Going back to the subject of chemistry, in this post I will talk about what an atom is made up of (its structure). With all of this talk about atoms and Dalton’s atomic theory, you might be wondering (and would be justified in doing so), what exactly is an atom? So far, a lot of mystery has surrounded the word. And I wouldn’t be telling the truth if I said I knew exactly what an atom is, off of the top of my head. So, while we are still on the more qualitative aspects of general chemistry, what is an atom anyway?
Though I might have mentioned that Dalton originally thought atoms to be indivisible particles, it has been shown in science that atoms actually consist of particles themselves. Specifically, two kinds of particles exist within an atom. The first that I’ll go over is the nucleus. The nucleus is at the central core of an atom, and it is positively charged. Most of the mass of an atom exists within the nucleus. Electrons also contribute to the mass of an atom, but much less so. The electron is a negatively charged particle that is very light and exists around the positively-charged nucleus of the atom (Ebbing and Gammon 2009).
J. J. Thomson was a physicist who demonstrated in the late 1890s that tiny negative particles could be emitted from the atoms of any given element, a point that he was particularly convinced of because he was able to show that atoms were repelled by the negative parts of an electric field. Thomson used a series of experiments to prove his theory (Zumdahl and DeCoste 2008).
Reasonably, Thomas inferred from his findings that atoms must contain another type of particle, because it was not possible for them to merely contain negatively-charged electrons, as the net charge of atoms is zero. He concluded that positively-charged particles also exist within atoms. These positive particles balance the negative charge that electrons within the atom carry (Zumdahl and DeCoste 2008).
William Thomson, another scientist with no relation to J. J. Thomson, developed a theory to try to explain the structure of an atom. This theory was known as the plum pudding theory, where it was thought that an atom might consist of a uniform distribution of positively-charged particles with enough negatively-charged particles to counterbalance the positive charge and make the net charge of the atom zero, likened to the random distribution of raisins throughout plum pudding. As you might guess, this was not the best theory around and in 1911, things changed drastically (Zumdahl and DeCoste 2008).
In 1911, Ernest Rutherford, a physicist who learned physics in the 1890s in J. J. Thomson’s laboratory, formulated his own ideas about the atom. Rutherford was particularly interested in studying alpha particles, which have a mass approximately 7,500 times that of the electron. Rutherford studied the flights of alpha particles through air and discovered that something in the air deflected some of the alpha particles. Rutherford was bewildered by this phenomenon and invented an experiment that would help him understand more about what was going on. The experiment involved directing alpha particles toward a thin metal foil. A detector with a substance that produced tiny flashes wherever an alpha particle made contact with it surrounded the foil (Zumdahl and DeCoste 2008).
Rutherford’s results were surprising to him. In fact, the way that he described the results of his experiment would surprise us even today: he compared the outcome of the experiment to shooting a gun at a piece of paper and having the bullet bounce back. At that point, Rutherford began to doubt the plum pudding model because, he knew, if the model was indeed correct, the gargantuan alpha particles would crash through the tin foil in a way similar to how cannonballs might crash through paper (Zumdahl and DeCoste 2008).
What Rutherford then concluded was that a center of concentrated positive charge was responsible for the large deflections of the alpha particles, which would cause the positively charge alpha particles to be repelled. He found that because the atom is mostly open space, most of the alpha particles passed directly through the foil. Deflected particles were found to have had a “close encounter” with the positive center of the atom (the nucleus). Thus, those that were reflected had experienced “direct hit” with the positive center of the atom. Essentially, what this brought Rutherford to conclude was that what he was observing could only be explained using the concept of the nuclear atom, or an atom with a dense, positively-charged center (the nucleus) around which electrons moved in an otherwise empty space (Zumdahl and DeCoste 2008).
This lead to Rutherford’s concluding that the nucleus, the positively-charged center of the atom, contained what he called the proton, which has the same size or magnitude as an electron, but has a positive charge rather than a negative charge. It has been established from these experiments and others that the proton has a charge of 1+ and the electron a charge of 1- (Zumdahl and DeCoste 2008).
Through more reasoning, Rutherford also came to the conclusion that the hydrogen atom consists of a single proton at its center and a single electron moving through space at a distance far from the proton at the core of the hydrogen atom (the hydrogen nucleus). From this, he went on to infer that other atoms contained nuclei that are compromised of many protons that are somehow bound to one another. James Chadwick, a coworker of Rutherford’s, and Rutherford, were further able to prove that neutrons exists. A neutron is a neutral particle contained within most nuclei, that is slightly larger than a proton, but with a neutral charge (no charge at all).
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