Antennating

Our world is filled with so much diversity, from non-living to living, from that which we can see and touch to that which we can’t feel, touch or properly quantify. We still need to learn a lot about our universe, it is filled with mysteries. One of such major questions is to understand what makes up our world and how it results in such diversity.  One of the most acceptable models of how the universe came to be, is the big bang, which is estimated to have occurred about 13.78 billion years ago, before it, it is thought that the world was a singularity, meaning there was no meaning of space and time. When it happened, 0 to 10⁻⁴³ seconds into the expansion, called the Planck epoch, it was very hot, about 10³² degrees Celsius, and the forces we know about right now: the electromagnetic force, the strong nuclear force, the weak nuclear force, and the gravitational force, were unified. After 10⁻⁴² seconds, the gravitational force separated from the other forces, which somehow led to a temperature drop, slowly and slowly different particles that make up our world started forming by combining with one another. Atom is one of the basic particles that make matter by combining atoms of different elements in different ways. A single type of atom, let us say Carbon can combine in multiple ways to give us various properties, graphite and diamonds, are made up of carbon, but are arranged differently, just like Lego blocks, you can use the same blocks to build up different things.

But the question remains, what are atoms and how are they able to combine? Well, atoms have been studied for decades now, and there were various studies done. Firstly, it was thought that the atom is the smallest unit of matter, but soon M. Faraday while doing a cathode ray experiment found evidence that there is a negative component to atoms, which was further studied by J.J. Thomson (determined mass) and later by R.A. Millikan (he performed an experiment called as oil drop experiment to determine the charge of an electron). The cathode ray tubes were also used later in the box television sets. A cathode ray tube is an airtight tube with electrodes on each end, whose pressure is controlled to be low using a vacuum and it can be filled with a gas of our interest to understand its properties.

Similarly, by modifying the cathode ray tube, they were able to observe the properties of atoms(gases) which were like a positive sub-atomic particle called a photon. As there were negative and positive sub-atomic particles, scientists wanted to know if there was a neutral particle too, thus Chadwick, by bombarding beryllium with alpha particles found a neutral subatomic particle, thus called a neutron. Now we have these sub-particles and also know that electrons, protons and neutrons are not the smallest particles, they can be further divided but I won’t be going there, so coming back we still didn’t quite understand the structure of atoms and how these sub-particles interact back then and we still don’t quite understand everything about it right now, let me try to brief in simple terms how we got where we are.

There were multiple theories given to try to explain the structure of an atom. J. J. Thomson proposed that the atom was like a watermelon, with the seeds being negative and the rest being a positive sphere. But this theory was not supported by further experiments. Later Rutherford and his student bombarded thin gold film with alpha rays (having positive charge), to his and his student’s surprise, they got a lower count than the expected count of particles on the photographic plate which was placed behind the gold film, they then placed a plate to an oblique angle and found out some particles were passing through, but most passed. They hypothesized that unlike Thomson’s model for atoms, the positive charge just cannot be spread across the atom, it must be condensed. Imagine throwing stones at a fence with the holes in it being big, much bigger than your stones, most of the stones you will throw will simply pass through, but those stones that do hit the fence will bounce back or get deflected, this experiment is the same. Thus, they concluded, the positive charge and the mass of the atom must be condensed at the centre of the atom and the electrons must be moving in orbits around it at a very fast speed, just like the Moon orbits the Earth, but relatively much faster.  But we know that if you spin a top, even at a very high speed, it still comes to a stop, it loses energy, same principles must work for an electron orbiting the nucleus, right?

During the same time as the study of models for atoms was going on, a lot of progress in physics like electromagnetism, the dual nature of electromagnetic radiation i.e., light has both particle-like and wave-like properties, photoelectric effect, Planck’s Quantum theory, etc was made, this helped us understand a lot about photons and energy. Later Bohr’s model of atoms used these modern theories and categorized orbits according to quantum theories, but mainly based on the light spectrum of hydrogen, and theorised how the electrons could change orbits if enough energy is provided, this was a very big advancement. But similar to Rutherford’s model, Bohr’s model couldn’t explain how bonds are formed, and how the line spectrum of different elements was so different from hydrogen. Till now most of the studies were based on how we study big objects, should we study something like an atom or electron the same way, or do they follow the same rules? Well, it doesn’t exactly follow, see when we go at a very small level, much smaller than cells, much smaller than viruses, we cannot study our system based on the classical laws of physics we use for big objects (classical mechanics that follows Newton’s Laws), here, the laws of quantum mechanics work, which is still a field we have to study a lot about and don’t quite understand. But, let us understand what we know so far. 

We now follow a modern Quantum mechanical model of atoms, but before it, we must understand why classical mechanics fails at the microscopic level. There are two main points, first, at the macroscopic level, we treat objects as particles moving, on which Newton’s law applies. Be it a ball or planets (although Einstein’s Theory of Relativity suggests new insights into how gravity, space and time are interrelated), but as we go towards the microscopic level, these laws do not hold, why? Because there the matter has dual properties. Dual properties here mean that they act both as a particle and a wave, this can be observed by double slit experiment or Young’s Experiment (by Thomas Young), where when a laser is passed through a plate with two parallel slits, the light acts as waves and they interfere with each other to form light and dark bands as they add up or cancel other waves. But when a detector is placed at the slits, it shows individual photons passing through. How can we explain this using classical mechanics? The electrons show the same properties as well. Well, we can’t(as of now) and thus theories for atomic models based on classical mechanics do not hold. Secondly, is the Heisenberg Uncertainty Principle, which states that we can only know either the position of an electron or the momentum, thus we cannot observe both together. What does this mean? It tells us that there is no definite path or trajectory the electron takes, we can only estimate the probability of where the electron can be in an atom and where it may go. If you find this confusing, it is because these properties are confusing. Quantum mechanics was further explained individually by Erwin Schrödinger and Werner Heisenberg.

Based on these theories and experiments, the Quantum Mechanical model of the atom was explained, in which there are shells and sub-shells, and angular momentum (how the electron is moving), and the spin on the electron. These all are determined by Principal Quantum Number(n), Azimuthal quantum number (l = 0 to n-1), Magnetic orbital quantum number (m_l or m= -l to +l), Electron spin (m_s or s= +1/2 or -1/2). The ‘n’ tells us how far the shell is from the center, ‘l’ represents the sub-shells and what the size of the orbital is, and ‘m_l’ or ‘m’ represents how the orbital is oriented and at last ‘s’ represents the spin or how the electron is oriented. Although we are saying that we can tell the exact position of an electron with this, it is not exactly true, the electrons cannot be easily observed directly, but we can observe the electron cloud, which is the probability graph of where an electron could be around the nucleus and these clouds are what defines the sub-shells. The main concept of this model is that it now explains the structure of an electron, not just as a particle but also as a wave using the wave function equation (Schrödinger’s equation) and also follows the Heisenberg Uncertainty Principle. This also gave us an understanding that the sub-shells of the last shell (the farthest from the nucleus) are mainly involved in bond formation. Also, there were certain rules assigned to how the sub-shells get filled, mainly Aufbau Principle and Pauli Exclusion Principle. 

The world doesn’t consist of only one type of element, we have heard about hydrogen, oxygen, carbon, nitrogen, sulfur and phosphorus which are very abundant in our bodies, but the universe consists of many more elements each with unique properties like the atomic number, mass, radioactivity, electronegativity, etc. These elements are been discovered till the modern day, and to understand these elements properly, we categorize them according to various properties, mainly based on their atomic number. 

Now we know a little about the atom, but the question remains the same, how is there so much diversity in the matter, as we till now mainly know about the structure of an atom, how does it correlate to molecular bonding? Well, atoms are not perfect, not all of them, most atoms don’t have electrons to fill up their shells and sub-shells, when the atom has a shell which has all the sub-shells completely occupied or even halfway occupied, meaning all sub-shells can have 2 electrons of opposite spins, so if all have either 1 or complete 2 electrons, they become happy, they are stable. But only noble gases have completely filled shells. All others want the configuration like that of noble gases. Let us assume, in the class, not everyone has all the stationary, only a few do have everything, and thus they don’t need to interact with others to get their work done, these are the noble gases. But for others, someone may have a pen, some may have a pencil and scale but not the sharpener, and so on. To survive the school, they have to group up in ways that benefit all, such that everyone can complete the tasks and be stable just like the noble student. Similarly, the atoms of different elements or the same element group up to share, give or take electrons in various ways. This is the basic thought behind the formation of molecular bonds and the diversity of the molecules. The type of bond and elements in a molecule and how they interact with each other determine the properties of a molecule.

There are various types of strong bonds, ionic bonds, covalent bonds and coordinate bonds, you may have heard about Hydrogen bonds, these have similar principles of attaining stable, low-energy configurations, they want to become relaxed just like the noble person, but they do this by weak electrostatic interactions and based on van der Waals radius. If we imagine atoms as spheres, then if two spheres’ come in contact, their van der Waals radius would irrespective of how big is one opposed to the other, be half the distance between them. If they come near then they would experience repulsion between them, there is not enough space to accommodate both of them, but if they both need something(electrons) from each other, they can accommodate and adjust a little like they are foam balls. If one atom needs an electron and another one has an extra one, they both want to take and give, they form an ionic bond. Like NaCl(salt), sodium, a soft and reactive alkali metal, wants to lose an electron and chlorine, a gas wants to gain an electron, thus they do so and form crystals of NaCl. Like, you have 2 pencils, and another person has none, so you want to lose the pencil, as that will make you more stable, you will have to carry one less pencil, thus you give a pencil and the other person takes the pencil, this creates a bond between you both.

Covalent bonds are the sharing of electrons, this can be sharing of one, two or three pairs of electrons between two atoms, making single, double and triple bonds. Similar to the previous examples, in methane (CH₄), carbon wants to gain four electrons and each hydrogen also wants to gain an electron to have a noble configuration, so they share electrons with each other, carbon shares 4 electrons and in return hydrogen shares 4 electrons this causes the formation of four covalent bonds. The coordinate bond is a type of covalent bond and is formed when you have a pencil and another person doesn’t, but you don’t want to lose the pencil, so you just share the pencil, but they share nothing in return. This is seen in NH₄⁺, the extra hydrogen forms a coordinate bond with nitrogen. All these types of bonds can also occur in the same molecule like NH₄Cl, here as explained, the fourth hydrogen forms a coordinate bond, other hydrogens form covalent bonds, and the chlorine and NH₄⁺ molecule forms an ionic bond.

There are multiple theories of how atoms share the electrons by mixing up the shape and configurations of their sub-shells, these theories include the Kössel-Lewis approach, Valence Shell Electron Pair Repulsion (VSEPR) Theory, Valence Bond (VB) Theory and Molecular Orbital (MO) Theory. The most acceptable of these is the molecular orbital theory, which in short is when the orbitals of the last shell combine in various ways while forming a molecule and the molecular orbitals thus formed are a hybrid and more stable than the atomic orbitals. One interesting way to see this is, when given a very small room to live in, two people would not get two beds, but rather a bunker bed which then gives them more space and relaxation while staying in the room.

This is not the end of these interesting theories, there is still work going on in understanding how atoms interact within a molecule, they show various changes in properties, from their individual states, and there are multiple applications of how electrons move in a molecule. Further defining their magnetism, conductivity, and state of matter. The world of atoms and molecules is a vast and interesting place to get lost in.