given options will have the largest atomic size.

Comparing their positions:

Al (Group 13) is to the left of Si (Group 14), P (Group 15), and S (Group 16).

Therefore, Aluminum (Al) is expected to have the largest atomic size among these four elements.

Conclusion

Based on the trend that atomic size decreases across a period, Aluminum (Al) is the largest sized atom among Silicon (Si), Aluminum (Al), Phosphorus (P), and Sulfur (S) because it is located furthest to the left in Period 3.

Revision Table: Key Periodic Trends

Additional Information: Effective Nuclear Charge ($Z_{eff}$) and Shielding

The decrease in atomic size across a period is primarily due to the increase in effective nuclear charge ($Z_{eff}$).

• Effective Nuclear Charge ($Z_{eff}$): This is the net positive charge experienced by a valence electron. It is calculated as $Z_{eff} = Z - S$, where $Z$ is the atomic number (number of protons) and $S$ is the shielding constant, representing the shielding effect of inner electrons.
• Shielding Effect: Inner electrons shield the valence electrons from the full positive charge of the nucleus. Across a period, electrons are added to the same valence shell. While the nuclear charge ($Z$) increases, the shielding effect ($S$) by inner electrons remains relatively constant because no new inner shells are being added.
• Impact on Size: As $Z$ increases and $S$ stays similar, $Z_{eff}$ increases across the period. A higher effective nuclear charge means the nucleus pulls the valence electrons more strongly, drawing them closer and reducing the atomic size.

For elements in Period 3 like Al, Si, P, and S, as you move from Al to S, the nuclear charge increases, leading to a stronger pull on the valence electrons in the 3rd shell, hence decreasing the atomic radius.