Thus, hydrogen has an atomic number of 1, while iron has an atomic number of 26. This number of protons is so important to the identity of an atom that it is called the atomic number of the element. All atoms of hydrogen have one and only one proton in the nucleus all atoms of iron have 26 protons in the nucleus. ![]() What makes atoms of different elements different? The fundamental characteristic that all atoms of the same element share is the number of protons. The modern atomic theory states that atoms of one element are the same, while atoms of different elements are different. (See Figure 3.4 “The Structure of the Atom”.) Figure 3.4 “The Structure of the Atom.” Atoms have protons and neutrons in the centre, making the nucleus, while the electrons orbit the nucleus. The electrons are outside the nucleus and spend their time orbiting in space about the nucleus. The relatively massive protons and neutrons are collected in the centre of an atom, in a region called the nucleus of the atom (plural nuclei). Experiments by Ernest Rutherford in England in the 1910s pointed to a nuclear model of the atom. How are these particles arranged in atoms? They are not arranged at random. Table 3.7 Properties of the Three Subatomic Particles Name Table 3.7 “Properties of the Three Subatomic Particles” summarizes the properties of these three subatomic particles. We now know that all atoms of all elements are composed of electrons, protons, and (with one exception) neutrons. The neutron is a subatomic particle with about the same mass as a proton but no charge. The proton is a more massive (but still tiny) subatomic particle with a positive charge, represented as p +. Later, two larger particles were discovered. It is often represented as e −, with the right superscript showing the negative charge. The first part to be discovered was the electron, a tiny subatomic particle with a negative charge. These concepts form the basis of chemistry.Īlthough the word atom comes from a Greek word that means “indivisible,” we understand now that atoms themselves are composed of smaller parts called subatomic particles. Atoms combine in whole-number ratios to form compounds.Atoms of the same element are the same atoms of different elements are different. ![]() ![]() The concept that atoms play a fundamental role in chemistry is formalized by the modern atomic theory, first stated by John Dalton, an English scientist, in 1808. Atoms are so small that it is difficult to believe that all matter is made from atoms-but it is. ![]() The period at the end of a printed sentence has several million atoms in it. It would take about fifty million atoms in a row to make a line that is 1 cm long. Rutherford needed to come up with an entirely new model of the atom in order to explain his results.The smallest piece of an element that maintains the identity of that element is called an atom. In a famous quote, Rutherford exclaimed that it was "as if you had fired a 15-inch shell at a piece of tissue and it came back and hit you." No prior knowledge had prepared them for this discovery. Some were even redirected back toward the source. Surprisingly, while most of the alpha particles were indeed undeflected, a very small percentage (about 1 in 8000 particles) bounced off the gold foil at very large angles. Rutherford found that a small percentage of alpha particles were deflected at large angles, which could be explained by an atom with a very small, dense, positively-charged nucleus at its center (bottom).Īccording to the accepted atomic model (Quantum Model), in which an atom's mass and charge are uniformly distributed throughout the atom, the scientists expected that all of the alpha particles would pass through the gold foil with only a slight deflection or none at all. (B) According to the plum pudding model (top) all of the alpha particles should have passed through the gold foil with little or no deflection. \): (A) The experimental setup for Rutherford's gold foil experiment: A radioactive element that emitted alpha particles was directed toward a thin sheet of gold foil that was surrounded by a screen which would allow detection of the deflected particles.
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