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Chemistry-140 Lecture 4 Chapter 2: Atoms & Elements Chapter Highlights atomic theory structure of the atom; electrons, protons, neutrons atomic mass, mass number and isotopes atomic number periodic table allotropes Chemistry-140 Lecture 4 Philosophy Chemistry is a pyramidal science, that is, all higher learning draws on the material already absorbed. A large base of knowledge provides a stable platform for understanding higher concepts. Chemistry-140 Lecture 4 Dalton’s Atomic Theory of Matter Each element is composed of small particles called atoms. All atoms of a given element are identical Atoms are neither created nor destroyed in chemical reactions. Compounds are formed when atoms of more than one element combine Chemistry-140 Lecture 4 Law’s From Dalton’s Theories Law of constant composition: all samples of a given substance must have the same composition (#4). Law of conservation of matter: matter cannot be created nor destroyed, only rearranged (#3). Law of multiple proportions: if two samples have different compositions, they must be samples of two different substances. Chemistry-140 Lecture 4 Consequences of Dalton’s Theories Combinations of elements are in ratios of small whole numbers. In forming water, 8.0 g of oxygen combines with 1.0 g of hydrogen. In forming hydrogen peroxide 16.0 g of oxygen combines with 1.0 g of hydrogen. Using atomic theory, we conclude that hydrogen peroxide contains twice as many atoms of oxygen per hydrogen. H2O2 versus H2O Chemistry-140 Lecture 4 The Discovery of Atomic Structure Cathode rays were discovered in the 1800s and were found to be beams of electrons streaming from the negatively charged cathode toward the positively charged anode. curvature is related to the mass and velocity of the particles Chemistry-140 Lecture 4 The Discovery of Atomic Structure J. J. Thompson: A balancing of the magnetic and electrical fields allows measurement of the charge-to-mass ratio of the electron Chemistry-140 Lecture 4 The Discovery of Atomic Structure Robert Millikan: a balancing of gravity with attraction to the positively charged plate allowed measurement of the charge on the electron (and the mass of the electron) Chemistry-140 Lecture 4 Radioactivty and Atomic Theory Radioactivity is the spontaneous emission of radiation from a substance. eg. X-rays, g-rays, a-particles, and b-particles X-rays and g-rays are high-energy light a-radiation is a stream of helium nuclei, He2+ known as aparticles. b-radiation is a stream of high speed electrons that originate in the nucleus (b-particles) Chemistry-140 Lecture 4 The Nuclear Atom Most of the mass and all of the positive charge was concentrated in the nucleus while the negative charge and most of the volume was accounted for by the electrons outside the nucleus. Chemistry-140 Lecture 4 The Nuclear Atom; Continued Subsequent experiments led to the discovery of both positive (protons) and neutral (neutrons) particles in the nucleus. Protons were discovered in 1919 by Rutherford. Neutrons were discovered by the British scientist James Chadwick in 1932 Chemistry-140 Lecture 4 Modern View of Atomic Structure In a chemical context, there are three important subatomic particles The proton is found in the nucleus charge = + 1.602 x 10-19 C (+1 electronic charge) mass = 1.67262 x 10-24 g (1.0073 amu) The neutron is found in the nucleus charge = 0 mass = 1.67495 x 10-24 g (1.0087 amu) The electron is found outside the nucleus charge = -1.602 x 10-19 C (-1 electronic charge) mass = 9.10939 x 10-28 g (5.486x10-4 amu) Chemistry-140 Lecture 4 Putting a Scale on Atoms The heaviest atom has a mass of only 4 x 10-22 g and a diameter of only 5 x 10-10 m. Useful units: 1 amu (atomic mass unit) = 1.66054 x 10-27 kg 1 pm (picometer) = 1 x 10-12 m 1 Å (Angstrom) = 1 x 10-10 m (not SI unit but very common) Chemistry-140 Lecture 4 A Simple Calculation With Atoms Question: The diameter of a penny is 19 mm. The diameter of a copper atom is 2.6 Å. How many copper atoms could be arranged side by side in a straight line across the diameter of a penny? Chemistry-140 Lecture 4 A Simple Calculation With Atoms Answer: The unknown is the number of Cu atoms. We can take the relationship that 1 Cu atom = 2.6 Å as a conversion factor relating number of atoms and distance. 103 m 1 A 1 Cu atom 10 1 mm 10 m 2.6 A Thus: # Cu atoms = (19mm)(3.8 x 106 Cu atom/mm) = 7.3 x 107 Cu atoms Chemistry-140 Lecture 4 Isotopes, Atomic Numbers and Mass Numbers Atomic number is the number of protons in the nucleus. All atoms of the same element have the same atomic number. Mass number is the sum of the number of protons & neutrons The number of neutrons in the nucleus is given by the mass number minus the atomic number. Isotopes are atoms of the same element that have different numbers of neutrons. Therefore isotopes have the same atomic number but different mass numbers. An atom of a specific isotope is called a nuclide. Chemistry-140 Lecture 4 Isotopes, Atomic Numbers and Mass Numbers In reference to a specific isotope, we use the symbolism; where mass number = (protons + neutrons) atomic number = (protons) usually do not include atomic number in symbol as the symbol implies a particular atomic number example: 14C is "carbon fourteen" also some isotopes have special names: 1H = hydrogen, 2H = deuterium, 3H = tritium Chemistry-140 Lecture 4 Measuring Atomic Masses A mass spectrometer is used to ionize samples and separate the ions produced according to their mass Chemistry-140 Lecture 4 Measuring Atomic Masses A sample of the element antimony (Sb) is separated into ions according to isotopic mass and natural abundance Chemistry-140 Lecture 4 A Question of Isotopes Example 2.3: Silver has two isotopes, one with 60 neutrons and the other with 62 neutrons. What are the mass numbers and symbols of these isotopes? Chemistry-140 Lecture 4 A Question of Isotopes Answer: Silver has an atomic number of 47, so it has 47 protons in the nucleus. The two isotopes therefore have mass numbers of Isotope 1: A = 47 protons + 60 neutrons = 107 Isotope 2: A = 47 protons + 62 neutrons = 109 The first isotope has a symbol the second is Chemistry-140 Lecture 4 Atomic Mass Atomic mass of an element is the average of all the isotopes weighted by their natural abundance Example 2.4: Bromine has two naturally occurring isotopes, one with a mass of 78.918336 amu and a percent abundance of 50.69%, the other with mass 80.916289 and a percent abundance of 49.31%. Calculate the atomic mass of bromine. Chemistry-140 Lecture 4 Atomic Mass Answer: The atomic mass of any element is the average of the masses of all the isotopes in a representative sample. To calculate the atomic mass, you multiply the mass of each isotopes by its percent abundance divided by 100. % abundance of isotope 1 Atomic mass = mass of isotope 1 100 % abundance of isotope 2 mass of isotope 2 ... 100 Average atomic mass of bromine = atomic mass of bromine Chemistry-140 Lecture 4 Textbook Questions From Chapter #2 Atoms: 24, 25 Isotopes: 30 Atomic Mass: 32, 34 Good Problems: 56, 62 Fun(?): 46 Chemistry-140 Lecture 5 The Periodic Table Elements are listed in ascending order of atomic number Periods are the horizontal rows in the periodic table. Groups (Families) are the vertical columns in the periodic table. Periodic Table of the Elements Chemistry-140 Lecture 5 The Periodic Table Metals: elements to the left of the periodic table (excluding hydrogen). Generally lustrous, ductile, malleable and conduct heat and electricity. Nonmetals: elements to the right of the periodic table. Lack the properties of metals, are brittle as solids, do not conduct well, and are often dull in appearance. Many are gases. Semimetals (metalloids): elements at the boundary between metals and nonmetals. Properties are intermediate between those of the metals and nonmetals. Boundary is often shown as a "stepped" line on the periodic table, from B to At Chemistry-140 Lecture 5 The Periodic Table Some Groups have special names: Examples Group 1 alkali metals Group 2 alkaline earth metals Group 17 halogens Group 18 noble gases 1 Group 1: Alkali Metals Group 2: Alkaline Earth Metals 2 Group 17: Halogens 17 Group 18: Noble Gases 18 Chemistry-140 Lecture 5 Allotropes Some elements exists in more than one form: Example: C: O: P: graphite, diamond, fullerene(s) dioxygen, ozone red, white, black Different forms of the same element are known as allotropes Chemistry-140 Lecture 5 Chemistry-140 Lecture 5 Chemistry-140 Lecture 5 C60 Fullerene