Electrons discoveries surrounding

Rutherford s discovery of the proton did not radically change the picture of the atom, but it did present a problem. The atom was still thought to be made up of a heavy, positively charged nucleus surrounded by electrons. The difference was that scientists now knew that the nucleus was composed of protons. Measurements showed that the electrical charge of a proton was identical to, but opposite of, the charge on an electron. The proton s charge was positive, the electron s negative. Because atoms are electrically neutral, the number of protons in the nucleus had to equal the number of electrons. And that was the problem. 

Several major discoveries at the turn of the 20 century ied to our current model of atomic structure. Cathode rays were shown to consist of negative particles (electrons) that exist in ail matter. J. J. Thomson measured their mass/charge ratio and con-ciuded that they are much smalier and iighter than atoms. Robert Miliikan determined the charge of the electron, which he combined with other data to calculate its mass. Ernest Rutherford proposed that atoms consist of a tiny, massive, positive nucleus surrounded by electrons. 

In 1911, the British physicist and Nobel laureate Ernest Rutherford (1871-1937) published the article The Scattering of Alpha and Beta Particles by Matter and the Structure of the Atom in Philosophical Magazine. In this article, Rutherford reported the results of an experiment that demonstrated that the protons and electrons in atoms are not distributed homogeneously. Instead, the protons are concentrated in a relatively tiny region Rutherford called the nucleus (from the Latin, meaning kernel ). The electrons are extranuclear electrons are located in a relatively much larger volume of space surrounding the nucleus. Rutherford s discovery of the nucleus was immediately accepted within the scientific community. However, the relationship, if any, between atomic structure and properties was still unclear. 

The dual discoveries of radioactivity and X-rays made possible the further discovery and identification of several new elements, such as radium and polonium, which needed to be accommodated, and thus provided further tests of the robustness of the periodic system and its ability to adapt to changes. Indeed, while it is the electron that is mainly responsible for the chemical properties of the elements, discoveries connected with the nucleus of the atom nevertheless have had a profound influence on the evolution of the periodic system. The exploration of the nucleus, along with further work on the nature of X-rays and radioactivity, led to the discovery of atomic number and isotopy, two developments that would together resolve many of the lingering uncertainties surrounding Dimitri Mendeleevs periodic system. 

Thirty years later, the first Fe(ll) spin-crossover complex, [Fe(phen)2X2] (phen = 1, 10-phenanthroline, X = NCS or NCSe), was discovered by Baker et al. [6]. In [Fe(phen)2(NCS)2], the Fe(ll) atom Is surrounded by six N atoms of phen and NCS ligand molecules, and the complex exhibits a first-order phase transition associated with an abrupt LS ( A g, S = 0)-HS ( T2g, S = 2) transition at I76K, where a small thermal hysteresis attributed to the spin transition was observed. Since the discovery of the spin-crossover phase transition for [Fe(phen)2X2] (X = NCS, NCSe), various kinds of spin-crossover complexes have been found for the electron configurations of 3d" (n = 4 — 7). Most of them are Fe(ll)... 

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