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Electron Affinity: Definition, Trends & Equation

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  • 0:00 What is Electron Affinity?
  • 1:35 Factors and Trends
  • 3:20 Exceptions to General Trends
  • 5:34 Lesson Summary
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Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

When an electron is added to an atom, a change in energy occurs. This change in energy is what we call the electron affinity. In this lesson, we will discuss electron affinity and its general trend in the periodic table.

What is Electron Affinity?

Imagine you're carrying a bag and adding things to it. Naturally, the bag becomes heavier, and there is a change in the energy you expend when the weight changes. In the same way, when an atom gains electrons, an energy change occurs. This energy change is what we call the electron affinity.

The electron affinity is defined as the energy change that occurs when an atom gains an electron, releasing energy in the process. Let's remember that an electron is negatively charged, so when an atom gains an electron, it becomes a negative ion.

An Atom Gaining an Electron

Since we are talking about a change in energy, when an electron is added to an atom, there is an equation used to determine the electron affinity:

Electron Affinity Equation

This equation shows that electron affinity is equal to the negative change in energy. Let's clarify the sign convention for the energy change associated with the gain of an electron. Remember that the definition of an electron affinity is the energy released, so that means that the reaction is exothermic. If a reaction is exothermic, the change in energy is negative. This means that the electron affinity is positive.

For example, the electron affinity of chlorine has the negative sign, which shows us the energy that is released to add one electron to an atom. The giving off of energy is shown with a negative sign.

Electron Affinity of Chlorine

Based on this sign convention, this means that a higher electron affinity indicates that an atom more easily accepts electrons. A lower electron affinity indicates that an atom does not accept electrons as easily.

Factors That Affect Electron Affinity

There are two factors that can affect electron affinity. These are atomic size and nuclear charge. With regard to atomic size, let's think about a magnet and a refrigerator. When a magnet is closer to the surface of the refrigerator, you can clearly feel the pull of the attraction between the magnet and the refrigerator. The farther the magnet gets away from the fridge, the less you feel the attraction or pull.

When looking at a drawing of a smaller atom side by side with a bigger atom, it can be seen that a smaller atom's outermost shell is closer to the nucleus than that of a bigger atom. Just like our magnet and refrigerator analogy, the electron will feel more attraction to the nucleus if it is closer.

Electron Affinity and Atomic Radius

The smaller the atom is, the closer the outermost shell is; therefore, it is a stronger attraction between the nucleus and the incoming electron. That means the electron affinity is higher for smaller atoms.

When looking at the periodic table the atomic radius increases from top to bottom, moving down a column; therefore, the electron affinity increases from the bottom to the top of the column.

Nuclear charge also affects electron affinity. The nuclear charge is also known as the atomic number, which is the same as the number of protons. Protons are positive subatomic particles. The more protons there are, the greater the attraction is to electrons.

Trends or patterns can be seen in the periodic table with regard to electron affinity. We are looking at this in terms of across the period, not down a group. From left to right, the nuclear charge increases, resulting in a greater attraction to incoming electrons. So, we can say that from left to right across a period, the electron affinity increases upward.

General Trend: Electron Affinity

Exceptions to the General Trend for Electron Affinity

The trends for electron affinity are generalizations, and so it's important to indicate a few exceptions. It's important to mention that noble gases are not included in the trend for electron affinity. Noble gases are inert, or nonreactive, and they like to stay the way they are. So, their tendency to attract electrons is very low, which is why their electron affinities are close to zero.

Looking at the periodic table, you'd think that fluorine would have the highest electron affinity. However, this is not the case. Imagine putting an object, let's say, an electron, in a small bag that is already packed with fluorine. It would be difficult to zip up the bag since there is barely any space left.

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