Video Transcript
Which of the following equations correctly shows the first electron affinity of an atom? (A) A negative gas ion plus an electron produces a gas atom. (B) A negative gas ion produces a gas atom plus an electron. (C) A gas atom plus an electron produces a negative gas ion. (D) A gas atom produces a positive gas ion plus an electron. Or (E) a gas atom plus a gas atom produces a positive gas ion and a negative gas ion.
This is a question about electron affinity. Electron affinity is defined as the energy released per mole of atoms when an electron is added to a neutral atom in the gas state to form a negative ion. To visualize this sentence, we can first show a neutral atom. When we combine an electron with that neutral atom, a negative ion forms and energy is released.
It is worth noting that this question specifically asks about the first electron affinity. The terms electron affinity and first electron affinity are often used interchangeably. So this definition is also the definition of the first electron affinity. Although it won’t come up in this problem, it’s worth noting that successive electron affinities, such as the second electron affinity, are a measure of the energy released as we add more electrons to the ion. With this definition and image in hand, can we select the answer choice that represents this process as a chemical equation?
First, we know that we must start with a neutral atom. Answer choice (A) and (B) begin with an ion, not a neutral atom, so we can eliminate them from consideration. Next, we know that an electron is added to this neutral atom. Answer choice (D) and answer choice (E) do not show an electron being added to the neutral atom. So we can eliminate them from consideration as well. We have only one answer choice remaining. Let’s make sure it matches the rest of the definition, and in fact it does. The last part of the definition shows that this process forms a negative ion. We can see that negative ion as a product in the definition, in the image, and in the chemical equation in answer choice (C). Choice (C) is the correct answer.
In this example, the X is a placeholder for any element. We could replace the X with an element such as fluorine to see a more specific example of this equation. We often see these electron affinity equations paired with an amount of energy in kilojoules per mole. This number is the amount of energy released during this process. In other words, it’s the electron affinity of the element in the equation. 328 kilojoules per mole of energy are released when we turn fluorine atoms into fluoride ions.
So, which of the following equations correctly shows the first electron affinity of an atom? That’s choice (C), a neutral gas atom plus an electron produces a one minus gas ion.
