Bohr's 'New' Model of the atom: What it is and why it matters
Since the very early days of our species, we have pondered the fundamental workings of the world and universe around us. This obsession with making sense of a seemingly chaotic and often scary world has led to some incredible revelations about the very nature of, well, nature.
One such discovery has been the concept that everything around us is made up of basic building blocks, atoms. While we know today that even atoms can be subdivided into other fundamental particles, this information was not yet known at the time of Danish physicist Neils Bohr.
However, his "New" model for the atom, developed with Ernest Rutherford, remains one of the most remarkable intellectual feats in physics and is still taught to millions of young minds every year. Let's take a closer look at this crucial stepping stone on the road to our current understanding of quantum physics.
What was Bohr's model of the atom called?
For anyone who has taken at least some basic lessons in chemistry, you are probably more than familiar with Bohr's "New" model for the atom. You may not know its name, but you are probably more than au fait with the basic concept.
In short, the Bohr Model consists of a central positively charged nucleus (usually depicted as small), surrounded by negatively charged electrons moving in discrete orbits. The model explained that the quantum of action could only determine the orbit occupied by an electron and that electromagnetic radiation from an atom occurred when an electron jumped to a lower-energy orbit. Now primarily considered obsolete for practicing scientists, it is still a fundamental component of any high school education in science.
This doesn't mean Bohr's Model is wrong, per se, only that it is not entirely correct. For example, it violates (an admittedly strong term) something called the Heisenberg Uncertainty Principle, as it states that electrons have a known radius and orbit. However, as we know it today, he correctly proposed that the energy and radii of the orbits of electrons in atoms are quantized (have a measurable amount of energy).
The model also provides an incorrect value for the ground state orbital angular momentum measurement and is less helpful in modeling larger atoms. In Bohr's defense, these phenomena had not yet been described when Bohr formulated his model.
What are the main points of Bohr's model?
The main takeaway points about the atom are relatively short and straightforward to understand. This is why, in part, it is still taught to students today.
The first point is that electrons orbit the nucleus in discrete levels, called shells, and they have a set size and amount (quanta) of energy.
The second main point is that the energy "needed" by the electron to maintain a 'larger' orbit (i.e., further away from the nucleus) is necessarily more than that required to maintain a smaller orbit.
And the final point is that radiation is absorbed or emitted when an electron moves from one orbit or shell to another. If an electron "jumps" a shell, it is said to have absorbed energy, and vice versa for electrons that "fall" to lower/closer orbits or shells.
Who discovered Bohr's Model?
Bohr's Model was discovered or rather formulated by the Danish physicist Niels Henrik David Bohr. Born in Copenhagen, Denmark, on the 7th of October 1885, Bohr would grow up to be one of the most critical thinkers in the then-nascent fields of atomic theory and quantum physics.
His work was so important that he was awarded the highly prestigious Nobel Prize in Physics in 1922.
In his later career, Bohr would establish the Institute of Theoretical Physics at the University of Copenhagen, now known as the Niels Bohr Institute, which opened in 1920. He would also mentor many other prominent physicists in their early careers, including Hans Kramers, Oskar Klein, George de Hevesy, Lise Meitner, Otto Frisch, and Werner Heisenberg.
Bohr was also able to successfully predict the existence of the element hafnium (based on the Latin name for Copenhagen, where it was discovered). The utterly synthetic element (i.e. does not occur in nature) bohrium was also named after him.
Bohr's accolades also extend into humanitarian work when, throughout the 1930s, he was very active in helping Jewish physicists escape the tentacles of National Socialist ideology. Bohr used his connections to offer physicists temporary positions at his institute and then helped them obtain permanent appointments elsewhere, often in the United States.
During the war, he met with Heisenberg (the head of the German nuclear weapons program) to discuss the possibility of developing a nuclear weapon. However, he felt that practical difficulties would delay the bomb's development until after the war.
In 1943, two years after Germany had occupied Denmark, Bohr was sent a secret message from British colleague James Chadwick, inviting him to come to England to do important scientific work. But Bohr remained, convinced that he could do more good in Denmark. However, a few months later, Bohr was warned that he was about to be arrested by the Germans, and he escaped by boat to Sweden with his family, and he was brought by a military airplane to England, where he joined the British Tube Alloys nuclear weapons project. He was also part of the British mission to the Manhattan Project.
He made significant contributions to the development of the bomb. Still, according to J. Robert Oppenheimer, his most outstanding contribution was to serve as “scientific father confessor to the younger [scientists].”
After the war, Bohr returned home to Denmark, where he was hailed as a hero. He continued to run his institute and helped establish a nuclear research facility at Risø, near Roskilde. He also called for international cooperation on nuclear energy. He was involved with CERN's establishment and the Danish Atomic Energy Commission and became the first chairman of the Nordic Institute for Theoretical Physics in 1957.
Bohr died of heart failure at his home in Carlsberg on November 18, 1962, at the ripe old age of 77. He was cremated, and his ashes were buried in the Bohr family plot in the Assistens Cemetery in Copenhagen.
What does Bohr's model explain?
In short, Bohr's Model of the atom proposes that electrons orbit their nuclear at fixed energy levels. If true, any electrons that orbit closer to the nucleus will have lower energy levels than those further away from it.
When electrons move from one orbit or shell to another, this will require either energy input or a release of energy. When electrons 'fall' from a higher orbit to another, this excess energy will be released from the atom in the form of radiation.
A very crude analogy would be the use of a ladder. To carry your mass up a single rung of it requires you to input energy. The higher up the ladder you go, the more energy is invested to overcome "build up" your potential energy the higher you go.
Coming back down the ladder releases that potential energy as you descend step by step. But, if you are not careful, you can release that potential energy all at once by falling off the ladder (obviously not desirable).
In addition, you take the climb or descent in steps. There is no "in-between" position on the ladder—your foot either hits a rung or hits space.
Depending on the original orbit/shell that an electron starts and then ends up will release a corresponding, and tell-tale, frequency of light.
Bohr's model also describes how different electron shells such as K, L, M, N, etc., can also "hold" different numbers of electrons. The larger the orbit or shell, the more electrons. We also know that these major shells also have subdivisions. For example, the L shell contains two subshells called 2s and 2p.
So, the electron shell (and subshells) closest to the nucleus has less energy, and the electron shell farthest from the nucleus has more energy.
How did Bohr discover the Bohr model?
Neils Bohr proposed his eponymous model of the atom, beginning with a series of articles published in 1913. This model was, in turn, a modification or improvement on earlier models for the atom proposed by Ernest Rutherford and other prominent scientists.
For this reason, it is not uncommon for the model to be called, by some, the Rutherford-Bohr Model.
Unlike the earlier "Cookie Dough" model (now largely rejected), Bohr included some elements of the emerging field of quantum mechanics to develop his revised model of the atom. While the Bohr Model does contain some significant errors (more on that later), it is essential because it describes most of the accepted features of atomic theory without all of the complex mathematical equations of the modern version.
For example, unlike many other models, like Rutherfords', that preceded it, Bohr's Model, while still inaccurate, can explain the Rydberg formula for the spectral emission lines of atomic hydrogen.
The Bohr Model is what is called a "planetary model" for obvious reasons — it has the negatively charged electrons (acting like tiny planets) orbiting a much smaller nucleus (vis-a-vis the Sun). The only difference is, contrary to what many people may think of the Bohr Model, and the electrons do not move in a single plane.
In this respect, the gravitational force of the solar system is mathematically akin to the Coulomb (electrical) force between the positively charged nucleus and the negatively charged electrons, sort of.
Why did Bohr create his model?
Like all scientific breakthroughs, big or small, they are all based on the previous work of a long line of scientists and thinkers over many centuries. The same is true for Bohr's Model.
Without going into too much excessive detail, the concept of the atom is a very long one. The ancient Greek philosopher Democritus, for example, famously postulated that if you kept spitting an object in half, eventually you'd reach a single piece that cannot be split any further.
The term "atom" was born.
Fast forward to the early-1800s, and great minds like the British chemist John Dalton, who developed the first modern "model" for an atom. Later, in 1904, another British scientist, J. J. Thomson discovered that atoms contain small negatively-charged particles that he called "electrons."
Like many great scientific discoveries, this was a complete revelation and one that occurred by accident while he was studying electricity. Since it was known that most atoms have an overall neutral charge, this must mean that another part of the atom is positively charged.
To this end, he proposed the so-called "Cookie Dough" or "Plum Pudding" model for the atom, where negatively-charged electrons "sit" on top of a positively charged ball of matter (like chocolate chips in cookie dough).
Then, around 1909, Ernest Rutherford proved that the “Cookie Dough” model of the atom was not entirely accurate. He showed this by firing a bunch of small, positively charged particles (known as alpha particles) at a sheet of gold foil. If the plum-pudding model was correct, the alpha particles should bounce back at Rutherford because they would hit the positive balls of atoms. Or so it was believed.
However, much to Rutherford’s surprise, most particles passed straight through the gold foil. Odd.
Therefore, Rutherford concluded that most of the atoms must be empty space. This was a revolutionary development at the time.
He also made the intellectual leap to suggest that the atom's positive charge was concentrated somewhere in the middle of the atom in a central "nucleus." The rationale was that any deflected particles must be the ones that had hit this central nucleus.
This led to Rutherford’s “Peach” model because it described a hard and dense center of the atom (i.e., the peach pit). The "flesh" of the peach in this model would be mostly empty space or, at least, less dense or heavy than the "core" or nucleus.
In 1885, Johann Balmer published two papers describing an equation for determining the emission spectra and the photoelectric effect. Emission spectra are the sequences of wavelengths characterizing the electromagnetic radiation emitted by energized atoms. The spectral line emissions of the hydrogen atom are called the Balmer series, which was another vital stepping stone in our understanding of the structure of the atom.
In 1911, Rutherford and his collaborators established experimentally that the atom is made up of a heavy, positively charged nucleus and lighter, negatively charged electrons circling around it. However, according to classical physics, this system would be unstable.
Bohr built on the work of Balmer and Rutherford in developing his solution to this instability, and in 1913, he came up with his widely known "New" model.
Bohr also made the intellectual leap to assert that electrons do not radiate energy randomly but do so according to states of constant energy, called stationary states. In other words, electrons "sit" in fixed orbits around a nucleus at fixed distances and only release energy when their stationary states are perturbed in some way.
What is Bohr's model of hydrogen?
The simplest atom known is the hydrogen atom or, for that matter, a hydrogen-like ion. These species of atoms consist of a single electron orbiting a positively charged nucleus.
Under such circumstances, electromagnetic energy will be absorbed or emitted if an electron moves from one orbit/shell to another. Remember that only certain orbits are permitted.
As we touched on earlier, the hydrogen emission spectra were one of the main inspirations for Bohr's model. The emission spectra were demonstrated experimentally by passing an electric current through a glass tube filled with hydrogen gas at low pressure.
When this is done, the tube emits a blue light that produces four narrow bands of bright light when passed through a prism. In turn, this light can be projected against a black background to give the observer a clear indication of the light frequencies emitted by the excited atoms.
This produces a spectrum of light in discrete bands of red (with a wavelength of 656 nanometers (nm), blue-green (wavelength of 486 nm), blue-violet (434 nm), and violet (410 nm).
The very fact that hydrogen atoms emit or absorb radiation at a particular number of frequencies indicates that these atoms can only absorb radiation with specific energies. It, therefore, follows that there are only a limited number of energy levels within the hydrogen atom. These energy levels are countable, and the energy levels of the hydrogen atom are quantized.
To help explain this, Bohr proposed that the possible orbit(s) in a hydrogen atom increase by n2, where n is the principal quantum number. According to Bohr's model, a shell 3 to shell 2 transition produces the first line of the Balmer series. For hydrogen, this makes a photon having a wavelength of 656 nm (or red light) - as seen in the emission spectra for hydrogen.
The other emission colors correspond to more significant leaps from higher energy states to the electron's "stationary state" and release a correspondingly shorter wavelength (more energy) than that for the red light.
Why is Bohr's Model of the atom wrong?
We've explained, albeit in brief, the principles and merits of Bohr's model for the atom, but what is wrong with it?
The main issue with Bohr's Model for the atom is that it works exceptionally well for atoms with only a single electron. This should not come as a surprise as it was, in part, formulated based on the emission spectra of hydrogen.
In addition, as we've seen, Bohr was able to predict the difference in energy between each energy level, allowing us to predict the energies of each line in the emission spectrum of hydrogen and understand why electron energies are quantized.
However, Bohr's model breaks down when applied to multi-electron atoms. It does not, for example, account for sublevels (s,p,d,f), orbitals, or electron spin. Bohr's model allows the classical behavior of an electron (orbiting the nucleus at discrete distances from the nucleus).
The application of Schrodinger's equation to atoms can explain the nature of electrons in atoms more accurately. This also tells us that the exact position of an electron can never be accurately known. Therefore, Bohr's concept of discrete "shells" cannot be the case.
Bohr's Model cannot explain the fine structure of the hydrogen spectrum and splitting of spectral lines due to an external electric field (Stark effect) or magnetic field (Zeeman effect).
It couldn't explain why some lines on the spectra were brighter than the others, i.e., why are some transitions in the atom more favorable than the others.
For these reasons, among others, Bohr's model today is venerated, but no longer quantitatively or qualitatively useful in atomic theory. You can think of it, fittingly, as akin to the replacement of Newtonian physics with Einstein's general theory of relativity.
And that, Bohr's New Model enthusiasts, is your lot for today.
While largely obsolete today in practical terms, Bohr's model for the atom is one of the most important discoveries in physics of all time. Just like the analogy of the electrons climbing up the rungs of a ladder, Bohr's model represents one such rung in our growing understanding of the atom and the fundamental secrets of the universe.
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