This term for science, I did a few ACE projects. They are mainly about chemistry. Chemistry is a very big and interesting topic given that there are over one hundred elements which give rise to millions of compounds. Of course, the other two sciences are just as big and interesting as chemistry. The most captivating part about chemistry is seeing how various chemicals react – whether a violent combustion or perhaps even an endothermic reaction which takes in heat. However, in order to understand chemical reactions, it is crucial that we understand the basics about chemistry, which forms the basis of the few science ACE which I have done this term.
The changing model of the atom
The first project was about the ever changing model of the atom. At first, in the ancient world, nobody understood anything about the atom. In fact, nobody even knew what an atom was. One day, a man named Democritus asked a question: What happens when you keep cutting an object into smaller and smaller bits? What do we get? He said that we would arrive with a particle that could not be further cut. He named the particles atoms. Many people thought that he was simply crazy and dismissed his idea. Then, a renowned philosopher named Aristotle, who said that all objects in the world was made up of four basic elements: water, earth, fire and air. Of course we now know that all of the objects in the world were not made of just water, earth, fire and air. Instead, they are made up of atoms.
In 1897, Joseph John Thomson discovered the electron. Since the electron was negatively charged, he thought that the rest of the atom had to be positively charged. Thus, he envisioned the atom to be like a plum pudding where electrons were stuck on the atom like plums on a pudding.
In 1911, Ernest Rutherford performed an experiment which shocked the whole scientific world. He fired alpha particles at a gold foil. At first, he expected most of the alpha particles which were positively charged helium nuclei to bounce back from gold atoms. However, most of the alpha particles passed right through the gold foil, some of the alpha particles changed course, and only a few of them bounced right back as if they hit something dense. That was when he discovered the proton. He realised that Thomson’s model was wrong and proposed that the atom was like our Solar System. It was mostly empty space with a positively-charged core in the centre and electrons orbiting the core like planets orbiting the Sun.
In 1913, Danish physicists Niels Bohr had been investigating the behavior of electrons when they gained and lost energy. When electrons gained energy, they emitted a flash of light. Bohr suggested that the electrons were arranged in energy levels around the nucleus. These levels are arranged in shells around the nucleus. Bohr suggests that each level could only accommodate a certain number of electrons. Evidence for this arrangement of electrons came from the chemical properties of the elements.
Bohr’s model of the atom was easy to understand and it is the model taught in school today. However, in 1924, French scientist Louis de Broglie realised that electrons could exist as both a particle and a wave. So instead of having electrons orbit the nucleus, the electrons move around like a wave.
By the 1920s, physicists had realised that the existing rules used to describe forces such as gravity could not be applied to atoms. The new rules they devised were called quantum theory. One of the rules states that it is impossible to measure exactly both the position and velocity of subatomic particles. This theory was published by Werner Heisenberg. This reminds me about a joke:
One day, Heisenberg was caught speeding. A traffic police asked him to pull over and said, “Do you know how fast you have been driving?” Heisenberg replied, “No, but I know exactly where I am.”
In the late 1920s, Heisenberg published the idea that the orbit of an electron was neither a circle nor a wave. Instead, it was a cloud covering the area where the electron was most likely to be. This allowed us to understand about various types of inter and intra molecular bonds such as dipole bonding.
Finally in 1932, the missing particle of the atom had been discovered – the neutron – by James Chadwick.
Here, we see that the model of the atom has been continuously changing and up till date, it still continues to evolve. I believe it is the same for science. Every day, new discoveries are being made. Old theories are being proven wrong. In the past, a famous chemist John Dalton thought that atoms were unbreakable. Scientists now are able to split the atom using nuclear fission. Science is always evolving and we should not only adapt to the change, we should also be part of the change. We should be the ones making the change.
A case study on covalent bonding
This ACE project deals with why aluminum chloride and beryllium chloride are covalent compounds and not ionic compounds. We know that a covalent bond is a bond formed by the sharing of a pair of electrons. We also know that ionic bonds are formed between metals and non-metals. The metal loses its electrons to the non-metal so that both can possess the electronic configuration of a noble gas.
Aluminum is a metal and chlorine is a non-metal. However, aluminum chloride seems to exhibit a more covalent nature rather than ionic nature. Anhydrous aluminum chloride sublimes, something strange for an ionic compound. Ionic compounds tend to have high melting and boiling points. For example, sodium chloride has a melting point of 801OC and boils at 1413OC. Aluminum chloride is soluble in organic solvents such as ethanol, unlike most ionic compounds.
So why is it that aluminum appears to behave like a covalent compound rather than an ionic compound? At first, I was actually quite puzzled. However, I read about electronegativity. Electronegativity describes how well an atom attracts electrons. Strongly electronegative elements such as chlorine tend to attract electrons and form negatively-charged ions. Weakly electronegative elements tend to lose their electrons and form positively-charged ions. Thus, elements with high difference in electronegativity tend to form ionic compounds and elements with low difference in electronegativity tend to form covalent compounds. Scientists estimate that elements with a difference of less than 1.7 will form covalent compounds.
Now we go back to our original question. Aluminum chloride has an electronegativity difference of 1.55 which is less than 1.7, thus aluminum chloride is a covalent compound. However, aluminum chloride does look like a salt and isn’t entirely covalent. As such, we classify it as a polar covalent compound. A polar covalent compound means that the electron isn’t shared very fairly: the electrons tend to stay on the chlorine side and not the aluminum side. This is because chlorine is more electronegative.
The same is for beryllium chloride. Beryllium chloride has an electronegativity difference of 1.59, thus it is a covalent compound just like aluminum chloride.
From doing this ACE project, I have learned that sometimes science is not as direct as it seems. There are often a few exceptions. I believe that in learning science, we have to be flexible and learn to accept new findings and integrate them with the old.
No comments:
Post a Comment