Chapter 2: Atoms and Elements
Chapter 2: Atoms and Elements Chem 105
Popular in Principles of Chemistry I
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This 5 page Class Notes was uploaded by Hayley Masterson on Monday September 21, 2015. The Class Notes belongs to Chem 105 at Washington State University taught by Z Heiden in Summer 2015. Since its upload, it has received 36 views. For similar materials see Principles of Chemistry I in Chemistry at Washington State University.
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Date Created: 09/21/15
Hayley Masterson Chem 105 Notes chp 2126 August 29 2015 1 Imaging and Moving individual Atoms a Gerd Binning and Heinrich Rohrer measured how an electrical current depended on separation between sharp metal tip and surface i Led to the development of the scanning tunneling microscopy that could image and move individual atoms and molecules ii Works by moving a sharp electrode over a surface and measuring the resulting tunneling current current that ows between tip and surface b An atom is the smallest identi able unit of an element i There are 91 different naturally occurring elements ii Scientist have made of 20 synthetic elements 2 Early Ideas about the building blocks of matter a Leucippus and his student Democritus rst posed that matter was made of small and indestructible parts They named them atomos b Plato and Aristotle believed made of bits of re wind water and air c Copernicus published a book that said sun was center of the universe not earth 3 Modern atomic theory and laws that led to it a The law of conservation of mass i In a chemical reaction matter is neither created nor destroyed ii Made by Antione Lavoisier b The law of de nite proportions i All samples of a given compound regardless of their source or how they were prepared have the same proportions of their constituent elements ii The ratio holds for every compound and the ratio of masses are the same c The law of multiple proportions i When two elements call them A and B form two different compounds the masses of element B that combine with 1 g of element A can be expressed as a ratio of small whole numbers d John Dalton and the Atomic theory i Each element is composed of tiny indestructible particles called atoms ii All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements iii Atoms combine in simple wholenumber ratios to form compounds iv Atoms of one element cannot change into atoms of another element In a chemical reaction atoms only change the way that they are bound together with other atoms 4 The discovery of the Electron a b Cathode Rays i JJ Thomson performed experiments to probe the properties of cathode rays He constructed a partially evacuated glass tube called a cathode ray tube ii He found that cathode rays travel from the negative cathode to the positive anode They travel in straight lines carry a negative electrical charge and are independent of the composition of the material from which they originate iii Electrical charge fundamental property of particles that compose atoms results in attractive and repulsive forces called electrostatic forces The area around a charged particle where these forces exist is called an electric eld iv He had discovered the electron a negatively charged low mass particle present within all atoms Millikan39s oil drop experiment the charge of the electron i Sprayed oil into ne droplets using an atomizer They were allowed to fall under gravity During the fall the drops acquired electrons that had been produced by using ionizing radiation He was able to slow down or reverse the free fall of the drops He determined the charge of each drop then reasoned that each drop must be combined in a whole number 5 The structure of the atom a b Radioactivity the emission of small energetic particles from the core of certain unstable atoms Three different types of radioactivityalpha particles beta particles and gamma rays Ernest Rutherford made experiment that used alpha particles to disprove Thomson39s plum pudding model Rutherford directed positively charged particles at gold foil which were to act like probes to the gold atoms structure Some particles were de ected He proposed the nuclear theory of the atom i Most of the atoms mass and all its positive charge are contained in a small core called the nucleus ii Most of the volume of the atom is empty space throughout which tiny negatively charged electrons are dispersed iii There are as many negatively charged electrons outside the nucleus as there are positively charged particles named protons within the nucleus so the atom is electrically neutral Chadwick determined that the unaccounted mass was due to neutrons neutral particles within the nucleus The mass of a neutron is similar to that of a proton 6 Subatomic Particles Protons Neutrons and Electrons in Atoms a The most common unknit to express masses of subatomic particles is the atomic mass unit amu de ned as 112 the mass of a carbon atom containing 6 protonsneutrons The mass of one proton or neutron is 1 amu The charge of the proton and the electron are equal in magnitude but opposite in sign The number of protons de nes the element The number of protons in an atoms nucleus is its atomic number and is given the symbol Z Each element is represented with a unique chemical symbol one or two letter abb All atoms of an element have the same number of protons but not neutrons i Atoms with different 5 of neutrons but same of protons are called isotopes ii Natural abundance of isotopes are the percentage of each isotope in a naturally occurring sample of a given element iii The sum of a number of neutrons and protons in an atom is its mass number iv A number of protons p number of neutrons n lons losing and gaining electrons i lons charged particles after atoms lose or gain electrons during chemical change i Positively charged ions are called cations and negatively charged ions are called anions 7 Class Notes a Hayley Masterson August 29 2015 Chapter 2 sections 2729 1 Finding Patterns The Periodic law and the Periodic table a Mendeleev summarized observations in periodic table by making periodic law i When the elements are arranged in order of increasing mass certain sets of properties recur periodically b Metals lie on the lower left side and middle of the periodic table i They are good conductors of heat and electricity ii Can be pounded into at sheets drawn into wires shiny tend to lose electrons c Nonmetals lie on the upper right side of the periodic table i Have varied properties some are solid others liquid or gases ii They tend to be poor conductors of heat and electricity and gain electrons d Elements that lie along the zigzag line are metalloids and exhibit mixed properties i Several are called semiconductors because of their electrical conduc v y e The periodic table is divided into maingroup elements whose properties tend to be predictable transition elementsmetals whose properties are less predictable Columns labled A are main group elements and labeled B are transition elements i Each column is a family or group of elements ii Group 8A are the noble gases and are unreactive iii Group 1A elements are called alkali metals are all reactive metals iv Group 2A elements are called alkaline earth metals are fairly reactive v Group 7A elements halogens are very reactive nonmetals f lons and the Periodic Table i A main group metal tends to lose electrons forming a cation with the same number of electrons as the nearest noble gas ii A main group nonmetal tends to gain electrons forming an anion with the same number of electrons as the nearest noble gas 2 Atomic Mass the average mass of an element s atoms a Atomic mass the average mass for each element b Atomic mass the sum of fraction of isotope n mass of isotope n i For however many isotopes there are ii the fraction is calculated as its abundance percent over 100 c Mass Spectrometry measuring the mass of atoms and molecules i Mass spectrometry a technique that separates particles according to their mass ii The atoms are injected and vaporized and then ionized by an electron beam These are then accelerated into a magnetic eld and experience a force that bends their trajectory The ions strike a detector and produce an electrical signal that is recorded The position of each peak on the x axis indicates the mass of the isotope that was ionized and the intensity indicates the relative abundance of that isotope 3 Molar Mass Counting atoms by weighing them a b The Mole the amount of material containing 60221410quot23 particles Avogadro39s number The value of the mole is equal to the number of atoms in exactly 12 grams of pure carbon12 12 g C 1 mol C atoms 602210quot23 C atoms The mass of 1 mol of atoms in an element is the molar mass i An elements molar mass in grams per mole is numerically equal to the elements atomic mass in atomic mass units ii The molar mass of any element is the conversion factor between the mass in grams of that element and the amount in moles of that element