The First Model of the Atom Was Developed Through

All thing is fabricated up of atoms. This is something we now take equally a given, and one of the things you larn right back at the beginning of high school or secondary school chemistry classes. Despite this, our ideas about what an atom
is are surprisingly recent: every bit niggling as one hundred years ago, scientists were still debating what exactly an atom looked similar. This graphic takes a look at the key models proposed for the atom, and how they changed over time.

Though our graphic starts in the 1800s, the idea of atoms was around long before. In fact, we have to go all the way back to Ancient Greece to notice its genesis. The discussion ‘cantlet’ actually comes from Ancient Greek and roughly translates as ‘indivisible’. The Ancient Greek theory has been credited to several dissimilar scholars, but is most oftentimes attributed to Democritus (460–370 BC) and his mentor Leucippus . Though their ideas about atoms were rudimentary compared to our concepts today, they outlined the idea that everything is fabricated of atoms, invisible and indivisible spheres of matter of infinite blazon and number.

These scholars imagined atoms equally varying in shape depending on the blazon of cantlet. They envisaged iron atoms as having hooks which locked them together, explaining why iron was a solid at room temperature. Water atoms were smooth and slippery, explaining why h2o was a liquid at room temperature and could exist poured. Though we now know that this is not the case, their ideas laid the foundations for future diminutive models.

It was a long expect, however, before these foundations were built upon. It wasn’t until 1803 that the English language pharmacist John Dalton started to develop a more than scientific definition of the cantlet. He drew on the ideas of the Ancient Greeks in describing atoms equally minor, hard spheres that are indivisible, and that atoms of a given chemical element are identical to each other. The latter point is ane that pretty much all the same holds true, with the notable exception being isotopes of different elements, which differ in their number of neutrons. Still, since the neutron wouldn’t exist discovered until 1932, we can probably forgive Dalton this oversight. He as well came up with theories about how atoms combine to brand compounds, and also came up with the kickoff ready of chemical symbols for the known elements.

Dalton’s outlining of atomic theory was a offset, simply information technology still didn’t really tell us much about the nature of atoms themselves. What followed was another, shorter lull where our noesis of atoms didn’t progress all that much. In that location were some attempts to define what atoms might look like, such as Lord Kelvin’s suggestion that they might have a vortex-similar construction, but it wasn’t until simply after the turn of the 20th Century that progress on elucidating atomic structure really started to pick upward.

The first breakthrough came in the late 1800s when English physicist Joseph John (JJ) Thomson discovered that the atom wasn’t every bit indivisible equally previously claimed. He carried out experiments using cathode rays produced in a discharge tube, and establish that the rays were attracted past positively charged metal plates just repelled by negatively charged ones. From this he deduced the rays must be negatively charged.

By measuring the charge on the particles in the rays, he was able to deduce that they were two thousand times lighter than hydrogen, and by irresolute the metal the cathode was made from he could tell that these particles were present in many types of atoms. He had discovered the electron (though he referred to it as a ‘corpuscle’), and shown that atoms were non indivisible, but had smaller elective parts. This discovery would win him a Nobel Prize in 1906.

In 1904, he put forward his model of the atom based on his findings. Dubbed ‘The Plum Pudding Model’ (though not by Thomson himself), it envisaged the atom equally a sphere of positive accuse, with electrons dotted throughout like plums in a pudding. Scientists had started to peer into the cantlet’due south innards, but Thomson’s model would not hang effectually for long – and it was one of his students that provided the show to export it to history.

Ernest Rutherford was a physicist from New Zealand who studied at Cambridge Academy nether Thomson. It was his later work at the Academy of Manchester which would provide farther insights into the insides of an atom. This work came afterwards he had already received a Nobel Prize in 1908 for his investigations into the chemistry of radioactive substances.

Rutherford devised an experiment to probe diminutive structure which involved firing positively charged alpha particles at a sparse sheet of gold foil. The alpha particles were and so pocket-size they could pass through the gold foil, and according to Thomson’due south model which showed the positive charge diffused over the unabridged atom, the should do and so with picayune or no deflection. By carrying out this experiment, he hoped to be able to confirm Thomson’due south model, just he concluded upwardly doing exactly the reverse.

During the experiment, most of the alpha particles did pass through the foil with petty or no deflection. Yet, a very small number of the particles were deflected from their original paths at very big angles. This was completely unexpected; equally Rutherford himself observed, “It was virtually as incredible every bit if y’all fired a 15-inch shell at a slice of tissue paper and it came back and hitting you”. The merely possible explanation was that the positive charge was not spread throughout the atom, merely concentrated in a small, dense centre: the nucleus. Most of the rest of the cantlet was merely empty infinite.

Rutherford’s discovery of the nucleus meant the diminutive model needed a rethink. He proposed a model where the electrons orbit the positively charged nucleus. While this was an improvement on Thomson’s model, it didn’t explain what kept the electrons orbiting instead of simply spiralling into the nucleus.

Enter Niels Bohr. Bohr was a Danish physicist who set nearly trying to solve the problems with Rutherford’s model. He realised that classical physics could not properly explain what was going on at the atomic level; instead, he invoked quantum theory to endeavour and explain the arrangement of electrons. His model postulated the being of energy levels or shells of electrons. Electrons could but be found in these specific energy levels; in other words, their energy was quantised, and couldn’t accept just whatsoever value. Electrons could move between these free energy levels (referred to by Bohr as ‘stationary states’), but had to do and then by either absorbing or emitting energy.

Bohr’due south suggestion of stable energy levels addressed the problem of electrons spiralling into the nucleus to an extent, but non entirely. The exact reasons are little more circuitous than nosotros’re going to hash out here, considering we’re getting into the circuitous world of breakthrough mechanics; and as Bohr himself said, “If breakthrough mechanics hasn’t profoundly shocked you, you lot haven’t understood it yet”. In other words, it gets kind of weird.

Bohr’s model didn’t solve all the atomic model bug. Information technology worked well for hydrogen atoms, but couldn’t explain observations of heavier elements. It as well violates the Heisenberg Doubtfulness Principle, one of the cornerstones of quantum mechanics, which states we tin’t know both the exact position
and
momentum of an electron. Still, this principle wasn’t postulated until several years after Bohr proposed his model. Despite all this, Bohr’s is probably still the model of the atom you lot’re most familiar with, since it’s often the i first introduced during high school or secondary school chemistry courses. It withal has its uses also; information technology’south quite handy for explaining chemical bonding and the reactivity of some groups of elements at a elementary level.

At whatsoever rate, the model still required refining. At this point, many scientists were investigating and trying to develop the breakthrough model of the atom. Master amidst these was Austrian physicist Erwin Schrödinger, who y’all’ve probably heard of earlier (he’south the guy with the cat and the box). In 1926 Schrödinger proposed that, rather than the electrons moving in fixed orbits or shells, the electrons carry as waves. This seems a little weird, but you probably already call up that light can bear as both a wave and a particle (what’s known as a wave-particle duality), and it turns out electrons can too.

Schrödinger solved a series of mathematical equations to to come upward with a model for the distributions of electrons in an atom. His model shows the nucleus surrounding by clouds of electron density. These clouds are clouds of probability; though we don’t know
exactly
where the electrons are, we know they’re probable to exist institute in given regions of space. These regions of infinite are referred to as electron orbitals. It’due south perhaps understandable why high schoolhouse chemistry lessons don’t lead in direct with this model, though information technology’due south the accepted model today, because it takes a petty more fourth dimension to get your head around!

Schrödinger’s wasn’t quite the last word on the cantlet. In 1932, the English physicist James Chadwick (a student of Ernest Rutherford) discovered the existence of the neutron, completing our picture of the subatomic particles that make upward an atom. The story doesn’t end there either; physicists have since discovered that the protons and neutrons that brand upwards the nucleus are themselves divisible into particles chosen quarks – only that’due south beyond the scope of this post! At any rate, the atom gives us a great example of how scientific models can change over time, and shows how new evidence can lead to new models.

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References & Further Reading

• The development of the atomic model – R Allain, Wired
• Models of the cantlet – M Fowler
• History and philosophy of scientific discipline through models: some challenges in the case of the atom (£) – R Justi and J Gilbert