Question Video: Identifying the Number of Electrons in Different Electron Shells Physics

The diagram shows electrons in different electron shells in an atom. How many electrons are there in the innermost shell? How many electrons are there in the outermost shell? How many electrons does the atom have in total?

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Video Transcript

The diagram shows electrons in different electron shells in an atom. How many electrons are there in the innermost shell? How many electrons are there in the outermost shell? How many electrons does the atom have in total?

Okay, so in this question, we’ve been given this diagram, and we’re told that it shows an atom with electrons in different electron shells. This red circle in the center represents the nucleus of the atom, so that’s where all the atoms, protons, and neutrons are found. Then around this nucleus, we’ve got these two black circles, each of which represents one of the atom’s electron shells. Each one of these small blue circles on each of these shells represents an electron that is in that shell in the atom.

In the first part of the question, we are asked “how many electrons are there in the innermost shell?” Now, we can recall that the innermost shell is the shell that’s closest to the atom’s nucleus. So, in our diagram then, that’s this inner black circle. Since each of the small blue circles in the diagram represents one electron, then to find the number of electrons in this innermost shell, we just need to count the number of small blue circles drawn on this shell in the diagram. We can see then that in this innermost shell, we’ve got this electron here and this one over here. In total then, we’ve counted two electrons drawn on this inner black circle. And this means that we know that there’re two electrons in this atom’s innermost shell.

Moving on now to the second part of the question, this asks us “how many electrons are there in the outermost shell?” In the same way that the innermost shell is the closest shell to the nucleus, an atom’s outermost shell is the shell that’s furthest away from the nucleus. In our diagram then, the atom’s outermost shell is represented by this outer black circle. To find the number of electrons in the outermost shell then, we just need to count the number of small blue circles drawn on this outer black circle.

To count up the number of blue circles drawn on this outermost shell, let’s number them off, starting at the top and then working our way around the circle in the clockwise direction, labeling each circle in turn. When we do this, we find that in total there are eight of them. We have found then that the number of electrons in the outermost shell is eight.

Lastly, in the third part of the question, we are asked “how many electrons does the atom have in total?” Since we can see from the diagram that this atom only has two electron shells, that’s an innermost shell and an outermost shell, then the total number of electrons in the atom must be equal to the number of electrons in this innermost shell plus the number of electrons in the outermost shell. In the first two parts of the question, we already found that there are two electrons in the innermost shell and eight electrons in the outermost shell. This means that we know that the total number of electrons in this atom is equal to the two electrons in the innermost shell plus the eight electrons in the outermost shell. Computing this sum gives us a result of 10.

Our answer to this third part of the question then is that the atom has 10 electrons in total.