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Greek philosophers (300 BC) proposed matter was made of 4 elements: earth, air, fire, water. Democritus coined the word “atom” meaning “cannot be broken.” Atom seen as a solid sphere John Dalton (1766-1844) 1. All matter is made of atoms 2. Atoms are indestructible and can’t be divided 3. All atoms of one element are exactly alike, but different from atoms of other elements. 4. Atoms of diff elements combine in simple, whole-number ratios to make compounds 5. In reactions, atoms are combined, separated, or rearranged. 1800s- scientists still thought atom was a tiny solid ball. THEN…JJ Thomson (1897) discovers the electron (e-) Vacuum tube (no air inside) w/ electrode on each end, attached to a terminal. He send electricity through the tube and saw A bright ray travelling from the negative end (cathode) to the positive end (anode). “cathode rays” Cathode ray bends toward a + end of a magnet. “ “ bends away from a - end of a magnet. Television Computer Monitor Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. Conclusion: 1. The cathode ray was actually ____ charged particles. 2. The atom could not be ________ as scientists had thought, but must contain charged particles. 1909-scientists now believe the atom is like chocolate chip cookie dough In 1910 Thomson discovered that neon atoms have different masses. Conclusion: there must be another particle that has no charge, called a neutron. (In 1932, James Chadwick confirms existence of the neutron) • • • 1911-Rutherford’s Gold Foil Experiment Shot “alpha Particles” (helium nuclei) at gold foil. Hypothesis: they would pass through unaffected. Expected results Actual results Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Conclusions: a) The nucleus is small b) The nucleus is dense c) The nucleus is positively charged d) Most of the atom is empty space In 1920, Bohr proposes that electrons around the nucleus exist at only distinct distances from the nucleus This is known as the Bohr model Particle Charge Mass (g) Location Electron (e-) -1 1/2000 AMU Electron cloud Proton (p+) +1 1 AMU Nucleus Neutron (no) 0 1 AMU Nucleus DON’T COPY The + charge on a proton is equal to the charge on an electron. Atoms are neutral (have no overall charge) Therefore, the # of protons = # electrons in an atom. determines the identity of the atom. tells us the # of protons in the atom. also tells us the # of electrons (b/c an atom is neutral in charge.) Ex: atomic number of carbon, C Question: how many protons? =6 How many electrons? How many neutrons?... Slide 2.2 Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. Element # of protons Atomic # (Z) Carbon 6 6 Phosphorus 15 15 Gold 79 79 Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0 p+ n0 e- Mass # 8 10 8 18 Arsenic - 75 33 42 33 75 Phosphorus - 31 15 16 15 31 Nuclide Oxygen - 18 (Def) Atoms of the same element w/different #s of neutrons. The number of neutrons can vary from atom to atom in an element. In order to know how many neutrons in an atom you must be told. The mass number tells you how much mass the atom has. Since p+ and n0 are the heavy parts, mass # =p+ + n0. Mass # 11 5 Atomic # B Symbol for element The atom in the prior slide can be called “boron-11” Name of element Mass # How many protons? How many electrons? How many neutrons? LET’S PRACTICE! Whiteboard Marker Paper towel Practice Problem #1 If an element has an atomic number of 34 and a mass number of 78, what is the: a) number of protons b) number of neutrons c) number of electrons d) complete symbol Practice Problem #2 If an element has 78 electrons and 117 neutrons what is the a) Atomic number b) Mass number c) number of protons d) complete symbol Atoms are weighed in a.m.u. 1 a.m.u. is based on the mass of a Carbon-12 atom. it has 6 p+ and 6 n0, 1 a.m.u = 1/12 the mass of a carbon-12 atom. (definition) Weighted average of all the isotopes of an element. See p 68 of text. calculating atomic mass Located below element symbol on periodic table. Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons. Isotope Protons Electrons Neutrons Hydrogen–1 (protium) 1 1 0 Hydrogen-2 (deuterium) 1 1 1 1 1 2 Hydrogen-3 (tritium) Nucleus Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons. Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results. If not told otherwise, the mass of the isotope is expressed in atomic mass units (amu) Atomic mass is the average of all the naturally occurring isotopes of that element. Isotope Symbol Carbon-12 12C Carbon-13 13C Carbon-14 14C Composition of the nucleus 6 protons 6 neutrons 6 protons 7 neutrons 6 protons 8 neutrons Carbon = 12.011 % in nature 98.89% 1.11% <0.01% Copper has the following isotopes ISOTOPE MASS # % ABUNDANCE Copper-63 63 69.15 Copper-65 65 30.85 Calculate the atomic mass (average mass) Finding Atomic Mass on Periodic Table Ch 5 – Chem IH Ch2.2 & 7 – Chem I • • • • Energy can travel in waves. There are high energy and low energy waves. The ones we can see are called “the visible spectrum.” ROY G BIV Red is the low energy end: violet is the high energy end. 1. Wavelength: distance between crests of a wave. Ex: radio waves = 102 m 2. Frequency: number of wave cycles to pass a point per unit time. Energy of Electrons Why electrons don’t crash into the nucleus: they have enough energy to keep them away. Why e-s (usually) don’t fly off of atoms: they have enough attraction to the nucleus to keep them in “orbit.” (Kind of like planets in orbit around the sun.) (Don’t write this!) DISCUSS WITH YOUR NEIGHBOR: You are an electron. If you have a lot of energy, will you stay close to the nucleus or will you move further from it? Answer: you may still stay in “orbit” but you will be able to move further away from the nucleus. Neils Bohr studied w/Rutherford His model is also called the planetary model He discovered that e-s could only exist at certain distances from the nucleus. (Energy Levels) "The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth." Neils Bohr See p 75 of text: electron energy levels are like rungs of a ladder. Ladder To climb to a higher level, you can’t put your foot at any level, you must place it on a rung Electron energy levels e-s must move to higher or lower e.l.’s in specific intervals Interactive Bohr Model Atoms are arranged in energy levels (e.l.’s), at different distances from nucleus Close to nucleus = low energy Far from nucleus = high energy e-s in highest occupied level are “valence e-s” Only so many e-’s can fit in energy levels e-s fill lower e.l.’s before being located in higher e.l.’s* (* There are exceptions we will learn later!) Only so many e-’s can fit in energy levels Energy Level 1st 2nd 3rd 4th # of electrons 2 8 18* 32* VALENCE ELECTRONS DETERMINE HOW ELEMENTS BEHAVE!!! Let’s practice drawing some atoms/ions In your teams, pick up enough of the following for your team: 1 white board per person 1 marker per person 1 paper towel per team (Please save a tree & share!) Electrons aren’t in perfect orbits. Energy levels are regions of space in which an e- is likely to be found most of the time. The area in which they move is like a cloud, an area of space surrounding the nucleus. Show # of protons and neutrons in the nucleus Draw e.l.’s and show each electron in the proper e.l. Ex: Bohr Model of BORON-11 Hydrogen-2 (Practice together) Helium-4 Lithium-6 Beryllium-8 Carbon-12 Magnesium-24 Have 2 parts 1. Chemical symbol of element 2. Valence e-s, represented by dots Are placed in one of four locations Above Below Right left Are not paired unless there is 1 e- in each location. Ex: Oxygen TEACHER DEMONSTRATION Hydrogen Helium Lithium STUDENT PRACTICE Beryllium Boron Carbon Bohr Models Lewis dot diagrams