The diagram shows an electrically neutral helium atom. How many of the electrons in the atom are in excited states?
Okay so, in this diagram we’ve been told that we’ve got a helium atom. And, we can see that this pink blob in the middle must represent the nucleus of the helium atom, which contains all the protons and neutrons in this atom. Additionally, we see two black circles which represent the electron energy levels in this atom. And, the blue blobs actually represent electrons themselves. Now, these electrons are occupying these energy levels.
Now, we’ve been told in the question that this atom is a helium atom and that it’s electrically neutral. If we look on the periodic table, we can see that a helium atom is defined as having two protons in its nucleus. And therefore, we know that there must be two protons in the nucleus, plus however many neutrons, which is not relevant to us right now.
Now, the fact that this atom is electrically neutral means that the number of electrons in this atom must be the same as the number of protons in the nucleus of this atom. And, the reason for this is that protons are positively charged and electrons are negatively charged. Therefore, the number of positive charges must balance the number of negative charges in order for the atom to be electrically neutral. And all this is telling us then is that there are two electrons in this atom.
But then we can see this from the diagram already. There are one, two electrons actually shown in the diagram. So anyway, what we’ve been asked to do is to find how many of the electrons in the atom are in excited states. To understand what we mean by excited state, we need to first recall that each electron energy level or shell can only hold up to a certain number of electrons.
Let’s recall that the very first shell, the shell nearest to the nucleus, which we will label as shell number one, can hold up to two electrons. But in this particular scenario, it’s only holding one electron. And yet there’s another electron in the second shell, shell number two.
Now, if these electrons were in the ground state, that is the lowest possible energy state that they can be in, then all of the inner shell, shell number one, will have to be filled completely before shell number two started filling up. In other words, both electrons in this atom would have had to be in the first shell, in order for them both to be in the ground state.
In this scenario though, it looks like one of those electrons has moved up to the second energy level. Thus, leaving behind a gap in a lower energy level that could be filled by an electron. Therefore, the electron that has popped up to a higher energy level is the one that is in an excited state. Whereas the other electron is in the very lowest shell possible. Therefore, that electron is in its ground state. Hence, the answer to our question is that there is one electron in this atom that is in its excited state.