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Unit2AtomsMolecules - ATOMS and MOLECULES ELEMENTS and...

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-28- ATOMS and MOLECULES; ELEMENTS and COMPOUNDS The Building Blocks of Everything Recall that atoms are the smallest entities that we deal with; they are the building blocks of all other matter. We will consider the atom to be a spherical species, with a nucleus in the centre. Atoms consist mainly of three types of particles: Protons and Neutrons (which are found in the nucleus) and Electrons (which move around the atom outside of the nucleus). Protons carry a positive ( + ve) charge; Electrons carry a negative charge ( - ve) and Neutrons do not carry a charge (said to be neutral ). Protons and neutrons have essentially the same mass; these particles account for almost all of the mass of the atom (recall this is inside the nucleus !). Electrons have a mass about 1/2000 that of the proton or neutron; but the negative charge on the electron is equal in size to the positive charge on the proton. The number of protons in an atom is called the Atomic Number (sometimes denoted by Z ); this number determines the identity of the element. The Atomic Number of Hydrogen is 1, so Hydrogen must have one proton in its nucleus. Chlorine has the atomic number 17, or it is element number 17; it has 17 protons. In a neutral atom (one that has no charge), the number of electrons outside the nucleus is equal to the number of protons inside the nucleus. A neutral Carbon atom (element number 6) has 6 protons and 6 electrons.
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-29- The Periodic Table has the elements arranged in order of their atomic numbers; this is the large number above the symbol for each element. What Does an Atom Look Like? We think of an atom as a sphere with a tiny nucleus at the centre; yet almost all of the mass of the atom is inside the nucleus. Most of the sphere is empty space; this is where we find the electrons. The electrons are always in motion outside the nucleus; but we can never say exactly where they are; the best “models” tell us where the electrons could be found about 90% of the time. Electrons are held within the volume of the atom by electrostatic forces (the negative electrons are attracted to the positive protons). Same signs repel: Z r r Y ; Z s s Y Opposite signs attract: r Y Z s So what are neutrons? These are particles contained in the nucleus; they have no charge, but they do have mass, and so add to the mass of the atom. All elements except “ordinary” hydrogen possess neutrons; and a very few hydrogen atoms possess a neutron as well. One form of Hydrogen (the ordinary one) has 1 proton in the nucleus and 1 electron outside the nucleus. Another form of Hydrogen (this one is actually called Deuterium ) has 1 proton in the nucleus and also has 1 neutron in the nucleus as well as 1 electron outside the nucleus.
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-30- (Sometimes we call this form Heavy Hydrogen ..... recall that the neutron has about the same mass as a proton, so it would weigh twice as much as ordinary hydrogen.) These two forms of Hydrogen are called Isotopes; elements which have the same number of protons but different numbers of neutrons.
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