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Studying Atoms Lecture 4.1 Objectives: • Describe ancient Greek models of matter • List the main points of Dalton’s atomic theory and describe his evidence for the existence of atoms • Explain how Thomson and Rutherford used data from experiments to produce their atomic models Daily Question • Do you know what makes up the parts of an atom? Overview • • Most natural materials are mixtures of pure substances “Pure” substances are either elements or compounds Ancient Greek Models • Compounds always contain the same elements in the same proportions - Law of Constant Composition • Democritus (460-370 B.C.): was the first to use the word “atomos”( which meant atoms); stated that matter is composed of atoms; atoms are indestructible and indivisible Ancient Greek Models • Aristotle (384-322 B. C.): said that empty space could not exist; matter is made of earth, fire, air, and water Dalton’s Theory • Dalton proposed that all matter is made up of individual particles called atoms, which cannot be divided. Dalton’s Atomic Theory (1808) 1) Elements are made of tiny particles called atoms 2) Atoms of any element are identical 3) Atoms of an element are different from atoms of any other element 4) Atoms of elements combine to form compounds –Always the same numbers and types of atoms Dalton’s Atomic Theory (1808) 5) Atoms are indivisible – Cannot be created or destroyed – Atoms rearrange during chemical reactions Importance of Dalton’s Theory • It explained many important observations –Conservation of mass –Constant composition Atoms and Conservation of Mass A + 1 mass unit A B 4 mass units B 3 mass units + A C 5 mass units A B 4 mass units A + A B C 2 mass units 7 mass units Thomson • Thomson took Dalton’s model, and expanded on it. • His experiments provided the first evidence that atoms are made of even smaller particles. Importance of Thomson’s Experiments • Confirmed existence of electrons • Implied existence of protons Schematic of a cathode ray tube. Cathode Ray Experiments • Proved the existence of electrons • Sir Joseph John Thomson – Mass of the electron (9.109 x 10-28 g) • Approximately 1/1800 the mass of the simplest hydrogen atom – The electron is the smallest possible negative charge • All other negative charges are wholenumber multiples of the electron Plum Pudding model of Thompson pg 114 Models of the Atom THOMSON “Plum Pudding” DALTON Indivisible - - The Nucleus • A dense, positively charged mass located at the center of the atom • Central portion of the atom – Contains most of the atom’s mass, but . . . – Occupies only a tiny fraction of its volume – The plural form of nucleus is nuclei 3min Earnest Rutherford (1899) • Discovered that uranium emits fast-moving particles with positive charges • He wanted to test Thompson’s Plum pudding model • He predicted that most of the particles would go straight through • According to Rutherford’s model, all of the atom’s positive charge is concentrated in its nucleus Rutherford’s Gold Foil experiment. Results of foil experiment… If the Plum Pudding model had been correct. Actual results Rutherford’s Model of an Atom Niels Bohr • (1885-1962) • Worked with Rutherford and agreed with Rutherford’s model • But his model focused more on the electrons which surrounded the nucleus Bohr’s Model • Electrons move with constant speed in fixed orbits around the nucleus – Like planets around a sun • Each electron in an atom has a specific amount of energy – Electron can gain or lose energy Bohr Model Picture Evidence of Energy Levels • Movement of electrons between energy levels explains the light you see from fireworks – Light is form of energy – No two elements have same set of energy levels , so different elements emit different colors of light Color Compound Red strontium salts, lithium salts lithium carbonate, Li2CO3 = red strontium carbonate, SrCO3 = bright red Orange calcium salts calcium chloride, CaCl2 calcium sulfate, CaSO4·xH2O, where x = 0,2,3,5 Firework Colorants Gold incandescence of iron (with carbon), charcoal, or lampblack Yellow sodium compounds sodium nitrate, NaNO3 cryolite, Na3AlF6 Electric White white-hot metal, such as magnesium or aluminum barium oxide, BaO Green barium compounds + chlorine producer + barium chloride, BaCl = bright green Blue copper compounds + chlorine producer copper acetoarsenite (Paris Green), Cu3As2O3Cu(C2H3O2)2 = blue copper (I) chloride, CuCl = turquoise blue Purple mixture of strontium (red) and copper (blue) compounds Silver burning aluminum, titanium, or magnesium powder or flakes Electron Cloud • Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus – Electrons move in a less predictable way Orbital • Is a region of space around the nucleus where an electron is likely to be found – The electron cloud represents all of the orbitals in an atom Figure 15 pg 117 Energy Level # of orbitals 1 1 Max. # of electrons 2 2 4 8 3 9 18 4 16 32 Electron Configuration • arrangement of electrons in the orbitals • The most stable electrons in orbitals with lowest possible energies • Electron moving up an higher energy orbital= configuration is in excited state 4.2 Structure of the Atom The Nucleus is: • Positively charged • Very Dense • the center portion of the atom – Contains most of the atom’s mass, but . . . – Occupies only a tiny fraction of its volume Composition of the Nucleus: • Proton – found in the nucleus of an atom – Positive charge – Mass of 1.673 x 10-24 grams or 1amu • Neutron – found in the nucleus of an atom. – No electrical charge (neutral) – Mass approximately equal to the proton Composition of the Nucleus: • Electron – moves around the nucleus – Negative charge Atom – Very small compared to proton and neutron 1/1800 – relatively far away from the nucleus • Most of the atom is empty space Electron Nucleus Protons, Neutrons, and Electrons Relative Mass Charge proton 1 +1 neutron 1 0 electron 1/1800 –1 Atomic number The atomic number (“Z”) = number of protons in an atom The atomic number identifies element – elements have different numbers of protons Mass number • Mass number (“A”) =protons + neutrons. • Mass number identifies the particular isotope • In an atom, electrons equal the number of protons makes atoms electrically neutral Nuclear Symbol Mass Number 65 29 Cu Atomic Number (Z) 29 Electrons Protons + Neutrons 36 Neutrons 65 29 Cu Protons Remember!! Atoms are neutral!! • https://phet.colorado.edu/sims/html/buildan-atom/latest/build-an-atom_en.html Isotopes Isotopes are atoms of the same element with different mass numbers – Same atomic number – Different mass numbers because they have different numbers of neutrons Two isotopes of sodium. How to calculate the number of neutrons in an atom? • Number of neutrons= mass number – atomic number Practice • Write the nuclear symbol for bromine-80 – How many protons, electrons, and neutrons are in this atom? • Write the hyphen notation for the element that has 6 protons and 7 neutrons – How many electrons does this atom have? – Write the nuclear symbol for this atom