notes-2 - B1) Atomic Chemistry and Bonding All matter is...

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B1) Atomic Chemistry and Bonding All matter is made up of atoms, and all atoms are made up of three main particles known as protons , neutrons and electrons . As summarized in the following table, protons are positively charged, neutrons are uncharged and electrons are negatively charged. The negative charge of one electron balances the positive charge of one proton. Both protons and neutrons have a mass of 1, while electrons have almost no mass. Elementary particle Charge Mass Electron -1 ~0 Proton +1 1 Neutron 0 1 The simplest atom is that of hydrogen, which has one proton and one electron. The proton forms the nucleus of hydrogen, while the electron orbits around it. All other elements have neutrons as well as protons in their nucleus. The positively-charged protons tend to repel each other, and the neutrons help to hold the nucleus together. For most of the 16 lightest elements (up to oxygen) the number of neutrons is equal to the number of protons. For most of the remaining elements there are more neutrons than protons, because with increasing numbers of protons concentrated in a very small space, more and more extra neutrons are needed to overcome the mutual repulsion of the protons in order to keep the nucleus together. The number of protons is the atomic number , the number of protons plus neutrons is the atomic weight . For example, silicon has 14 protons, 14 neutrons and 14 electrons. Its atomic number is 14 and its atomic weight is 28. The most common isotope of uranium has 92 protons and 146 neutrons. Its atomic number is 92 and its atomic weight is 238 (92+146). Electron orbits around the nucleus of an atom are arranged in what we call shells [see the first few pages of Chapter 2]. The first shell can hold only two electrons, while the next shell will hold only eight electrons. Subsequent shells can hold more electrons, but the outermost shell of any atom will hold no more than eight electrons. These outermost shells are generally involved in bonding between atoms, and bonding takes place between atoms that do not have the full complement of eight electrons in their outer shells (or two in the first shell for the very light elements). To be chemically stable, an atom seeks to have a full outer shell (i.e., 8 electrons for most elements, or 2 electrons for the very light elements). This is accomplished by lending, borrowing, or sharing electrons with other atoms. Elements that already have their outer orbits filled are considered to be inert; they do not readily take part in chemical reactions. These noble elements include the gases in the right-hand column of the periodic table: helium, neon, argon etc. Sodium has 11 electrons, 2 in the first shell, 8 in the second, and 1 in the third. Sodium readily gives up this third shell electron, and because it loses a negative charge it becomes positively charged [pages 38 and 39]. Chlorine, on the other hand, has 17 electrons, 2 in the first shell, 8 in the second, and 7 in the third. Chlorine readily accepts an eighth electron for its third shell, and thus becomes negatively charged. In changing their number of electrons these atoms become
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notes-2 - B1) Atomic Chemistry and Bonding All matter is...

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