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 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 firstname.lastname@example.org.