Nucleus, atomic alpha-particle model

A series of episodes in the historical development of our view of chemical atoms are presented. Emphasis is placed on the key observations that drove chemists and physicists to conclude that atoms were real objects and to envision their stracture and properties. The kinetic theory of gases and measmements of gas transport yielded good estimates for atomic size. The discovery of the electrorr, proton and neutron strongly irtfluenced discttssion of the constitution of atoms. The observation of a massive, dertse nucleus by alpha particle scattering and the measrrrement of the nuclear charge resrrlted in an enduring model of the nuclear atom. The role of optical spectroscopy in the development of a theory of electronic stracture is presented. The actors in this story were often well rewarded for their efforts to see the atoms. 

The atom was once thought to be the smallest unit of matter, but was then found to be composed of electrons, protons, and neutrons. The question arises are electrons, protons, and neutrons made of still smaller particles In the same way that Rutherford was able to deduce the atomic nucleus by bombarding atoms with alpha particles (Chapter 3), evidence for the existence of many other subatomic particles has been obtained by bombarding the atom with highly energetic radiation.This research over the past centmy has evolved into what is known as the "standard model of fundamental particles, which places all constituents of matter within one of two categories quarks and leptons. 

Ernest Rutherford studied atomic structure in 1910-1911 by firing a beam of alpha particles at thin layers of gold leaf. According to Thomson s model, the path of an alpha particle should be deflected only slightly if it struck an atom, but Rutherford observed some alpha particles bouncing almost backwards, suggesting that nearly all the mass of an atom is contained in a small positively charged nucleus. 

Rutherford concluded that the plum pudding model was incorrect because it could not explain the results of the gold foil experiment. He set out to develop a new atomic model based upon his findings. Considering the properties of the alpha particles and the electrons, and the frequency of the deflections, he calculated that an atom consisted mostly of empty space through which the electrons move. He also concluded that there was a tiny, dense region, which he called the nucleus, centrally located within the atom that contained all of an atom s positive charge and virtually all of its mass. Because the nucleus occupies such a small space and contains most of an atom s mass, it is incredibly dense. Just how dense If a nucleus were the size of the dot in the exclamation point at the end of this sentence, its mass would be approximately as much as that of 70 automobiles ... 

The concentrated positive charge in the nucleus explains the deflection of the alpha particles—the repulsive force produced between the positive nucleus and the positive alpha particles causes the deflections. Alpha particles closely approaching the nucleus were deflected at small angles, while alpha particles directly approaching the nucleus were deflected at very large angles. You can see in Figure 4-12 how Rutherford s nuclear atomic model explained the... 

Rutherford s nuclear model of the atom explains the results of the gold foil experiment. Most alpha particles pass straight through, being only slightly deflected by electrons, if at all. The strong force of repulsion between the positive nucleus and the positive alpha particles causes the large deflections. 

Alpha particles bounced back, Rutherford reasoned, because the atom is not a pudding. The atom is mostly empty space with an incredibly small, incredibly dense, positively charged core. Rutherford proposed a new model—the planetary model—in which negatively charged electrons in huge orbits circled a tiny, dense, positively charged nucleus. 

The alpha-particle scattering experiments of Rutherford established that the atom contains a dense, positively charged nucleus. The later work of Chadwick demonstrated that the atom contains neutrons, which are particles with mass, but no charge. Rutherford also noted that light, negatively charged electrons are present and offset the positive charges in the nucleus. Based on this experimental evidence, a model of... 

Rutherford thought this scattering happened when positive nuclei of the test particles collided and were then repelled by heavy positively charged gold nuclei. It was later proven that Rutherford s dense pit model was correct. When an accelerated alpha particle colUded with an electron of a gold atom in a gas, a proton was knocked out of the nucleus. 

The nucleus of an atom also has a structure the nucleus is composed of two different kinds of particles, protons and neutrons. The type of alpha-particle scattering experiment that led to the nuclear model of the atom was also instrumental in clarifying this structure of the nucleus. 

Rutherford proposed the nuclear model of the atom to account for the results of experiments in which alpha particles were scattered from metal foils. According to this model, the atom consists of a central core, or nucleus, around which the electrons exist. The nucleus has most of the mass of the atom and consists of protons (with a positive charge) and neutrons (with no charge). Each chemically distinct atom has a nucleus with a specific number of protons atomic number), and around the nucleus in the neutral atom are an equal number of electrons. The number of protons plus neutrons in a nucleus equals the mass number. Atoms whose nuclei have the same number of protons but different number of neutrons are called isotopes. 

In 1911, Rutherford s alpha-particle scattering experiments were controversial. In the Rutherford model of the atom, all of the positive charge was crammed into the dense, tiny nucleus. Like charges repel, so the nucleus of the atoms should not be stable, yet it was. The relationships of classical physics that worked so well in explaining large-scale systems did not work on atom-sized systems. Thus, someone had to develop a new approach to understanding the atom. The breakthrough that was needed was the development of the field of study now known as quantum mechanics. 

FIGURE 9.9 (a) A schematic of Rutherford and Marsden s experimental apparatus with platinum foil, (b) The nuclear model of the atom, based on the experiments. Three paths of alpha particles through the atom show how the alpha particles are influenced by a massive and heavily charged nucleus. Although some details of the model have been modified, the general idea remains intact a massive nucleus with lighter electrons moving around it... 

The model derived from these observations is called the Rutherford model, or the nuclear model, as shown by the circles in Figure 4.3. The scientists concluded that most of the mass of the atom had to be concentrated into an extremely small volume of space inside the atom, which they called the nucleus. That was the only way to explain the small number of alpha particles that were found reflected off the front side of the foil. In addition to this small volume of space where most of the mass is found, they concluded that the nuclei of neighboring atoms were surprisingly distant from one another and that the masses of any particles found there had to be extraordinarily small. This was the only way to explain finding the vast... 

Based on the scattering of alpha particles on platinum foil, Ernest Rutherford proposed a nuclear model of the atom. Later work showed that the nucleus of an atom contains particles ealled protons, each of which has a charge of +e (responsible for the positive charge), and neutrons... 

Joseph John Thomson had supposed that an atom was a uniform sphere of positively charged matter within which electrons were circulating (the plum-pudding model). Then, around the year 1910, Ernest Ruthorford (who had discovered earlier that alpha rays consisted of positively charged particles having the mass of helium atoms) was led to the following model for the atom Protons and neutrons exist in a very small nucleus, which means that the tiny nucleus contains all the positive charge and most of the... 

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