Name of particle Relative charge Relative mass Proton +1 1 Electron -1 1/1840 Neutron 0 1

Annotation: The following information may also be noted for each of the fundamental sub-atomic particles, of which a typical atom consists.

Let’s engage in a simple, if not basic, study of the above table.

Firstly, in that the proton, neutron and electron are three fundamental subatomic particles, there are also other basic subatomic particles. An example of other known particles include alpha and beta particles. But bracketing the existence of these other particles for a moment, what is interesting about the humble proton, neutron and electron is how they interact with one another.

Although the Bohr model is outdated, replaced with the more accurate model of an electron cloud (diagram below, courtesy of Berry Perry), the former offers a more intuitive model directly relatable with human experience. This is one reason why it is still taught in some undergraduate courses. When people thinking of electron orbitals, they seem to often imagine examples in the form of planetary orbits or something similar, with little particles – almost like subatomic marbles – orbiting around the nucleus of an atom. Indeed, the earliest model of electron orbitals – the Rutherford model – was based on the classic solar system model.

The problem with this classic model – and, similarly, the idea that microscopic objects obey classical laws – is that the electron would inevitably crash into the proton, causing the atom to collapse. Niels Bohr stepped in with an alternative. This alternative is based on the once radical idea that the circular orbits of electron must have certain sizes: n=1, n=2, etc. and that there is a discrete set of orbit energies. This model, called the Bohr model, helped explain electron orbitals for a stable atom that doesn’t collapse. This Bohr model is the one taught in introductory science class, such that one will discuss the way in which electrons jump orbitals, and as energy is conserved, the extra energy when an electron jumps down is emitted as a photon. One will also know that for an electron to jump to a higher orbital, it must absorb the extra energy from an incoming photon.

The problem with the Bohr model is that it only stands as being an accurate description for the hydrogen atom. With the Bohr model also follows the idea that we can accurately calculate the velocity of an orbiting electric and its position. But this is not correct. To explain why it is not correct, one might refer to the Heisenberg Uncertainty Principle. This is something I will write more about in a separate post, as there seems to be confusion in popular accounts between this principle and the observer effect (it is wrong to conflate the two). But the main idea I want to grab at here is that, just as the more intuitive Bohr model is not correct, science often teaches us there is a limit to the knowledge of human experience. This is why, in my estimation, a lot of people seem to struggle with understanding quantum mechanics and things like the Schrödinger atom. And it is this theory of the Schrödinger atom that now comes into focus.

The Schrödinger atom is not intuitive, and it generally does not corroborate with what one often observes in their daily experiential experience. Moreover, in the classical model, there is a much closer representation to what one wants to see (and what one wants to conceptually apply via the senses) than what is the actual essence of the electron. The first radical idea comes from Louis de Broglie: namely, the wavelike properties of the electron. Along with the work of Schrödinger, including his development of the wavefunction, the classical orbital model and the Bohr model were replaced with a picture in which there is only a single electron. This single electron is in many places at once, in what is called an electron cloud. This electron cloud, and its shape, is given by the wavefunction. But more accurately still, if we take it one step further: in quantum theory, we learn that even the electron cloud is outdated, as the depiction of many points is not accurate insofar that the electron cloud is theorised to be more smooth and diffuse. The electron, again, is not in any single place within that cloud – although, one might observe an electron in a particular place. The essence of the electron is that cloud. Or, to phrase it another way, the quantum state of the electron is the cloud.

Quantum states, the wave function, and the nature of the orbital are things we’ll discuss in a lot more detail in future posts. Meanwhile, and to summarise a rather simplistic introduction: the key idea to begin thinking about is how instead of the Newtonian, mechanical view of the configuration of electrons, which is modelled from a more classical orbital configuration, similar even in picture as the Bohr model, electronic orbitals and sub-orbitals are much more diffuse. It is from within the quantum model of electron configuration that we understand how, instead of the electron being in a classically defined and fixed orbit around the nucleus (think of a planet around a star or a particle with fixed positions circulating a track), the description is one of orbitals as probability density functions that can be imagined as a sort of dispersed cloud. In fact, in thinking of electrons, one can think of them as standing matter waves (or at least there is an analogy here) within the limit of certain energy values. But, again, the purpose here is maintain an introductory tone.

In the sense described above, an orbital, which is a wave function, therefore helps account for in this case any volume of space around the nucleus in which there is x probability of finding an electron. Additionally, and before moving on to the illustration, it is important to highlight that there are two primary features of the orbital: the energy shell, n, and the corresponding periods on the periodic table. But even the word “orbital” – much like the word “shell” or “energy shell” – can be susceptible to misinterpretation, wherein one may be inclined to resort to some classical mechanical definition. It is worth reemphasising: electron orbitals aren’t so neatly defined as to be clean edged containers in which the electron absolutely resides and orbits.

What is useful about the illustration of the model of the electron cloud is how, if one were to take many snapshots of electrons, the image like the one above would form insofar that the probability density would be closer to the nucleus. The further out the less likely (a much lower probability) an electron might be found.

Having said that, and understanding that there are theoretical reasons as to why an electron can occupy space behind the probability density orbital, or electron shell, what the Bohr model highlights is how an atom can be divide into protons, neutrons and electrons. The proton, which is positively charged (+1 relative charge), is also a nucleon. In other words, if most of the mass of an atom is in the nucleus, this area – and I use the word “area” cautiously – is made up or constituted by protons and neutrons, which we can also call nucleons (Wang, ND).

Furthermore, neutrons have no relative charge – hence they are neutral. For this reason, they do not interact with protons and electrons (although along with protons they make up most of the mass of an atom). Additionally, the neutrality of the neutron is why an atom has a neutral charge. It also explains the existence of electron orbit, as this has to do, in simplistic terms, with the interaction between the positively charged proton and the negatively charged electron. The positively charged proton within the nucleus is important with respect to the fundamental relation of the atomic dynamic, because electrons, which are negatively-charged, interact outside the nucleus with the proton. That is to say there is a force of attraction due to the opposite charges, and this force enables the relative orbit of the electron around the positive nucleus.

If an atom is electrically neutral, this is due to how the charges of the positive proton and the negative electron are of equal magnitude. Thus, they balance or cancel out. Hence, the ratio that a neutral atom will have one electron for every proton.

Having said all that, in particle physics we learn that even protons and neutrons are “composite objects”, as physicists delve deeper into the nature of matter and into the study of such particles as quarks and leptons (among others). In my field of study, particle physics, there is an incredibly exciting frontier of science to potential be revealed as we move further into the 21st century. At CERN and elsewhere troves of data await analysis. The hunt for new particles continues, and there remains much to be accomplished in particle physics in the decades ahead. The Standard Model is an incredible achievement; but it’s also incomplete and the exciting possibility of a new physics stares back at us from the furthermost distant horizon.

In a future post I’ll expand on this simple discussion toward a more deepened and complex view.

References

Adams, S. and Allday, J. (2013). Advanced Physics. Oxford University Press. Oxford, UK.