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Ch. 3: Atoms 3.1 Foundations History Democritus named the most basic particle atom- means “indivisible” Aristotle didn’t believe in atoms thought matter was continuous (thought you could keep dividing it) History by 1700s, all chemists agreed: on the existence of atoms that atoms combined to make compounds Still did not agree on whether elements combined in the same ratio when making a compound Law of Conservation of Mass mass is neither created or destroyed during regular chemical or physical changes Law of Definite Proportions any amount of a compound contains the same element in the same proportions by mass No matter where the copper carbonate is used, it still has the same composition Law of Multiple Proportions applies when 2 or more elements combine to make more than one type of compound the mass ratios of the second element simplify to small whole numbers 15.9994 15.9994 15.9994 15.9994 31.9988 Law of Multiple Proportions Dalton’s Atomic Theory 1. 2. 3. 4. 5. All matter is made of atoms Atoms of same element have the same size, mass, and properties Atoms can’t be subdivided, created or destroyed Atoms of different elements combine in whole number ratios to make compounds In chemical reactions, atoms can be combined, separated, and rearranged. Modern Atomic Theory Some parts of Dalton’s theory were wrong: atoms are divisible into smaller particles (subatomic particles) atoms of the same element can have different masses (isotopes- atoms of the same type with different masses) Most important parts of atomic theory: all matter is made of atoms atoms of different elements have different properties Ch. 3: Atoms 3.2 Structure of Atom STRUCTURE OF THE ATOM ALTHOUGH DALTON THOUGHT ATOMS WERE INDIVISIBLE, INVESTIGATORS IN THE LATE 1800’S PROVED OTHERWISE IT SOON BECAME CLEAR THAT ATOMS ARE ACTUALLY COMPOSED OF SEVERAL BASIC TYPES OF SMALLER PARTICLES AND IT’S THE NUMBERS AND ARRANGEMENTS OF THESE SUBATOMIC PARTICLES THAT DETERMINES THE IDENTITY OF THE ATOM. STRUCTURE OF THE ATOM THE FIRST DISCOVERY OF A SUBATOMIC PARTICLE RESULTED FROM THE INVESTIGATIONS INTO THE RELATIONSHIP BETWEEN ELECTRICITY AND MATTER. IN THE LATE 1800’S, MANY EXPERIMENTS WERE PERFORMED IN WHICH ELECTRIC CURRENT WAS PASSED THROUGH VARIOUS GASES AT LOW PRESS CARRIED OUT IN TUBES CALLED CATHODE-RAY TUBES STRUCTURE OF THE ATOM INVESTIGATORS NOTICED THAT WHEN CURRENT WAS PASSED THROUGH A CATHODE RAY TUBE, THE SURFACE OF THE TUBE DIRECTLY OPPOSITE THE CATHODE GLOWED. STRUCTURE OF THE ATOM THEY HYPOTHESIZED THAT THE GLOW WAS CAUSED BY A STREAM OF PARTICLES THEY CALLED THE STREAM A CATHODE RAY STRUCTURE OF THE ATOM THE CATHODE RAY TRAVELED FROM THE CATHODE TO THE ANODE WHEN CURRENT WAS PASSED THROUGH THE TUBE. THE CATHODE RAY TUBE OPENED THE DOOR FOR J.J. THOMPSON electron e -1 0 9.11x10-28 proton + p +1 1 1.67x10-24 neutron n0 0 1 1.67x10-24 THE ATOMIC NUCLEUS WHEN SUBATOMIC PARTICLES WERE DISCOVERED, SCIENTISTS WONDERED HOW THESE PARTICLES WERE PUT TOGETHER IN AN ATOM. THIS WAS A DIFFICULT QUESTION TO ANSWER, GIVEN HOW TINY ATOMS ARE. MOST SCIENTISTS THOUGHT IT LIKELY THAT THE ELECTRONS WERE EVENLY DISTRIBUTED THROUGHOUT AN ATOM FILLED UNIFORMLY WITH POSITIVELY CHARGED MATERIAL. • BASED ON THE RESULTS, RUTHERFORD SUGGESTED A NEW THEORY OF THE ATOM. • HE PROPOSED THAT THE ATOM IS MOSTLY EMPTY SPACE –THIS EXPLAINED THE LACK OF DEFLECTION OF SOME OF THE PARTICLES • HE CONCLUDED THAT ALL THE POSITIVE CHARGE AND ALMOST ALL THE MASS IS CONCENTRATED IN A SMALL CORE –HE CALLED THIS REGION THE NUCLEUS • IT’S THE NUMBERS OF PROTONS IN AN ATOM THAT MAKES ATOMS DIFFERENT. – AN OXYGEN ATOM HAS 8 PROTONS IN ITS NUCLEUS – A HYDROGEN ATOM HAS 1 PROTON IN ITS NUCLEUS • THE ATOMIC NUMBER OF AN ELEMENT IS THE NUMBER OF PROTONS IN THE NUCLEUS OF AN ATOM OF THAT ELEMENT – SINCE OXYGEN’S NUCLEUS HAS PROTONS ITS ATOMIC # IS 8 • THE PROTONS IDENTIFY THE TYPE OF ATOM, THE ONLY ATOM WITH 8 PROTONS IS OXYGEN 8 ATOMS OF THE 1ST TEN ATOMS Lithium H He Li 1 2 3 1 2 3 0 2 4 1 4 7 1 2 3 Beryllium Be 4 4 5 9 4 Boron B C N O F Ne 5 6 7 8 9 10 5 6 7 8 9 10 6 6 7 8 10 10 11 12 14 16 19 20 5 6 7 8 9 10 Hydrogen Helium Carbon Nitrogen Oxygen Fluorine Neon • FOR EACH ELEMENT LISTED, THE NUMBER OF PROTONS EQUALS THE NUMBER OF ELECTRONS – REMEMBER ATOMS ARE ELECTRICALLY NEUTRAL • IN AN ATOM, THE NUMBER OF ELECTRONS MUST EQUAL THE NUMBER OF PROTONS • A HYDROGEN ATOM HAS 1 ELECTRN • AN OXYGEN ATOM HAS 8 ELECTRNS • THE MASS OF AN ATOM IS CONCEN. IN ITS NUCLEUS AND DEPENDS ON THE NUMER OF PROTONS AND NEUTRONS • THE TOTAL NUMBER OF PROTONS AND NEUTRONS IN AN ATOM IS CALLED THE MASS NUMBER – A CARBON ATOM, WHICH HAS 6 PROTONS AND 6 NEUTRONS, HAS A MASS NUMBER OF 12 • IF YOU KNOW THE ATOMIC NUMBER AND MASS NUMBER OF AN ATOM OF ANY ELEMENT, YOU CAN DETERMINE THE ATOM’S COMPOSITION • THE COMPOSITION OF ANY ATOM CAN BE REPRESENTED IN SHORTHAND NOTATION: CHEMICAL COMPOSITION SHORTHAND # OF PROTONS + # OF NEUTRONS MASS NUMBER 35 ATOMIC NUMBER 17 Cl NUMBER OF PROTONS ISOTOPES • EVERY CHLORINE ATOM HAS 17 PROTONS, WITHOUT EXCEPTION, – HOWEVER, NOT EVERY CHLORINE ATOM HAS 18 NEUTRONS. – ATOMS WITH THE SAME NUMBER OF PROTONS BUT CONTAIN DIFFERENT NUMBERS OF NEUTRONS ARE CALLED ISOTOPES. • BECAUSE ISOTOPES OF AN ELEMENT HAVE DIFFERENT NUMBERS OF NEUTRONS THEY HAVE DIFFERENT MASS NUMBERS. • ISOTOPES ARE CHEMICALLY ALIKE BECAUSE THEY HAVE IDENTICAL NUMBERS OF PROTONS AND ELECTRONS – IT’S THE ELECTRONS AND PROTONS THAT ARE RESPONSIBLE FOR CHEMICAL BEHAVIOR BERYLLIUM ISOTOPES proton neutron electron EXAMPLE OF AN ISOTOPE ATOMIC MASS Cl Cl 35 37 17 17 18 NEUTRONS 20 NEUTRONS ATOMIC NUMBER AN IONS ELEMENT’S ATOMS ARE NOT ALWAYS NEUTRAL IN CHARGE. WHEN AN ATOM LOSES OR GAINS ONE OR MORE OF ITS ELECTRONS IT BECOMES ION. AN ION THAT HAS MORE ELECTRONS THAN PROTONS HAS A NEGATIVE ELECTRICAL CHARGE AN ION THAT HAS FEWER ELECTRONS THAN PROTONS HAS A POSITIVE ELECTRICAL CHARGE NOTE: IT’S THE PROTONS THAT DEFINE THE TYPE OF ATOM IT IS, BUT THE ELECTRONS DEFINE THE ATOM’S CHARGE. SOME ATOMS GAIN ELECTRONS ATOM’S IONIC CHARGE = # PROTONS - # ELECTRONS - - - - - - - O - - - - - - - O-2 - ATOMS, IONS, AND ISOTOPES ATOMS NEUTRAL AND ARE DEFINED BY THE # OF PROTONS IN THEIR NUCLEUS 3 p+ = Li ATOM, ETC. IONS HAVE AN ELECTRICAL CHARGE DETERMINED BY PROTONS - # ELECTRONS N-2 = 7 p+ - 9 e- ; ETC. ISOTOPES TWO ATOMS WITH THE SAME # OF PROTONS, BUT DIFFERENT #’S OF NEUTRONS OR MASSES CALCIUM-40 & CALCIUM-44 #