### Video Transcript

The graph shows the probability of finding an electron at a distance from the nucleus for the first three s orbitals of an atom of hydrogen. Which plot corresponds to the 1s orbital?

We’re generally used to describe electrons as point particles that are located at a single point in space. Now, we describe electrons with three-dimensional mathematical expressions that describe the most likely location of an electron in an atom. We describe electrons using atomic orbitals. The four key atomic orbitals are s, p, d, and f. The different orbital types have different shapes. The s orbital is spherical. The question refers to the first three s orbitals, which are 1s, 2s, and 3s. The 2s orbital is larger than the 1s orbital. It consists of two regions of electron density, represented using the color blue, separated by a radial node. Both regions of electron density are part of the 2s orbital.

A node is a point in space where the probability of finding an electron is zero. There isn’t just one radial node in the 3s orbital, but two radial nodes. We can use the presence of these nodes to find out which plot corresponds to the 1s orbital. At a node, the probability of finding an electron is zero. So, if we look at the 𝑦-axis of the graph, which is the probability of finding an electron, we can determine that when the 𝑦-value is zero, there will be a node.

Let’s start by looking at the green line C. It has two nodes, where there is zero chance of finding an electron. There is a nonzero chance of finding an electron outside of the orbital. So, for each orbital, although the tail of the line appears to have a value of zero, it doesn’t. So that part is not counted as a node. And it has three regions where there is a probability of finding an electron. We can see that this green line corresponds to the 3s orbital, not the 1s orbital as specified in the question. Thus, plot C is not the answer to this question.

Plot B, the red line, has one node where there’s no probability of finding an electron and two regions where there is a probability of finding an electron. So we can see that the red line B corresponds to the 2s orbital. Therefore, plot B is not the answer to this question.

The final plot, plot A, has one region where there’s a probability of finding an electron. Therefore, it must represent the 1s orbital. So the answer to the question “Which plot corresponds to the 1s orbital?” is A.