Ch.9 - Periodic Properties of the Elements

Tro - Chemistry: A Molecular Approach 5th Edition

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Tro 5th Edition

Ch.9 - Periodic Properties of the Elements

Problem 77a

Chapter 9, Problem 77a

Choose the element with the more negative (more exothermic) electron affinity from each pair. a. Na or Rb

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Understand that electron affinity refers to the energy change when an electron is added to a neutral atom in the gaseous state. A more negative electron affinity indicates a greater tendency to gain an electron.

Recall that electron affinity generally becomes more negative across a period from left to right and less negative down a group in the periodic table.

Identify the position of Na (Sodium) and Rb (Rubidium) in the periodic table. Na is in the third period, while Rb is in the fifth period.

Consider the trend: as you move down a group, the electron affinity becomes less negative due to increased atomic size and electron shielding, which makes it harder for the nucleus to attract additional electrons.

Based on the periodic trend, predict that Na, being higher up in the group than Rb, will have a more negative (more exothermic) electron affinity compared to Rb.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Electron Affinity

Electron affinity is the energy change that occurs when an electron is added to a neutral atom in the gas phase. A more negative electron affinity indicates that the process is more exothermic, meaning energy is released when the atom gains an electron. This property is crucial for understanding how easily an atom can accept an electron and form an anion.

Trends in the Periodic Table

Periodic trends refer to the predictable patterns observed in the properties of elements as you move across or down the periodic table. For electron affinity, it generally increases (becomes more negative) across a period from left to right and decreases down a group. Understanding these trends helps in predicting the electron affinity of elements based on their position in the periodic table.

Group 1 Elements

Group 1 elements, also known as alkali metals, include lithium, sodium, potassium, rubidium, cesium, and francium. These elements have low ionization energies and tend to lose one electron easily, making their electron affinities less negative compared to other groups. When comparing sodium (Na) and rubidium (Rb), it is essential to consider their positions in the group and how their atomic size affects their ability to attract additional electrons.

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