Subatomic Particles In An Atom: Their Discovery and Properties
Before talking about subatomic particles, let’s know what an atom is. Atoms are the building blocks of matters. It is the smallest unit of an element retaining its properties.
Democritus was the first to propose that all matters were made up of indivisible particles called atoms. The word, “atom” comes from the Greek word, “Atomos” which is divided into, “a” and, “Tomos”. “A” means, “not” and, “Tomos” means, “to cut”. Thus combined, “atom” means “uncuttable” or “indivisible”.
Suppose, if you were to break your phone (do not do it actually), you will see different parts that made up the phone. Now, grind it, and it will turn into very small particles. You can tell by looking at or touching what those originally were- plastic, metal, etc. and you can isolate these particles individually. Let’s not stop here and grind this even more and more. How much far can you go? By normal means, this process can’t go past a point. But, hypothetically, if you keep dividing those particles, you will reach a point that dividing it will make the particles lose their properties and you will be left with elementary particles and that is the smallest possible unit. In this case, carbon, hydrogen from plastic, copper, other metals, and silicon, oxygen from glass, etc. We are down to a scale that can’t be seen even with the strongest of microscopes. If you were to put 10 million hydrogen atoms in a line, it will only be 1 meter long.
And this is what you can call an atom.
Particles in an Atom
But things don’t end here. People started to wonder if this is unbreakable or is it made of even smaller particles. And the answer is, “Yes”.
An atom is made up of about 200 different types of particles such as proton, neutron, electron, muon, positron, tau, and blah blah blah. These are called subatomic particles. Among these only protons, neutrons, and electrons are permanent and contribute to the structure of the atom while the others act as force carriers and not permanent. You can compare the protons, neutrons, and electrons to bricks, rods, and cement. And other particles can be compared to paints, tiles, or such.
Many scientists have worked hard to discover the structure of the atom. Particularly, Rutherford’s alpha-particle scattering experiment’s contribution is noteworthy.
Primarily, an atom can be divided into two parts: 1. The nucleus and 2. Electron orbits. Protons and neutrons make up the nucleus of the atom while electrons move around the nucleus in certain orbits. Protons and neutrons are also called nucleons as they make up the nucleus. Let’s elaborately talk about the atomic structure some other say. Today is for subatomic particles only.
If are very much shocked about the fact that atoms can be divided then hold tight. There is an even bigger surprise for you. Even the particles proton and neutron can be divided. They are composite particles meaning they are made up of even smaller particles.
A proton is made up of two up quarks and one down quark while a neutron is made up of two down quarks and one up quark. The quarks themselves are elementary particles and so is an electron. There are mainly six types of quarks in the universe.
One of the blessings of the modern world is electricity and electrons are the particle that carry it through wires. It is one of the twelve elementary particles meaning it can’t be divided anymore. Electrons were first discovered by J. J. Thompson in the year 1897 through the cathode tube ray experiment. He passed high voltage electricity through a vacuum tube. When electricity flows, a ray is produced known as the cathode ray. This ray was attracted to a positively charged plate placed on the tube. Later it was discovered that this ray was, in fact, a flow of negatively charged particles named, “electron” by J. J. Thompson.
Electrons are denoted by, “e”. It has a mass of 0.000549 a.m.u. or 9.11×10-31 kg and it has a charge of – 4.8×10-10 e.s.u. or – 1.6×10-19 C. An electron is almost 1836 times lighter than a proton.
Protons are positively charged particles that can be found in the nucleus of an atom. Protons were discovered by Rutherford in 1917. In the year 1886, German physicist Eugen Goldstein discovered a positively charged ray in gas discharge which travels in a direction opposite to the cathode ray. He named them, “Canal ray”. This beam consists of positively charged ions. Using this the charge-to-mass ratio of various positive ions was calculated. It was then proved that the hydrogen ion had the smallest size among all ionized gases of elements.
In 1911 Rutherford conducted his alpha-particle-scattering experiment and concluded that all the positive charge of an atom was concentrated in the center of an atom and named it nucleus. He also observed that hydrogen nuclei were produced when the alpha beam was shot through the air. After investigation, he proceeded to bombard nitrogen gas with alpha particles which produced a greater number of hydrogen nuclei. He concluded that the hydrogen nucleus was a part of all the atoms and named it proton.
Protons are denoted by, “p”. As a hydrogen nucleus without an electron is but a proton, it is also expressed as H+.
The mass of a proton is 1.007 a.m.u. or 1.6725×10-27 kg. It has the same charge as an electron but opposite i.e., 4.8×10-10 e.s.u. or 1.6×10-19 C.
Neutrons are as the name suggests, electrically neutral particles. They make up the nucleus along with protons. British physicist Sir James Chadwick was the one to discover neutrons in the year 1932. After the alpha-particle scattering experiment, Rutherford had already predicted the existence of a neutral particle. It was then known that the atomic number was the number of protons in the nucleus. But atomic numbers and relative mass were different. So, where did this extra mass come from? In 1913 isotopes were discovered which raised more questions about the difference in mass. It was thought that there were excess protons in the nucleus, with an equal number of electrons to cancel out the additional charge. But there was no evidence.
In 1928, German physicist Walter Bothe and Herbert Becker found out that if alpha particles emitted from polonium is incident on beryllium, it gives off a penetrating and electrically neutral radiation. This radiation was interpreted as high-energy photons.
But then something interesting happened. In 1932, Irene Joliot-Curie, one of Madame Curie’s daughters, and her husband, Frederic Joliot-Curie, studied the then-unidentified radiation from beryllium for further investigations. They found that this radiation ejected protons of high velocity from a paraffin target. They tried to associate with the Compton Effect. Here Compton Effect is a phenomenon, where, if photons with high enough energy are incident on a metal surface, they knock out protons from the metal. The Compton effect was observed by Arthur Holly Compton in 1923 and was named after him.
Now, the problem is that protons are about 1836 times heavier than electrons. Yes, an ant might be able to carry weights six times their own, but imagine an ant trying to knock away an elephant. So, the neutral beam being a high energy photon is quite unlikely.
This is where Sir Chadwick comes in. He could not accept Compton Effect as the conclusion and tried similar experiments with other elements in addition to the paraffin wax, including helium, nitrogen, and lithium as targets. By measuring the kinetic energies of ejected protons, he found out that the beryllium emissions contained a neutral component with a mass approximately equal to that of the proton and named this particle neutron.
He won the Nobel Prize in the year 1935 from Physics for this discovery. Though the Juliot-Curie pair missed their chance of getting a Nobel prize, in this case, they got a Nobel prize for discovering artificial radioactivity. So, there is no reason to feel sorry.
But this is not the end. For all we know, there might be other particles playing hide and seek only to be found by you. By the way, the Nobel prize is worth about 1.5 million US dollars 😉.
- Judy on Atomic Structure: Relation to Nucleon numbers । Isotopes and Isotopic Notation
- Brittney on Atomic Structure: Relation to Nucleon numbers । Isotopes and Isotopic Notation
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