Atom, the smallest unit of a chemical potential. In the philosophy of ancient Greece, the word "atom" was used to refer to the smallest part of matter that could be conceived. This "fundamental particle" was considered indestructible. In fact, atom is Greek for "not divisible." Knowing the size andnature of the atom proceeded very slowly over the centuries as people are merely speculating about it.
With the advent of experimental science in the sixteenth and seventeenth centuries, advances in atomic theory became more rapid. Chemists realized very soon that all liquids, gases and solids can be decomposed into its ultimate constituents, or elements
1 .- The atom in the ancient Greek philosophers argued a lot about the nature of matter and concluded that the world was simpler than it seemed. Some of his ideas more relevant were:Leucippus
In the V century BC C., Leucippus contended that a single type of matter and thought that if we divided the material into ever smaller parts, we get a piece that could not cut more. Democritus called these pieces atoms ( "no division"). Atomistic philosophy of Leucippus and Democritus was summed up: 1 .- The atoms are eternal, indivisible, homogeneous and invisible. 2 .- The atoms are distinguished by their shape and size. 3 .- The properties of matter vary according to the grouping of atoms. Empedocles
In the fourth century BC BC, Empedocles postulated that matter consisted of 4 elements: earth, air, water and fire.
Aristotle then postulated that matter consisted of these 4 elements but denies the idea of atoms, a fact which was maintained until 200 years later in human thought.
1.1 .- The atomic theory of Dalton the early nineteenth century and explored ways in which various elements combine to form chemical compounds. Although most scientists, starting with the ancient Greeks, had claimed as the smallest units of a substance were atoms, Dalton is considered as one of the most significant figures because the atomic theory became quantitative. Dalton showed that atoms joined together in definite proportions. TheResearch shows that the atoms tend to form groups called molecules.
All atoms of a given element have the same chemical properties. Therefore, the atom is the smallest entity to be considered. The chemical properties of elements are very different from each other, their atoms combine in many ways to form numerous different chemical compounds.
In 1808, John Dalton published his atomic theory, which incorporated the old ideas of Leucippus and Democritus. Dalton's theory:
1 .- The elements are formed by tiny particles, indivisible and unalterable callsatoms.
Dalton established a system to designate each atom so that it could distinguish between different elements:
2 .- The atoms of the same element are all equal in mass, size and other physical or chemical properties. By contrast, atoms of different elements have different mass and properties. 3 .- The compounds are formed by the joining of atoms of the corresponding elements according to a simple numerical relationship and constant. From Dalton's atomic theory are available, the following definitions: - An atom is the smallest particle of an element that retains its properties. - An element is a pure substance that consists of equal atoms. - A compound is a substance that consists of different atoms combined in a simple numerical relationship and constant.2 .- The atom is divisible Once accepted atomic theory of matter, the phenomena of electrification and electrolysis showed, on the one hand, the electrical nature of matter and, secondly, that the atom was divisible, ie , which was composed of other smaller fundamental particles. On this page you can see examples of electrification phenomena.
Electrical phenomena are a manifestation of their electrical charge. The unit of electric charge in SI is the coulomb (C). There are 2 types of electric charges: positive and negative. two bodies which have acquired a load of the same type repel each other, while if they have loads of different kinds attract. The matter is electrically neutral, ie has the same amount of each type of load. when it takes charge, whether positive or negative, is because it has more of one type than another. In the late nineteenth and early twentieth centuries, a series of experiments helped identify the particles responsible for the negative charge (theelectron) and positive charge (the proton). These experiments provided the following data on the structure of matter: - The atom contains subatomic particles of matter. - The electrons have negative electric charge and mass. Each electron has an elementary electric charge. - Protons have positive electric charge and larger mass. - As the atom is electrically neutral, one must assume that the number of negative electric charges (electrons) equals the number of positive charges (protons) 3.1 .- Plum pudding model Being so small the mass of the electrons, the physical English JJ Thomson assumed in 1904 that most of the mass of the atom correspond to the positive charge, which therefore should occupy most of the atomic volume. Thomson imagined the atom as a kind of continuous positive area in which electrons are embedded (like raisins in a pudding).
This model allowed to explain several experimental phenomena as the electrification and the formation of ions. - The electrification: The excess or deficiency of electrons has a body and is responsible for its positive or negative electric charge. - The formation of ions: An ion is an atom that has gained or lost electrons. If gains electrons has a net negative charge and is called anion and if it loses electrons has net positive charge and is called a cation. 3.2 .- Rutherford model of Thomson's model was widely accepted until in 1911 the English chemist and physicist Ernest Rutherford and his colleagues conducted the "Rutherford Experiment This model allowed to explain several experimental phenomena as the electrification lto ion formation. - The electrification: The excess or deficiency of electrons has a body and is responsible for its positive or negative electric charge. - The formation of ions: An ion is an atom that has gained or lost electrons. If gains electrons has a net negative charge and is called anion and if it loses electrons has net positive charge and is called a cation. 3.2 .- Rutherford model of Thomson's model was widely accepted until in 1911 the English chemist and physicist Ernest Rutherford and his colleagues conducted the "Rutherford Experiment".
The experiment was bombarding a thin gold foil with alpha particles (positive) from a radioactive material and noted that: - Most of the alpha particles passed through the blade without changing direction, as expected. - Some alpha particles deviated considerably. - A few alpha particles rebounded towards the emission source. The Rutherford model or nuclear model states that:- The atom has a central core in which are concentrated positive charge and nearly all the mass. - The positive charge of protons in the nucleus is offset by the negative charge of electrons outside the nucleus. - The nucleus contains, therefore, protons in an equal number of electrons in the atom. - Electrons move at high speed around the nucleus and are separated from it by a great distance. 3.3 .- The mass of protons neutrons and electrons did not match the total mass of the atom, so Rutherford thought it had to be another type of subatomic particles in the nucleus of atoms. These particles were discovered in 1933 by J. Chadwick. Having no electric charge were called neutrons. Neutrons are uncharged particles and mass slightly larger than the mass of a proton. 3.4 .- Structure of the atom is accordingly the atom was composed as follows: - a central or core where the total charge is positive (the lI protons) and most of the mass of the atom, provided by the protons and neutrons. - An outer zone or cortex where are the electrons orbiting the nucleus. There are the same electrons in the crust that protons in the nucleus, so that the entire atom is electrically neutral. 4 .- Identification of the atoms The atoms are identified by the number of protons in its nucleus containing, as it is fixed for the atoms of the same element. Atomic number: The number of protons in an atom. It is represented by the letter Z and is written as a subscript to the left of the symbol of element: ZX. Mass number: The sum of the number of protons and number of neutrons in an atom. It is represented by the letter A and is written as a superscript to the left of the symbol of element: AX. - If a cation has lost electrons and you have to subtract the number that appears with the positive charge: - If you have won an anion electrons and must add the numberor to appear with the negative charge: 4.1 .- Isotopes In the early twentieth century it was discovered that not all atoms of the same element have the same mass. That is, the number of neutrons can vary for atoms of the same element. Isotopes are atoms of the same element have the same atomic number but different mass numbers. That is, have the same number of protons but different numbers of neutrons. 5 .- Relative atomic mass relative atomic mass of an element which corresponds to one of its atoms and is practically equivalent to the sum of the masses of its protons and neutrons, since the electron is so small it can be neglected. Thus, most of the mass of the atom is at the core. The mass of an atom as compared to the mass of carbon-12 is called the atomic mass. It is contained in the periodic table value for each element 6 .- New facts, new models The Rutherford model could not explain certain facts: - Thenegative charge of the moving electron would lose energy to fall against the core and this would make the atoms were unstable. - The visible radiation passing through a prism, the light breaks into the colors of the rainbow, this is called a continuous spectrum of visible light: Well, the light emitted by atoms of elements lead to discontinuous spectra: The fact that each atom has a different spectrum, discontinuous lines is related to its structure. This could not be explained with the model of Rutherford.