In the atomic heart
Aditi Chandrasekar
On a cold mountain, a magnificent glacier moves down the sloping faces. In the glacier there are water molecules frozen in their places. In the molecules there are atoms held together, in the atoms there are. . . .
A funny thought! Atoms are the “building blocks of matter.” We don’t worry about what is inside a brick, then why should we bother about a tiny atom? They are just a bunch of spheres that hold stuff together. No, that is not all. When we do probe the inside of these spheres, we discover a whole new world. A world with new forces, new interactions, and enormous energy, like nothing we have seen before.
Atoms consist of a nucleus at the centre containing neutrons and positively charged protons. Negatively charged electrons are attracted by the nucleus and orbit it analogous to the sun and the planets in our solar system.
When the electronic orbits are farther away from the nucleus, electrons experience less attraction and hence are more loosely bound than those with closer orbits. Hence there arises a discontinuous gradation of energy of these electrons depending on their orbit distances and the net positive charge of each nucleus. Electrons can be excited to higher energy orbits by absorbing light photons, or de – excite to lower energy states by giving out light photons. This property is extensively exploited in the field of spectroscopy, in which various regions of the electromagnetic spectrum are used as a probe to study matter and its structure. A breathtaking example in nature of electronic de – excitation to give light is the Aurora Borealis also called Northern Lights. It is a natural display of vibrant colour in the sky which can be seen at the poles when ions from the solar wind interact with the earth’s magnetic field they enter the earth at the poles. These charged particles strike the molecules in the atmosphere and excite their electrons. When these electrons get to lower energy states they emit light. The colours depend on which element they belong to. (At the south pole, the phenomenon is called Aurora Australis.)
The electrons around the nucleus are attracted by the positively charged protons by a Coulombic attraction which is a familiar concept. However, the nucleus contains uncharged neutrons and a number of protons as well concentrated together in a small volume. If the same reasoning of coulombic forces are applied to this, a nucleus with anything more than one proton could never have existed. Nevertheless, all nuclei except hydrogen and its isotopes contain more than one proton; clearly suggesting that there is bound to be an alternate explanation for the existence of these nuclei. At extremely short distances at which protons interact within an atomic nucleus, an entirely different force operates. It is an attractive force that is stronger than the coulombic repulsions. The force is simply and conveniently termed “strong interaction.” The name is deceptively simpler than the nature of the force itself.
The number of protons and neutrons inside a nucleus is not a random distribution. Nuclear forces and parameters govern the ratio of protons and neutrons such that for each element only certain isotopes are stable. Other isotopes are radioactive and loose particles and energy from the nucleus to attain stability. Mainly: alpha, beta, and gamma decay occur.
Alpha particles are positively charged and are emitted when there are too many protons or in heavy nuclei. When there are too few protons neutrons are eventually converted to protons in turn emitting negatively charged beta particles. Gamma is an electromagnetic ray having no mass or charge. Gamma decay accompanies beta emissions or sometimes highly energetic nuclei emit gamma rays to gain stability. These radioactive isotopes find extensive use in medicine, agriculture, and industry. They have most popularity in the diagnosis and treatment of cancers. Gamma rays are targeted to kill cancerous cells at long range in radiation therapy. Beta particles can do the same at short range and in certain cases a beta emitter is kept within the cancerous region in the body where it kills the surrounding tumour with minimal damage to healthy cells due to the short range effect. Boron Neutron Capture Therapy (BNCT) is an ingenious method by which a molecule which specifically binds to cancer cells is tagged with inactive boron. Once the boron cells reach the tumour, neutrons are used to induce alpha particle emissions from the boron nucleus to irradiate the cancer. By a similar method a radionuclide can be preferentially adsorbed in the cancer locations within the body and its emissions can be used to image the location, size, and shape of the tumour.
Just like atoms and molecules undergo chemical reactions to produce new compounds, nuclei can undergo nuclear reactions to produce new elements. Nuclei can fuse together undergoing a fusion reaction, or break apart to form smaller nuclei in what is called a fission reaction. Chemical reactions are balanced by knowing that the total mass on the reactants side is equal to the total mass on the products side of the reaction. In nuclear reactions something odd happens. Mass goes missing! Be it in fusion reactions or fission reactions, the entire mass of the products is less than what reacted. Another striking difference between chemical and nuclear reactions is that an enormous amount of energy is released in nuclear reactions. Energies incomparable to that released even in the most exothermic reactions, like an explosion. The solution to this mystery of the missing mass, and large energies, lies in Einstein’s famous equation:
E=mc2
Where m is the missing mass, c is the velocity of light in vacuum and E is the energy released.
This equation reveals that mass is converted into energy. By substituting the value for the velocity of light, it can be worked out and shown that a mass as small as 1 milligram can be converted to 9×1010 Joules or ninety billion Joules of energy.
In the stars, lighter elements like hydrogen fuse to give heavier elements like helium, which in turn fuses to give the even heavier elements like lithium, boron, carbon, etc. The mass of the products of each fusion reaction is less than the total mass of the individual fusing nuclei. The deficient mass is converted to energy that is released in the form of heat. The amount of heat is so much that it is converted to light. Red stars are less hot and blue stars are hotter. The heat and light from the sun, which life on earth is so dependent on, arises from the fusion reaction of hydrogen to give helium.
Some people envisaged that if nuclear reactions could be induced to occur, so much energy could be released and utilized on earth. A lot of energy from such little material makes nuclear fuel one with highest energy density per unit mass than any other energy source. Although fusion reactions are very difficult to induce artificially, fission reactions have been induced in the actinide elements; mainly uranium and plutonium. When a neutron is incident on a uranium or plutonium nucleus, the actinide nucleus fragments to give lighter elements like krypton, molybdenum, barium, etc., as fission products and releases large amounts of energy due to some mass defect. Additionally, more neutrons are released in each fission event and further fission more atoms of actinides, which in turn produce neutrons on fission, finally culminating in a self-sustaining chain reaction. Energy thus produced is used to generate various forms of power. Thermal, electrical and unfortunately, even political power. In August of 1945, America dropped two bombs one made of uranium and the other of plutonium, on the Japanese cities of Hiroshima and Nagasaki. Even today, the mere thought of the event, brings a lump to my throat and shatters my belief in humanity. The effect of a bomb made of fissionable elements is disastrous and also long lived due to the radioactivity that gets indiscriminately spread around the area, affecting generations to come.
It pains me to dwell further on explanations about a nuclear bomb. Moving to a nuclear reactor, many people wonder why a reactor does not explode like a bomb when the same reaction is going on in a reactor. One reason for this is that the ratio of fissile elements is much less in the reactor fuel than that in a bomb. The other reason is that the neutrons are absorbed by control rods placed in the core of the reactor.
Atoms can create and atoms can destroy. Inside the atom lies immense power. With great power, great responsibility must follow. We must make a humane and judicious choice.
Seeing colours of electronic transitions between atomic energy levels: Flame tests
The light emission due to the electronic transitions between the Bohr orbits of some metal ions can be seen through flame tests. While performing flame tests, samples of mainly alkali and alkaline metals salts are supported on a platinum wire loop dipped in concentrated hydrochloric acid, and are directly held into a Bunsen flame. The heat energy from the flame excites electrons to higher energy orbits. When the electrons return to their original orbits they emit this excess energy as light. Some of this light is in the visible region and appears as bright and intense colours. The energy gap between electronic orbits is characteristic to each ion, and hence the colours observed in a flame test is used to identify specific ions.
Examining Bohr’s model of a hydrogen atom
In this experiment the electronic transitions between Bohr orbits in a hydrogen atom can be visualized. A high voltage is applied across a hydrogen discharge tube and an electric discharge occurs in the tube producing light. The light focussed on a triangular prism through a slit and the prism splits the light into individual wavelength bands. The light transmitted through the prism is detected on a photographic plate where the Bohr transitions of the hydrogen atom are seen.
The author is working on her Ph.D. on fuel chemistry at Indira Gandhi Centre for Atomic Research (IGCAR) at Kalpakkam. She has also worked with the Azim Premji University on the undergraduate curriculum in science. She can be reached at aditic2003@gmail.com.
