Is it true that the interior of an atom is mostly vacuum or empty space?

I'm not sure why people write such excessive comments on this one... The atom is surrounded by an electron cloud. But the electron cloud's mass density is comparatively small to that of the core. Most of the mass you interact with, is located at the core. In this picture, the electron cloud is simply considered "vacuum". But the electron cloud communicates the forces you need to interact with other objects.

A simple way to look at the "interior of an atom is mostly vacuum" sentence is to think about mass distribution. In some sense the sentence is true: most of the mass is concentrated in a very small volume (the nucleus). Compared to the nucleus, the rest of the space is occupied by electron orbitals, which matter density (or better, the energy density) is many orders of magnitude lower (so almost "vacuum" from this point of view).

It is just a non-technical way of conveying the idea that the atom structure is "similar" to a "dense" planet that is extremely small but is surrounded by a rarefied atmosphere that contributes almost nothing to the total mass. Maybe a ineffective analogy for someone, but useful to give to the general public the idea that you do not need to be massive to occupy space.

I would take your sentence

It is important to the goal of conveying correct physical intuition and thus encouraging correct physical insight.

as a central guideline in my answer, as you'll see, my conclusions are quite different.

The origin of the statement about the almost empty interior of an atom comes from the thorough Rutherford's analysis (1911) [1] of the Geiger and Marsden experiment of alpha particles scattering by thin gold foils. Rutherford did not write explicitly such expression, but the key point of his analysis was to show that a model of the atom made by a spatially concentrated heavy particle whose charge should be proportional to the atomic weight was able to account for the experimental data much better than a diffuse density of the scattering component. Therefore, the main emphasis in Rutherford's paper was more on the "concentrated" character of the scattering component than on the presence of an "empty space."

However, such last inference was quite natural once one considers the positive nuclei as the main source of scattering if the second atomic component, the electrons, are considered point-like particles. An unavoidable step more than 10 years before the birth of wave mechanics.

So, from a historical perspective, the sentence corresponds to a well definite and historically justifiable point of view.

Now, let's examine the situation with today's eyes. What could be a correct statement? The interior of the atom is empty? Is it full of something? Electrons? Fields?

Here, I'm afraid I have to disagree with your idea that

The main point is that the electrons are smoothly spread throughout the interior of each atom, and they carry enough mass and charge to make it misleading to compare the situation to empty space.

I am afraid that such a statement would enforce a common misconception which tries to maintain the original idea of the "waves of matter" proposed by de Broglie and eliminated from the possible interpretations of QM since the analysis of scattering processes made by Born and resulting in the present-day probabilistic interpretation of QM.

From a pedagogical point of view, the key point is to insist that all the existing experiments agree on a point-like structure of the electron. Measuring charge or mass density would be misleading, since it would convey the idea that it is possible to detect part of an electron.

In summary, I would not insist too much on the sentence about "empty space" in the atom if not connected with Rutherford's analysis. But certainly, I would not substitute it with potentially wrong ideas about an extended electron.

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[1] Professor E. Rutherford F.R.S. (1911) LXXIX. The scattering of α and β particles by matter and the structure of the atom, The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 21:125, 669-688, DOI: 10.1080/14786440508637080

I will answer the question by comparing empty space and vacuum with the properties of the interior of an atom. I will write at the level of high school physics in order to make my answer accessible. My conclusion will be that the answer is "no": it is not true that the interior of an atom is mostly vacuum or empty space, and furthermore such an idea conveys a thoroughly misleading picture of the nature of atoms. The main point is that the electrons are smoothly spread throughout the interior of each atom, and they carry enough mass and charge to make it misleading to compare the situation to empty space.

First let's see what we mean by empty space. We mean of course "nothing is there". To flesh this out a little, consider the case of an ordinary gas at standard temperature and pressure (STP). This is not a vacuum, clearly, since the pressure is quite high. We might speak of "vacuum" once the pressure is below a millibar (100 Pa); at STP the pressure is about a bar ($10^5$ Pa). On the other hand it is true to say that such a gas is "mostly empty space" in that the mass is concentrated in the molecules, with almost no mass in between the molecules, and the volume occupied by the molecules is a small fraction (about a thousandth) of the total. On the other hand, if you place a mass-detector anywhere in the gas, then it will register some mass very quickly, because the molecules will soon hit it. So to say that a gas is "mostly empty space" is helpful to get the intuition that the molecules can move freely with a large mean free path, but it can mislead in some respects.

The density of an ordinary gas at STP is about 1 kg/m$^3$.

Now let's think about the interior of an atom. I have in mind some ordinary atom such as carbon, and ordinary locations inside the atom, so not at the nucleus and not far away; one could pick a location about half a Bohr radius from the centre, for example. Let's calculate some properties.

First, the mass density. This is the mass density owing to the electrons which are present. Their mass is spread throughout the atom via their extended wavefunctions, and the average mass density can be estimated by noting that an electron is about 2000 times lighter than a proton, and a typical atomic nucleus has as many neutrons as protons, so the electrons contribute about one part in 4000 of the total mass. The density of a solid element such as carbon is about 2000 kg/m$^3$ so we can estimate that the density owing to the electrons at a typical place in an atom is about $0.5, {rm kg}/{rm m}^3$. An estimate using the atomic radius of carbon gives the value 8 kg/m$^3$, suggesting that our previous value was an underestimate because there is some space between the atoms. Anyway the main conclusion here is that mass density at a typical spot inside an atom is similar to the average mass density of a gas at STP.

To get the charge density, we multiply the mass density by $q/m_e$, the charge to mass ratio of an electron, which gives about $10^{12}$ coulombs per cubic metre. This is a huge charge density by everyday standards. (For comparison, a typical 1 microfarad capacitor charged to 1 volt carries a micro-Coulomb in a volume of order $10^{-7}$ m$^3$, giving a charge density $10$ C/m$^3$.)

Next let's consider the flux of matter---the rate at which mass will approach and hit a detector if we were able to place a mass-detector inside our atom. The electrons have speeds of order a few times $alpha c$ where $alpha simeq 1/137$ is the fine structure constant and $c$ is the speed of light. The flux (mass crossing unit area per unit time) is therefore around $8 times 3times 10^8 / 137 simeq 10^7$ kg per second per square metre. Needless to say, this is a huge value in everyday terms.

Next let's enquire into whether or not there is "empty space" in the sense that there is room to put stuff inside an atom. The original statement perhaps comes from a desire to compare an atom to a gas, using some notion that electrons are point-like in some sense, with room in between them.

To address this question we need some more advanced physical ideas, to do with the Pauli exclusion principle. This is an important result in quantum physics, which says that particles such as electrons cannot overlap one another completely. To be precise, in any given spatial situation there can be at most two electrons having that particular combination of position and momentum.

What this means in practice is that there is no more room for low-energy electrons in any atom. If the atom is a hotel, then all the rooms on the lower floors are occupied---completely full up. Thus the space inside an atom is completely unavailable to further electrons unless they move quickly. This is about as far from "empty space" as you can get. It is "complete and utterly full-up space", as far as low-energy electrons are concerned. But this does not exclude electrons altogether, as I already said. If they are moving quickly then there is room for further electrons to get into an atom. They won't stay there---they would have to form a beam passing through; they are visitors to the guests staying in the hotel. For an extra electron bound to an atom (making a negatively charged ion), the wavefunction of the extra electron does get inside the atom a bit (it penetrates the core, as we say), and this can be compared with a visitor rapidly visiting again and again.

What about other types of particle---say, neutrons? They can more easily enter an atom. But is the experience of a neutron sitting inside an atom anything like the experience of a neutron sitting in empty space? Hardly. They would be continually bombarded by that high flux of electrons we calculated just now, and they would notice because although they carry no electric charge, neutrons carry a substantial magnetism, and this leads to an electromagnetic interaction between the neutron and all the nearby electrons.

Now let's summarize.

Electrons in atoms behave in ways that classical physics cannot account for; we require quantum physics. As a result, when we talk about atoms in everyday language, we are trying to convey in everyday terms what quantum physics says is going on. Among the things that quantum physics tells us about the interior of an atom is that the electrons are smoothly spread out, such that the probability of an electron being present at any given moment is non-zero throughout the interior of an atom. We can flesh this out a little by calculating properties such as mass density and charge density and flux. The mass density of the electron cloud of a typical atom is similar to the average mass density of an ordinary gas at standard temperature and pressure. The flux is huge and the charge density is enormous. Also, it is strictly impossible to introduce further slow-moving electrons into the inside of an atom, but it is possible for fast-moving electrons to pass through. Neutrons can also enter atoms, and when inside they interact with the electrons which are there.

In view of the above, it seems to me that it is misleading to say that the interior of atoms is anything like either vacuum or empty space. It really isn't. But it seems that this idea has become lodged into popular presentations of science. It will take some effort to dislodge it.

I now wonder where this idea came from in the first place. I think possibly it might have originated in the early attempts to model atoms via classical point-like particles, or perhaps it is descended from the "fly in a cathedral" image, which is a correct statement about the relative sizes of the atomic nucleus and the whole atom. The "fly in a cathedral" seems to imply that the rest of the "cathedral" is empty, but it is not. It is full of electrons.