Question

According to general trends in the periodic table, predict which element in each of the following pairs has the larger atomic radius: (a) \(\mathrm{Rb}\) or \(\mathrm{Sr}\) (b) As or Se (c) Pb or Bi (d) I or Xe

Short answer

(a) Rb, (b) As, (c) Pb, (d) I

Step-by-step solution

Understand Atomic Radius Trends in the Periodic Table

The atomic radius generally increases as you move down a group due to the addition of electron shells. Conversely, it decreases across a period from left to right because of increased nuclear charge pulling the electrons closer to the nucleus.

Analyze Pair (a): Rb vs. Sr

Rubidium (Rb) and Strontium (Sr) are in the same period (5), with Rb being a Group 1 element and Sr a Group 2 element. Since atomic radius decreases across a period, Rb, being to the left of Sr, has a larger atomic radius.

Analyze Pair (b): As vs. Se

Arsenic (As) and Selenium (Se) are in the same period (4), with As in Group 15 and Se in Group 16. Atomic radius decreases across a period, so As, positioned to the left of Se, has a larger atomic radius.

Analyze Pair (c): Pb vs. Bi

Lead (Pb) and Bismuth (Bi) are in the same period (6), with Pb in Group 14 and Bi in Group 15. Following the periodic trend that atomic radius decreases across a period, Pb, being to the left of Bi, has a larger atomic radius.

Analyze Pair (d): I vs. Xe

Iodine (I) and Xenon (Xe) are in the same period (5), with I in Group 17 and Xe in Group 18. The atomic radius decreases across a period from left to right, so Iodine, being to the left, has a larger atomic radius than Xenon.

Key concepts

Periodic Table Trends

The periodic table is a powerful tool for understanding the properties of elements, as it organizes them based on similar characteristics. There are several trends that can be observed when looking at the periodic table which help predict the behavior and properties of elements.

One important trend is the variation in atomic size—specifically, the atomic radius. As we navigate through the table, shifting from one element to the next, these trends in atomic size give insight into how elements are structured and how they might interact with one another.

Generally, when moving down a group in the periodic table (from top to bottom), the atomic size increases. This occurs because each row down the group adds a new electron shell. Each additional shell pushes the outermost electrons further from the nucleus, increasing the size of the atom. Conversely, when we move from left to right across a period (a row in the table), the atomic size decreases. As we progress across a period, protons are added to the nucleus, increasing its positive charge. This stronger nuclear attraction pulls the electrons in the same shell closer to the nucleus, reducing the atomic radius.

Atomic Radius Trends

The trend in atomic radius across the periodic table is primarily influenced by two factors: the number of electron shells and the nuclear charge of the atoms. Atomic radius trends help predict such features of elements and their compounds.

• As you move down a group, a new electron shell is added for each subsequent element, resulting in a larger atomic radius. This is because the electron clouds extend further from the nucleus, despite the increase in nuclear charge, which is somewhat shielded by the additional inner electron shells.
• Across a period, the atomic radius decreases because electrons are added to the same shell while the number of protons increases. The enhanced positive charge of the nucleus attracts the electron cloud closer, effectively shrinking the atom's size. Though each additional electron slightly increases electron-electron repulsion, the stronger nuclear pull generally surpasses this repulsion, reducing the radius.

Understanding these trends aids in the prediction and comparison of atomic sizes among elements, providing a vital insight into the chemical behavior and interaction of the elements in different chemical contexts.

Element Comparison

Comparing elements to determine which has a larger atomic radius involves applying an understanding of periodic trends. By knowing where elements sit on the periodic table, we can make educated guesses about their relative sizes.

• For example, in a pair of elements that reside in the same period, such as Rubidium ( 2mathrm{Rb} 2) and Strontium ( 2mathrm{Sr} 2), Rubidium is further to the left in Group 1. Generally, elements on the left of the periodic period have larger atomic radii than those on the right, meaning Rubidium exhibits a larger atomic radius than Strontium.
• Similarly, Arsenic ( 2mathrm{As} 2) is larger than Selenium ( 2mathrm{Se} 2), Lead ( 2mathrm{Pb} 2) is larger than Bismuth ( 2mathrm{Bi} 2), and Iodine ( 2mathrm{I} 2) is larger than Xenon ( 2mathrm{Xe} 2). These comparisons follow the principle that atomic size decreases as you move from left to right within a period.
• An easy way to predict the larger atomic radius is to consider both the period trends and the group trends, allowing for logical deductions about element sizes based on their position within the periodic table.

This precision in comparing atomic radii is crucial for understanding not only the properties of individual elements but also their interaction in compounds.