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2.4. Electron Affinity
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Today, we're going to discuss electron affinity, which is essentially the energy change when an electron is added to an atom in the gas phase. Can anyone tell me how this can be represented mathematically?
Isn't it X(g) + e⁻ → X⁻(g)?
That's correct! And we denote the energy change as ΔE = EA. Now, why do you think this energy change is important?
I think it helps us understand how easily an atom can gain an electron!
Exactly! So, does anyone know how we categorize whether this process is exothermic or endothermic?
We say it's exothermic if energy is released, right? So, it would have a negative value.
Yes! And if energy is absorbed, we get a positive value. So, remember, electron affinity gives us insight into the reactivity of elements and how they form anions.
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Now, let's look at how electron affinity trends across a period. What happens to the electron affinity as we move from left to right, and why?
I think it becomes more negative, like it's more exothermic.
Exactly! This is primarily due to the increase in effective nuclear charge, or Z_eff, as well as the decrease in atomic radius. Can anyone explain what that means?
It means that the added electron feels a stronger pull from the nucleus, right?
Yes! Now can anyone name any exceptions to this trend?
Group 2 elements like Be and Mg have less negative electron affinities!
Great! Those exceptions occur because they have a filled s orbital which makes gaining an electron less favorable.
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Create a free accountNext, let's explore the trend down a group. What happens to electron affinity as we descend the periodic table?
It becomes less exothermic, or less negative.
Exactly! Why do you think that is?
Because the added electron goes into a higher energy level that's further away from the nucleus?
That's right! And because it’s farther from the nucleus, the attraction isn’t as strong, resulting in lower energy release. Can anyone give me an example of a group with positive electron affinities?
Noble gases have positive electron affinities since it's unfavorable to add an electron to their filled shells.
Excellent point! Remember, this is a crucial aspect of how we understand the behavior of different elements.
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Create a free accountTo wrap up our session, let's summarize the key points about electron affinity. Can anyone recall what electron affinity measures?
It measures the energy change when an electron is added to an atom!
Correct! And how does electron affinity generally trend across a period?
It becomes more exothermic from left to right.
Good job! And what about down a group?
It becomes less exothermic.
Exactly right! Remember these trends are critical for understanding element reactivity. Any final thoughts or questions?
Nothing from me, but I appreciate how it all fits together!
Glad to hear that! Keep these concepts in mind as they will tremendously help when we discuss chemical bonds next.
Overview
Short Summary
Electron affinity refers to the energy change that occurs when an electron is added to a gaseous atom, forming an anion.
Medium Summary
Electron affinity measures the tendency of an atom to accept an electron. This section discusses the definition, trends across periods and groups, exceptions, and the significance of electron affinity in understanding element reactivity.
Detailed Summary
Electron Affinity
Electron affinity (EA) quantifies the energy change associated with the addition of an electron to a neutral atom in the gas phase, resulting in the formation of an anion. The reaction can be expressed as:
X(g) + e⁻ → X⁻(g) ΔE = EA.
Typically, the value of EA is reported as negative when energy is released (exothermic process) and positive when energy must be absorbed (endothermic).
Trends in Electron Affinity
- Across a Period:
- Electron affinity generally becomes more exothermic (more negative) from left to right across a period. This is attributed to an increase in effective nuclear charge (
Audio Book
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Create a free account● Electron affinity (EA): The energy change (often released) when an electron is added to a gaseous atom, forming an anion:
X(g) + e⁻ ⟶ X⁻(g) ΔE = EA.
● Generally reported as the negative of ΔE if energy is released (exothermic process) or positive if energy must be absorbed (endothermic).
Detailed Explanation
Electron affinity is a measure of the tendency of an atom to gain an electron. When an electron is added to a gaseous atom, it can either release energy (exothermic, reported as a negative value) or absorb energy (endothermic, reported as a positive value). Essentially, this process tells us how much an atom wants to capture an electron and the energy changes that accompany this process.
Examples & Analogies
Think of electron affinity like a game of catch. If you are excited to catch a ball (gain an electron), you might run towards it, and that excitement (energy release) makes you feel good. However, if the ball is heavier than you expected and you have to put in extra effort to catch it (absorb energy), you might not be as happy about it. Just like some atoms release energy when they capture an electron while others need energy to do the same.
Key Concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
Electron affinity measures the energy change when an electron is added to an atom in the gas phase.
Electron affinity tends to become more exothermic across a period due to increased effective nuclear charge.
Electron affinity generally becomes less exothermic down a group due to added electrons entering higher energy levels.
Noble gases often show positive electron affinities due to filled outer electron shells.
Examples
Step-by-step examples to apply the section's ideas and test your understanding.
When chlorine (Cl) gains an electron to form Cl⁻, it releases energy, indicating a negative electron affinity.
Beryllium (Be) has a positive electron affinity, requiring energy to add an electron because it has a filled 2s shell, making it less favorable.
Memory Aids
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