Unit2AtomsMolecules

Unit2AtomsMolecules - -28-ATOMS and MOLECULES; ELEMENTS and...

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Unformatted text preview: -28-ATOMS and MOLECULES; ELEMENTS and COMPOUNDSThe Building Blocks of EverythingRecall that atoms are the smallest entities that we deal with; they arethe building blocks of all other matter.We will consider the atom to be a spherical species, with a nucleusin thecentre.Atoms consist mainly of three types of particles: Protonsand Neutrons(which are found in the nucleus) and Electrons(which move around theatom 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 particlesaccount for almost all of the mass of the atom (recall this is inside thenucleus !).Electrons have a mass about 1/2000 that of the proton or neutron; butthe negative charge on the electron is equal in size to the positivecharge 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 theelement.The Atomic Number of Hydrogen is 1, so Hydrogen must have one protonin its nucleus.Chlorine has the atomic number 17, or it is element number 17; it has 17protons.In a neutral atom (one that has no charge), the number of electronsoutside the nucleus is equal to the number of protons inside the nucleus.A neutral Carbon atom (element number 6) has 6 protons and 6electrons.-29-ThePeriodic Tablehas the elements arranged in order of their atomicnumbers; 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; yetalmost 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 neversay exactly where they are; the best models tell us where theelectrons 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: ZrrY; ZssYOpposite signs attract:rYZsSo what are neutrons? These are particles contained in the nucleus; theyhave no charge, but they do have mass, and so add to the mass of theatom.All elements except ordinary hydrogen possess neutrons; and a veryfew hydrogen atoms possess a neutron as well.One form of Hydrogen (the ordinary one) has 1 proton in the nucleus and1 electron outside the nucleus.Another form of Hydrogen (this one is actually called Deuterium) has 1proton in the nucleus and also has 1 neutron in the nucleus as well as 1electron outside the nucleus.-30-(Sometimes we call this form Heavy Hydrogen.....recall that theneutron has about the same mass as a proton, so it would weigh twice asmuch as ordinary hydrogen.)These two forms of Hydrogen are called Isotopes;elements which havethe same number of protons but different numbers of neutrons.the same number of protons but different numbers of neutrons....
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This note was uploaded on 10/23/2011 for the course CHEMISTRY 1027 taught by Professor Calvin during the Fall '10 term at UWO.

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Unit2AtomsMolecules - -28-ATOMS and MOLECULES; ELEMENTS and...

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