Rutherford, Ernest, atomic model

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. 

A FIGURE 2.9 J. J. Thomson s plumpudding model of the atom. Ernest Rutherford proved this model wrong. 

The heginning of the science of electron spectroscopy can only be equated to the experiments of British physicist and Nobel laureate J.J. Thomson in 1897. These experiments first identified electrons and protons as the electrically charged particles of which atoms were composed, according to the atomic model propounded by British chemist and physicist Ernest Rutherford. Thomson s experiments were also the first to measure the ratio of the charge of the electron to the mass of the electron. This feature must be known to utilize the interaction of electrons and electromagnetic fields quantitatively. 

M Figure 2.9 J. J. Thomson s "plumpudding" model of the atom. He pictured the small electrons to be embedded in the atom much like raisins in a pudding or like seeds in a watermelon. Ernest Rutherford proved this model wrong. 

In 1897, J. J. Thomson had shown that cathode rays consist of extremely small particles, negatively charged. They were given the name electrons . They formed part of the first modern atom model. In 1911, the British physicist Ernest Rutherford (1871-1937) discovered that the atom is not a soHd piece of matter but mostly empty space. The mass is concentrated in the center, in an infinitesimally small nucleus . 

RBS is based on collisions between atomic nuclei and derives its name from Lord Ernest Rutherford who first presented the concept of atoms having nuclei. When a sample is bombarded with a beam of high-energy particles, the vast majority of particles are implanted into the material and do not escape. This is because the diameter of an atomic nucleus is on the order of 10 A while the spacing between nuclei is on the order of 1 A. A small fraction of the incident particles do undergo a direct collision with a nucleus of one of the atoms in the upper few pm of the sample. This collision actually is due to the Coulombic force present between two nuclei in close proximity to each other, but can be modeled as an elastic collision using classical physics. 

The first detailed model of the atom, proposed by J. J. Thomson in 1898, was based upon the expectation that the atom was a sphere of positive electricity in which electrons were embedded like plums in a pudding. This picture of the atom was not particularly satisfying because it was not useful in predicting or explaining the chemical properties of the atom. Finally, in 1911, a series of experiments performed in the McGill University laboratory of Ernest Rutherford showed that Thomson s picture of the atom had to be abandoned. 

Sir Ernest Rutherford (1871-1937 Nobel Prize for chemistry 1908, which as a physicist he puzzled over) was a brilliant experimentalist endowed with an equal genius of being able to interpret the results. He recognized three types of radiation (alpha, beta, and gamma). He used scattering experiments with alpha radiation, which consists of helium nuclei, to prove that the atom is almost empty. The diameter of the atomic nucleus is about 10 000 times smaller than the atom itself. Furthermore, he proved that atoms are not indivisible and that in addition to protons, there must also be neutrons present in their nucleus. With Niels Bohr he developed the core-shell model of the atom. 

Elements are made of tiny particles called atoms, which can combine in simple numerical ratios according to the law of multiple proportions. Atoms are composed of three fundamental particles Protons are positively charged, electrons are negatively charged, and neutrons are neutral. According to the nuclear model of an atom proposed by Ernest Rutherford, protons and neutrons are clustered into a dense core called the nucleus, while electrons move around the nucleus at a relatively large distance. 

By the early twentieth century, chemists and physicists recognized that the atoms of which chemical elements are composed are themselves made up of electrons and protons, of electrically negative and positive subatomic particles that were the universal constituents of all chemical elements. Sir Joseph Thomson had discovered the electron in 1897. Ernest Rutherford postulated the existence of a positive nucleus in atoms in 1911, and he used this in developing his planetary model of the atom, with a positive center and orbiting electrons. He discovered the proton in 1919, in experiments on the disintegration of atomic nuclei. Much later, in 1932, the British physicist James Chadwick (1891— 1974) discovered a third subatomic particle, the electrically neutral neutron. 

Ernest Rutherford a, (3, and -y emissions gold-foil experiment nuclear model of the atom Determined the nature of radioactive particles. His gold-foil experiment established the presence of a positively charged nucleus and that the atom is mostly empty space. 

The plum pudding model, a batter of positive charge with minute negative currants embedded in it, appeared to be consistent with experiments which showed that a beam of electrons could pass undeflected through a thin metallic foil. In other words, one might conclude, as Philipp Lenard (1862-1947) did in 1903, that the atom was mostly empty space. These data as well as the larger question about the inner structure of the atom prompted a most provocative line of experimentation by Ernest Rutherford (1871-1937). Manchester University was the site of these historical experiments, which Rutherford initiated soon after he arrived in 1907 to assume his responsibilities as Langworthy Professor of Physics. 

Consider Ernest Rutherford s alpha-particle bombardment experiment illustrated in Figure 2.11. How did the results of this experiment lead Rutherford away from the plum pudding model of the atom to propose the nuclear model of the atom ... 

Thomson proposed that the electrons of an atom were embedded in a positively charged ball of matter. His picture of an atom, which is shown in Figure 7, was named the plum-pudding model because it resembled plum pudding, a dessert consisting of a ball of cake with pieces of fruit in it. Ernest Rutherford, one of Thomson s former students, performed experiments in 1909 that disproved the plum-pudding model of the atom. 

To learn more about the discovery of atomic structure, visit the Chemistry Web site at chemistrymc.com Activity Research Ernest Rutherford s work. Write a newspaper article announcing his model of the atom. 

Soon after Thomson developed his model, tremendous insight into atomic structure was provided by one of Thomson s former students, Ernest Rutherford (1871-1937), who was the outstanding experimental physicist of his time. 

In 1909, a team of scientists led by Ernest Rutherford in England carried out the first of several important experiments that revealed an arrangement far different from the cookie-dough model of the atom. Rutherford s experimental setup is shown in Figure 2.9. 

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