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Atoms: The Building Blocks of Matter Do Now • What is a theory? • What is a model? • How do you make inferences about things you can not see? Atom • From Greek, meaning “indivisible” • Atomic theory (from 400 BCE) = atoms are the building blocks of matter • There wasn’t any evidence for nearly 2000 years. Atomic Size • At sea level, one cubic centimeter of air (size of a sugar cube, or marble) will have 45 billion billion atoms within it. – 45,000,000,000,000,000,000 • If you tried to count to 45,000,000,000,000,000,00 0 it would take you 400,000 years • Fill Rye COMPLETELY with 45,000,000,000,000,000,00 0 marbles. Atomic Size • To see the atoms in a drop of water, you would need to enlarge the drop until… it is 24 kilometers wide! • Think of a line 1 millimeter long. If this line were blown up to the size of the empire state building, an atom would be… a tenth the thickness of a sheet of paper. Some History • Democritus – 460-371 B.C. – ancient Greek philosopher – believed all matter consisted of extremely small particles that could not be divided – atoms, from Greek word atomos, means “uncut” or “indivisible” • Aristotle – believed all matter came from only four elements—earth, air, fire and water Some Scientific Laws • The Law of Definite Proportions • The Law of Conservation of Mass Law of Definite Proportions • Two samples of a chemical compound contain the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound. – NaCl = 39.3% sodium 60.7% chlorine – H2O = 11.2% hydrogen 88.8% oxygen – C2H6O2 = 38.7% carbon, 9.7% hydrogen, 51.6% oxygen Law of Conservation of Mass • Mass is neither created nor destroyed during ordinary chemical reactions or physical changes • Thus, the mass of the reactants equals the mass of the products Dalton’s Atomic Theory • Dalton proposed an explanation for the law of conservation of mass, the law of definite proportion, and the law of multiple proportions. • He reasoned that elements are composed of one kind of atom and that only whole numbers of two or more kinds of atoms can combine to form compounds. • He proposed the solid sphere model Dalton’s Atomic Theory 1. All matter is composed of extremely small particles called atoms, which cannot be subdivided, created, or destroyed. 2. Atoms of a given element are identical in physical (size/mass) and chemical properties. 3. Atoms of different elements differ in physical (size/mass) and chemical properties. 4. Atoms of different elements combine in simple, whole-number ratios to form chemical compounds. 5. In chemical reactions, atoms are combined, separated, or rearranged, but never created, destroyed, or changed. Modern Atomic Theory • Atoms are divisible into even smaller particles. These smaller parts of the atom are called subatomic particles – Electrons – Protons – Neutrons Discovering Electrons • In 1897, J.J. Thomson used a cathode ray tube (passes electricity through a glass tube with little pressure) to deduce the presence of a negatively charged particle. J.J. Thomson’s cathode ray tube • He knew that rays must have come from the atoms of the cathode because most of the atoms in the air had been pumped out of the tube. Because the cathode ray came from the negatively charged cathode, Thompson reasoned that the ray was negatively charged. J.J. Thomson’s cathode ray tube • He observed that when a small paddle wheel was placed in the path of the rays, the wheel would turn. This observation suggested that the cathode ray consisted on tiny particles that were hitting the paddles of the wheel. • His experiments showed the cathode ray consists of particles that have mass and a negative charge. These were called electrons. • He proposed the plum pudding model. • Cathode Ray Tube http://www.bbc.co.uk/history/british/victorians/launch_ani_paddle_steamship.shtml Discovering the Nucleus • In the 1900s, Ernest Rutherford performed his gold foil experiments – He directed small, positively charged alpha particles (that are helium nuclei) at a thin gold foil. – Particle hits on the detecting screen (film) were recorded and deflected angles were measured. Rutherford’s gold foil experiments This diagram shows the expected result of Rutherford's experiment if the "plum pudding" model of the atom is correct. This diagram shows the actual result. Most of the alpha particles are only slightly deflected, as expected, but occasionally one is deflected back towards the source Only a very concentrated (dense) positive charge in a tiny space within the gold atom could possibly repel the fast-moving, positively charged alpha particles enough to reverse the direction of the alpha particles. Rutherford’s gold foil experiments • His experiments showed that the nucleus is very small and positively charged. • He also hypothesized that the mass of the nucleus must be larger than the mass of the alpha particles, otherwise the alpha particles would have knocked the nucleus out of the way. • He also argued that most of the alpha particles were not deflected, because most of the atom was empty space. • He proposed a planetary model or nuclear model Rutherford’s Gold Foil Experiment • http://www.mhhe.com/physsci/chemistry/ess entialchemistry/flash/ruther14.swf • Rutherford's Gold Foil Experiment RUTHERFORD ACTIVITY HALLWAY Pennies Rolled Marbles Rolled Marbles http://www.learner.org/resources/series61.html Pennies Equal Distance From each other The Nucleus • Using measurements from Rutherford’s experiment, scientists calculated the radius of the nucleus to be less than 1/10,000 of the atom. – If the nucleus were the size of a marble, the atom would be the size of a football stadium The Nucleus • Protons = positively charged particles – The charge of a proton was calculated to be equal but opposite to the charge of an electron – The mass of a proton is almost 2000x the mass of an electron • Neutrons = neutral particles – The mass of a neuron is almost equal to the mass of a proton • The sum total of masses of protons, neutrons, and electrons equals the mass of the atom. Mass of atoms are measured in Atomic Mass Units! 1 amu = 1/12 mass the Carbon-12 (amu) Atoms • All living things are made up of tiny units called ATOMS. ATOMS consist of electrons orbiting around a nucleus. ELECTRONS • (-) negative electrical charge found in the space around the nucleus NUCLEUS • PROTON (+) has a positive electrical charge. • NEUTRON has a neutral charge (no charge) Subatomic Particles ATOM NUCLEUS ELECTRONS PROTONS NEUTRONS POSITIVE CHARGE NEUTRAL CHARGE NEGATIVE CHARGE Atomic Identity • If the number of electrons equals the number of protons, the atom is electrically neutral. (No electrical charge) • Elements differ in their number of protons and therefore in the amount of positive charge their nuclei possess. • The number of protons determines an atom’s identity. Atomic Number Atomic # = p+ • Atomic Number (Z)= number of protons of each atom of that element – Atoms of different elements have different numbers of protons (different atomic numbers) – Atoms of the same element all have the same number of protons (same atomic numbers) • The atomic number identifies the element – 113 elements have been identified, with 113 different atomic numbers Atomic Number • Because atoms are neutral, they must have the equal numbers of protons and electrons. • Therefore, the atomic number tells us how many protons and also how many electrons an atom has. How many PROTONS and ELECTRONS are in: • • • • • • • Silver Hydrogen Neon Gold Boron Sodium Tungsten 47 1 10 79 5 11 74 Mass Number Mass # = p+ + n0 • Mass number = the total number of protons and neutrons (total number of particles in the nucleus) • Mass numbers can vary among atoms of a single element, because atoms of the same element can have different numbers of neutrons. • Different elements can have the same mass numbers, because the mass number does not help you identify the element, the atomic number does! Try this: Mass # = p+ + n0 Element p+ Oxygen 33 Phosphorus n0 e- Mass # Nuclear Symbols • • 235U 92 235 is the mass number of Uranium 92 is the atomic number of Uranium • A uranium nucleus has 92 protons. • It also has a total of 235 neutrons and protons in its nucleus (mass number). • How many neutrons in an atom of Uranium-235? • Mass # – Atomic # = # of Neutrons • 235 (protons + neutrons) – 92 protons = 143 neutrons FIND THE NUMBER OF NEUTRONS: • • • • • • • Sodium Calcium Nitrogen Iron Argon Lithium What does this tell you? 12 6 C Modern Atomic Theory • Atoms of a particular element do share the same atomic number (number of protons) and identical chemical properties but the atoms of a given element may differ in their mass numbers (number of protons and neutrons). • Elements occur in nature as mixtures of isotopes. Isotopes • Isotopes = atoms of the same element with different numbers of neutrons and mass numbers Nuclear symbol: Mass # Atomic # 12 6 Hyphen notation: carbon-12 C Isotopes • Isotopes = atoms of the same element with different numbers of neutrons and mass numbers Nuclear symbol: Mass # Atomic # 14 6 Hyphen notation: carbon-14 C Isotopes © Addison-Wesley Publishing Company, Inc. Try to determine information about these isotopes: • Chlorine-37 – atomic #: 17 – mass #: 37 – # of protons: 17 – # of electrons: 17 – # of neutrons: 20 37 17 Cl Isotopes of Hydrogen Isotope Hydrogen–1 (protium) Hydrogen-2 (deuterium) Hydrogen-3 (tritium) Protons Electrons Neutrons Nucleus Using Mass Numbers • How many protons, neutrons, and electrons make up an atom of Br-80? • • • • Protons + Neutrons = 80 Protons = 35 Electrons = 35 Neutrons = 80 – 35 = 45 • How many protons, neutrons, and electrons make up an atom of C-14? • • • • Protons + Neutrons = 14 Protons = 6 Electrons = 6 Neutrons = 14 – 6 = 8 Ions Are created when an atom loses or gains one or more electrons; it acquires a charge http://web.visionlearning.com/custom/chemistry/animations/CHE1.3-an-ions.shtml Charge of Ion = number of protons – number of electrons More electrons than protons = negative charge (anion) More protons than electrons = positive charge (cation) 12 6 C +1 # of protons # of electrons Total charge PRACTICE IONS Ion Li +1 Ni +2 Pb +2 Ca +2 Cs +1 # protons # neutrons # electrons Chemical Symbol Number of protons Number of electrons Number of neutrons 35 36 45 Atom or Ion? I 11 12 atom 55 78 atom 14 12 Zr 12 50 Br 53 atom 44 atom Ce ion 27 25 32 84 80 125 73 68 108 50 71 Sc Pb Ni atom Do Now • If your first quarter grade is based 10% on homework, 20% on labs, 20% quizzes, and 50% tests, what should your grade be if you averaged 100 on homework, 90 on labs, 80 on quizzes, and 70 on tests? – – – – 10% x 100 = 10 20% x 90 = 18 20% x 80 = 16 50% x 72 = 36 – Total = 80 % Atomic Mass • Atomic mass = the weighted average of the masses of the existing isotopes of an element. • Don’t get these confused! – Mass number = the total number of protons and neutrons that make up the nucleus of an atom. – Atomic mass includes the masses of the protons, neutrons and electrons of atoms and isotopes. Weighted Average • You have a box containing two sizes of marbles. • 25% of the marbles have masses of 2.00 g each • 75% of the marbles have masses of 3.00 g each • Calculate the weighted average…. Calculate the weighted average • Assume you have 100 marbles – 25%, or 25, have a mass of 2.00 g – 75%, or 75, have a mass of 3.00 g – – – – 25 marbles x 2.00 g = 50 g 75 marbles x 3.00 g = 225 g Total mass = 50 + 225 = 275 g 275g / 100 marbles = 2.75 g/ marble – A simpler method is as follows: 25% = 0.25 75% = 0.75 (2.00g x 0.25) + (3.00g x 0.75) = 2.75g Atomic Mass • 1 amu = 1/12 the mass of a 12C atom Carbon = 12.011 Isotope Symbol Carbon-12 12C Carbon-13 13C Carbon-14 14C Composition % Abundance in of the nucleus nature 6 protons 98.89% 6 neutrons 6 protons 1.11% 7 neutrons 6 protons <0.01% 8 neutrons Calculating Atomic Mass Avg. Atomic Mass (mass)(% ) (mass )(% ) 100 • STEP 1: Take the mass # (in amu) of each element and multiply by its percent abundance (%) • STEP 2: Add all of these values together • STEP 3: Divide by 100 Calculating Atomic Mass • Boron exists as 2 isotopes: B-10 or B-11 • B-10 10 • B-11 11 5B 5B % Abundance 19.78% 80.22% Calculating Atomic Mass Atomic Mass of Boron • STEP 1: 10 x 19.78 = 197.8 11 x 80.22 = 882.42 • STEP 2: 197.8 + 882.42 = 1080.22 • STEP 3: 1080.22 = 10.802 amu 100 Calculating Atomic Mass • Calculate the Atomic Mass of Chlorine: % Abundance • Chlorine – 35 75.53% • Chlorine – 37 24.47% Calculating Atomic Mass • Calculate the Atomic Mass of Silicon: % Abundance • Si – 28 92.21% • Si – 29 4.70% • Si – 30 3.09% Calculating Atomic Mass • Calculate the Atomic Mass of Oxygen: % Abundance • O-16 99.762% • O-17 0.038% • O-18 0.200% Calculating Atomic Mass • What is the average atomic mass of Cu which is found in nature as 69.15% Cu-63 (62.929601 amu) and 30.85% Cu-65 (64.927794 amu) ? (0.6915 x 62.929601 amu) + (0.3085 x 64.927794 amu) = 63.55 amu Relating masses in grams to numbers of atoms • Mole = the amount of a substance that contains as many particles as there are atoms in exactly 12 grams of carbon-12. – The mole is a counting unit, like a dozen. – The mole relates to masses of atoms and compounds. • Avogadro’s number = The number of particles in exactly one mole of a pure substance. – 6.022 x 1023 particles • Molar mass = the mass of one mole of a pure substance; numerically equal to the atomic mass of the element in atomic mass units (g/mol) • 1 mole of any element is its atomic mass in grams What is the molar mass of Li? 6.94 g/mol What is the molar mass of Hg? 200.59 g/mol The molar mass of an element contains one mole of atoms. 4.00g He, 6.94g Li, and 200.59g Hg all contain one mole of atoms. How many atoms is this? Avogadro’s number: 6.02 x 1023 particles (atoms) What is the mass in grams of 3.50 mol of Cu? 3.50 mol Cu x 63.55g Cu 1 mol Cu = 222g Cu What is the mass in grams of 3.42 mol Ag? What is the mass in grams of 0.876 mol Pb? A chemist produced 11.9 g of Aluminum. How many moles of Al were produced? 11.9 g Al x 1 mol Al 26.98 g Al = 0.441 mol Al How many moles of Na are in 4.01 g Na? How many moles of Zn are in 0.674 g Zn? How many moles of Ag are in 23 3.01 x 10 atoms of Ag? 3.01 x 1023 atoms Ag x 1 mol Ag 6.02 x 1023 atoms Ag = 0.500 mol Ag How many atoms are in 32 g of S? Gram Formula Mass (GFM) • Gram formula mass is the molar mass, or atomic mass of a compound • Units are grams/mole (g/mol) Example: H2O 1) What is the molar mass of H? O? • H = 1.007 • O = 15.994 2) Multiply by the subscripts • For Hydrogen (1.007) x (2) = 2.014, of 2 • For Oxygen (15.994) x (1) = 15.994, or 16 3) Add the masses together • 2+ 16 = 18, the molar mass of H2O • What is the gram formula mass (GFM) of salt (NaCl)? • What is the gram formula mass (GFM) of sugar (C6H12O6)? Discoveries about the atom Dalton 1. All matter is composed Of extremely small particles which cannot be subdivided, created or destroyed. 2. Atoms of a given element are identical in physical and chemical properties. 3. Atoms of different elements have different physical and chemical properties. 4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. 5. In chemical reactions, atoms are combined, separated, or rearranged, but never created, destroyed, or changed. JJ Thomson What did he discover: Electron His Experiment: Cathode Ray Tube His findings: Electrons are negatively charged embedded in a positive charge. Rutherford What did he discover: The Nucleus His experiment: GOLD FOIL EXPERIMENT (1900’s) His findings: The atom is mostly empty space. The nucleus is small. The nucleus is dense. The nucleus is positively charged Niels Bohr •Electrons revolve around the nucleus in specific orbits, or energy levels. • An atom has energy levels. Electrons can only exist in these energy levels, not in between. •When an atom is in the ground state, the electrons exist in the energy levels closest to the nucleus. •GROUND STATE: the lowest energy state of an atom; the electrons occupy energy levels closest to the nucleus. •If an atom receives, energy, the atom becomes excited and electrons jump to higher energy levels. •EXCITED STATE: an atom with higher potential energy than in the ground state because electrons have “jumped” to a higher energy level. Solid Sphere Model Electron Cloud Model/ Quantum Model This model suggested that electrons could be considered waves confined to the space around a nucleus. Electron cloudsregions where electrons are likely to be found Refining Nuclear Models • In 1913, Danish physicist, Niels Bohr, refined Rutherford's idea by adding that the electrons were in orbits around the nucleus. Rather like planets orbiting the sun. With each orbit only able to contain a set number of electrons. • He proposed a Bohr model or Orbit model Bohr’s ATOM Atom HELIUM _______ _______ _________ + _________ N N + - __________ The Bohr Model • 1. Electrons revolve around the nucleus in specific orbits (shells), or energy levels. • 2. An atom has energy levels. Electrons can only exist in these energy levels, not in between. • 3. When an atom is in the ground state, the electrons exist in the energy levels closest to the nucleus. • 4. If an atom receives, energy, the atom becomes excited and electrons jump to higher energy levels. http://www.visionlearning.com/library/flash_viewer.php?oid=1347&mid=51 Ground State • The lowest energy state of an atom • Electrons in the first energy level have the lowest potential energy since they are located closest to the nucleus. Excited State • An atom has a higher potential energy than in the ground state because electrons have “jumped” (moved up) to a higher energy level. • Electrons with higher potential energy occupy orbits farther from the nucleus. The further an electron is from the nucleus, the greater its energy! Modern Atomic Theory Bohr Model—shows electrons in orbit around protons and neutrons Quantum-mechanical model—doesn’t show exact location of electrons, just probable place Current Atomic Model • Electrons act like particles (because they have a mass) and waves (because they have certain frequencies corresponding to their energy levels) • Electrons are located in orbitals around the nucleus that correspond to specific energy levels • Electron clouds = orbitals that do not have sharp boundaries, but shows 3D region where electrons are most probable to be found. • Electron Cloud Model or Quantum Model proposed by Louis de Broglie & Erwin Schrodinger Electron Configuration • Arrangement of electrons • Each atom has a distinct electron configuration. • The ground state electron configuration is found on the periodic table in the lower left hand corner of each box. Classify the following as ground state electron configurations or excited state electron configurations. Element ground state electron configurations or excited state electron configuration lithium 1-2 calcium 2-8-7-3 excited chlorine 2-8-7 ground aluminum 2-7-4 excited neon 2-7-1 excited sodium 2-8-1 ground potassium 2-8-7-2 excited exited The configuration listed on the periodic table is the ground state electron configuration. In the chart below, draw the ground state and excited state electron configurations Element He O Na F Al Mg Br Ground State Excited State Element Ground State Electron Configuration Ion Ion’s Electron Configuration Na Na + Mg Mg +2 Fe Fe +3 Al Al +3 Li Li +1 Quantum • Electrons can only absorb or release energy in discrete, specific amounts. • The amounts, or bundles of energy are called quanta (or photons) corresponding to differences in energy levels of the orbitals/shells. • The greater the radius of an orbit (the farther from the nucleus), the greater the energy of the electrons in that orbit. The orbits or shells are known as principal energy levels. Electrons and Light • An atom emits energy when the electron falls from high energy levels to lower energy levels. This energy is in the form of electromagnetic radiation. • If the wavelength is in the visible light spectrum, the energy can be seen as color. Light Emission • Each move from a particular energy level to a lower energy level will release light of a specific wavelength. • When certain elements are excited, they give off energy of a distinctive color as the electron fall back down to lower energy levels. These colors are specific and can be used to identify the elements (Flame Test). Spectroscopic analysis of the visible spectrum… …produces all of the colors in a continuous spectrum …produces a “bright line” spectrum Spectral lines • If high voltage is applied to hydrogen gas confined in a gas tube, called a gas discharge tube, light is emitted. If this light is passed through a prism, a series of bright lines of distinct colors is produced. Bohr reasoned that these different colored bands of light were actually quanta of corresponding energy. These quanta were emitted as electrons of hydrogen atoms returned from their higher levels in the excited state to their lower levels in the ground state. Bright Line Spectra • Bright line spectrum = the series of bright lines produced when excited electrons return to their original energy levels • Each element has its own unique set of spectral lines which can therefore be used to identify the elements presence. Valence Electrons Electrons that occupy the valence energy level Valence Electrons= found in outer most energy level Na 2-8-1 Cl 2-8-7 Atoms can have a maximum of 8 valence electrons Lewis Dot Diagrams (Electron Dot Diagrams) • Represent the arrangement of electrons around the nucleus. • Electrons are the DOTS. • Nucleus is the symbol. • ONLY REPRESENT VALENCE ELECTRONS!! • Fill one side first, then one on each side before you pair electrons. Lewis Dot Diagrams (Electron Dot Diagrams) Na B O Mg Cl Ne Si H N