John Dalton• An English scientist and teacher• He knew of these observations and offered anexplanation for them• His explanation is known as:
DALTON’S ATOMIC THEORYThe main ideas of his theory include:1. Elements are made of tiny particles called atoms.2. All atoms of a given element are identical. Atoms of a givenelement are different from those of any other element.3. Atoms of one element can combine with atoms of otherelements to form compounds. A given compound alwayshas the same relative number and types of atoms4. Atoms are indivisible in chemical processes.** Atoms are not created or destroyed in chemical reactions.A chemical reaction simply changes the way atoms aregrouped together.
* Dalton’s Theory offered simple explainations for some basiclaws of chemistry such as:The Law of Conservation of Mass Mass is neither created or destroyed.If atoms are conserved in a reaction then mass must also beconservedThe Law of Constant Composition Tells us that a cmpd regardless of its origin or method ofpreparation always contains the same elements in the sameproportions by weightLaw of Multiple Proportions When 2 elements combine to form more than 1 cmpd themasses of one element which combines with a fixed mass ofthe other elelment are in a ratio of small whole numberssuch as 2:1
Law of Multiple ProportionsExample:Element A + B combine to form 2 cmpds.Cmpd 1 Cmpd 2A2B ABThe weight of A combined with a fixed amt ofB in cmpd 1 would be 2x that of the second.
Dalton’s Atomic Teory Like all new ideas Dalton’s theory was notaccepted immediately- this did not dissuade Dalton- Dalton predicted that N and O could combineto form several compounds:NO , N2O and NO2- when the existence of these compounds wasverified…..Dalton’s Theory became widely accepted
A Look at CompoundsA compound is a distinct substance composedof atoms of two or more elements that alwayscontains exactly the same relative masses ofthose elements• H2O always has two H atoms and one O atom
Chemical Formulas Chemical formulas- used to show the types andnumber of each type of atom• Atoms are indicated by the elements symbol• The number of each type of atom is represented by asubscript (appearing to the right and below theelement symbol)ie. SO3 contains 1 atom of sulfur and 3 atoms of oxygenWrite the formula for the compound containing 2atoms of nitrogen and 5 atoms of oxygen.Ans. N2O5
Chemical Formulas continued• Glucose has 6 carbon atoms, 12 hydrogenatoms and 6 atoms of oxygen.Write its formula.Ans. C6H12O6• The unique formula or arrangement of atomsin a molecule makes one compounddifferent from another
Compounds with the same chemicalformula:CH3– CH2 –OH CH3-O-CH3C2H6O C2H6Oethyl alcohol dimethyl ether• The difference lies in the order of the bonded atoms• Formulas that show the order and arrangement ofthe specific atoms are called structural formulas
• Dalton’s Theory provided a convincing explanation forthe composition of compounds• Scientists believed that elements consistedatoms but…….What did the atom look like?
The Structure of the Atom• Much pondering about the structure occurredduring the 1800’s• It was not until 1900 that convincing evidenceregarding the structure became available
First Convincing Evidence for SubatomicParticles• Came from experiments involving theconduction of electricity through gases at lowpressures using a device called a Cathode RayTube
• In 1897 J. J. Thomson (English physicist) took this apparatuspartially evacuated it and connected it to a high voltagesource (spark coil)• An electric current flows thru the tube• Associated with this flow are colored rays of light whichoriginate at the (-) end of the cathode* Thomson found that these rays were bent by both electricand magnetic fields* Careful study of the nature of this deflection demonstratedthat the the rays consisted of a stream of negatively chargedparticles which he called electrons.
2.2• In the absence of any field, the cathode ray (which is -) strikesat B.• Cathode ray strikes at A in the presence of a magnetic field• Electric plates create an electric field perpendicular to thedirection of the cathode rays (cathode ray is -) causing themto strike the screen at C when the field is on.
Cathode Ray Tube2.2Passing anelectric currentmakes a beamappear to movefrom thenegative to thepositive end.
•Went on to measure the mass to charge ratio of theelectron finding it to be:m/e = 5.69 x 10-9g/Ce/m = -1.76 x 108C/g•Since he found the ratio to be the same regardlessof what gas was in the tube, this implied that theelectron was a fundamental particle commonto all atoms•Provided the first evidence that atoms are made ofeven smaller particlesJ.J. Thomsonand the Cathode Ray Tube1897(1906 Nobel Prize in Physics)
J. J. Thomson• Thomson showed that atoms of any element can bemade to emit tiny negative particles• He called these negative particles electrons• Thomson new that although the atoms containednegative particles the atoms overall charge had to bezero• He postulated that the atom must also containpositive particles to balance exactly the negativecharge of the electrons
Thompson’s Model• Found the electron– 1 unit of negative charge– Mass 1/2000 of hydrogen atom– Later refined to 1/1840• Concluded that there must be apositive charge since atom wasneutral• Atom was like plum pudding– A bunch of positive stuff, withelectrons able to be removed.
Millikan1909• Millikan determined the charge on an electronwith his famous Oil Drop Experiment
• In this experiment small droplets of oil which had picked upextra electrons were allowed to fall between 2 electricallycharged plates• The drops were observed: When the voltage between the plates was increased thenegatively charged drop fell more slowly (it was attracted tothe + plate) At some point the drop will be balanced and stationary.
Millikan’s Experiment continued:• Knowing this voltage and the mass of the drop, itwas possible to calculate the charge on the drop• Millikan found the charge to always be an integralmultiple of a smallest charge• Assuming the smallest charge to be that on anelectron he arrived at a value of 1.60 x 10-19C• Combining this value with the charge/mass ratio hecalculated the mass of an electron
e-charge = -1.60 x 10-19CThomson’s charge/mass of e-= -1.76 x 108C/ge-mass = 9.10 x 10-28gMeasured mass of e-(1923 Nobel Prize in Physics)2.2
Henri Becquerel1896• Studying a uranium mineral called pitchblende hediscovered that it spontaneously emitted highenergy radiation.• Further studies by the Curies and ErnestRutherford revealed 3 types of Radiation1. Alpha (α) - positively charged(+2) and heavy2. Beta (β)- negatively charged (-1)3. Gamma (γ)- neutral
• With the growing evidence that the atom wascomposed of even smaller particles, attentionwas being given to how they fit together• Recall, early in the 1900’s JJ Thomsonproposed the “plum pudding” model of theatom
Ernest Rutherford• A student of J. J.Thomson• Believed in the “PlumPudding” Model• In 1911 he performedhis now famous“Gold Foil Experiment”
“Plum Pudding” Model of the Atom• Developed by J.J.Thomson• The atom was a uniformpudding of positivecharge with enoughnegative electronsscattered within tocounter balance thepositive chargePlum Pudding Model orRaisin Bun Model
Rutherford’s Experiment• Rutherford and his students bombarded apiece of gold foil using alpha particles• Here’s what it looked like…….
Rutherford’s “Gold Foil” Experiment• Rutherford directed α-particles 7500 times themass of an electron at athin sheet of gold foilonly a few atoms thick• The detector producedtiny flashes if it was hitby an α- particle
• What he expected to see
– Alpha particles should pass through withoutchange in direction– Positive charges were spread out evenly.Alone they were not enough to stop an alphaparticle
• What he got
• How he explained it– Atom is mostly empty– Small dense, positive piece at the center– Alpha particles are deflected if they get closeenough to positive center
A picture of the Atom Evolves• Based on his alpha-particle scatteringexperiment on gold,Rutherford concludedthat the atom consistedof a hard central corewhere most of the massof the atom rested.
Other Subatomic Particles• Protons were discovered by Rutherford in1919.• Neutrons were discovered by James Chadwickin 1932.
Subatomic Particles• Protons and electrons are the only particles thathave a charge.• Protons and neutrons have essentially the samemass.• The mass of an electron is so small we ignore it.
Nuclear Structure• Facts about the nucleus:Protons and neutrons haveroughly the same mass(1amu=1.67x10-4g), and each isabout 2000 times as massive asthe electron.The number of protons is thesame as the number of electrons(not shown) which orbit thenucleus at a distance of about 10-8cm.• **If the nucleus were a grape theelectrons would be 1 mileaway!!!No charge.Help holdprotons inthe nucleus.Withoutneutronsthe + chargeswould repelone another.+ charge
If all atoms contain the same components(protons, neutrons and electrons) why do differentatoms have different chemical properties?The answer lies in the number and arrangementof the electrons.***It is the electrons that are responsible for thechemical properties of an atom of an element.
Representing Elements• Each and every atom of an element is described on the basisof the number and types of nuclear particlesA QX – atomic symbolZA = Mass numberThe number of protons and the number of neutronsZ = Atomic numberThe number of protonsIn a neutral atom the number of protons = the numberof electronsQ = Charge if not neutral
Atomic Symbols23ie. Na11Sodium-23 contains 11 electrons, 11 protonsand 12 neutrons.24Sodium 24: Na contains 11 electrons, 11 protons11 and 13 neutrons. It is anexample of an isotope
Isotopes• Atoms of the same element that contain anidentical number of protons but differ in thenumber of neutrons12 13 14• C(carbon -12) C(carbon-13) C(carbon-14)6 6 66e, 6p, 6n 6e, 6p, 7n 6e, 6p, 8n- three different forms of carbon- differ only in the number of neutrons inthe nucleus- illustrates the concept of isotopes
More Isotopes1 2 3H H H1 1 1Hydrogen-1 Hydrogen-2 Hydrogen-3deuterium tritium• In nature elements are usually found as a mixture of isotopes• An isotopes mass is determined by comparison to a standard,carbon-12 (has a mass of 12 atomic mass units or amu)• An elements atomic mass is obtained by taking a weightedaverage of the atomic masses of all isotopes of that elementpresent in nature
Putting it all together• How many protons , neutrons and electronsare present in 96Mo ? 42Plan: The number of p, n and e are determined fromthe atomic # and mass #Atomic # = 42 and Mass # = 96Atomic # = # protons → 42p# protons = # electrons in a neutral atom → 42eMass # = # protons + # neutrons → 96 – 42 = 54n
In any room where chemistry is taught,or practicedyou are certain to finda chart called the……….
Periodic Table:• A systematic catalogof elements.• Elements arearranged in order ofatomic number.
The Periodic Table• Shows all the knownelements• Each box contains theelement’s atomic number(# of protons = # ofelectrons) written over theone or two letter symbol• Each box contains theelement’s atomic mass (#of protons + # of neutrons)written below the symbol
Symbols and much, much more• Not only does thePeriodic table providesymbols, atomicnumber and atomicmass of each element,……• The periodic table tellus a good deal abouteach element
As you peruse the table you cannot helpbut note that:• The elements arearranged in order ofincreasing atomicnumber in nicehorizontal and verticalcolumns
PeriodicityWhen one looks at the chemical properties ofelements, one notices a repeating pattern ofreactivities.
Dimitri Mendeleev• In 1869 he arranged theelements based onsimilarities in chemicalproperties into“families”• Mendeleev listed thefamilies vertically andcalled them groups- groups are referred to bythe number over thecolumn- many groups also havenames
• Mendeleev called thehorizontal rows periodsthey are designated withnumbers• Elements in a period eachhave commoncharacteristics• Each period ends with amember of the family ofelements called the NobleGasesNoble Gases - chemically un-reactive elements that existin nature as individualatoms
GroupsThese five groups are known by their names.
Periodic TableNonmetals are on theright side of theperiodic table (withthe exception of H).
Periodic TableMetalloids border thestair-step line (withthe exception of Aland Po).
Periodic TableMetals are on the leftside of the chart.
Chemical FormulasThe subscript to the right ofthe symbol of an elementtells the number of atomsof that element in onemolecule of the compound.
Molecular CompoundsMolecular compounds arecomposed of molecules andalmost always contain onlynonmetals.
Diatomic MoleculesThese seven elements occur naturally asmolecules containing two atoms.
Types of Formulas• Empirical formulas give the lowest whole-number ratio of atoms of each element in acompound.• Molecular formulas give the exact number ofatoms of each element in a compound.
Types of Formulas• Structural formulas show theorder in which atoms arebonded.• Perspective drawings also showthe three-dimensional array ofatoms in a compound.
Ions• When atoms lose or gain electrons, they becomeions.– Cations are positive and are formed by elements onthe left side of the periodic chart.– Anions are negative and are formed by elements onthe right side of the periodic chart.
Ionic BondsIonic compounds (such as NaCl) are generallyformed between metals and nonmetals.
Writing Formulas• Because compounds are electrically neutral, onecan determine the formula of a compound thisway:– The charge on the cation becomes the subscript on theanion.– The charge on the anion becomes the subscript on thecation.– If these subscripts are not in the lowest whole-numberratio, divide them by the greatest common factor.
Important groups in thePeriodic Table• Group IA – VIIA – known as The Main Group orRepresentative ElementsGroup IA- Alkali Metalsmost highly reactive metalsGroup IIA – Alkaline Earth Metalshighly reactive metalsGroups IIIA, IVA and VA - are not generally referred to by afamily nameGroup VIA – ChalcogensGroup VIIA – Halogenshighly reactive nonmetals
Group VIIIA• Group VIIIA – known as the Noble or InertGases- Noble Gases form few chemical compounds- He, Ne, and Ar do not form any compounds…all exist in nature as individual atoms
Group B Elements• Called the Transition Metals• They include many familiar metals ie. Fe, Cr, Ni, Sn…• They also include the noble metals Cu, Ag, and Au (Grp IB)-Noble metals are the rare metals of coins and jewelry.-Noble metals are comparatively chemically inert to rust and corrosion
Inner Transition Metals• Lanthanides – group between 57La and 72Ha• Actinides – group between 89La and 104Rf
The Periodic Table at a Glance• Allows us to classify an element very broadly into twoclasses:1. Metals2. Non-metals- the point of separation is the heavy stair step line- B, Si, Ge, As, Sb and Te which border the line are calledmetalloids or semimetals
Physical State and thePeriodic Table• the periodic table tells us about an elements physical state at T= 25oC orstandard reference temperature• Except for hydrogen all gaseous elements are found at the extreme rightof the tableHeN O F NeCl ArKrXeRn• There are only two liquids….metal Hg and non-metal Br2• ****all other elements are solids
• Of the non-metals many exist as diatomic moleculesrather than individual atoms• This includes all gaseous elements except the NoblegasesH2, N2,O2, F2, Cl2, Br2, I2• Natural form of P is P4• Most common form of S is S8• Carbon exists in three different forms calledallotropes- all three allotropes of carbon have differentproperties- the 3 allotropes are diamond, graphite andbuckminsterfullerene
A molecule is an aggregate of two or more atoms in adefinite arrangement held together by chemical forcesH2 H2O NH3 CH4A diatomic molecule contains only two atomsH2, N2, O2, Br2, HCl, COA polyatomic molecule contains more than two atomsO3, H2O, NH3, CH42.5
Formation of Cations from a Neutral Atom• Cations or positively charged ions are formed whenone or more e-are lost from a neutral atom to createa species with a + charge-1e-Na → Na++ 1e-11p+, 11e-11p+, 10e-sodium ion-2e-Mg → Mg+2+ 2e-12p+, 12e-12p+,10e-magnesium ionCations are named by retaining the elements original name
Formation of Anions from Neutral Atoms• Anions are negative ions formed when a neutralatom gains extra electrons to create a species with a- chargeCl + 1e-→ Cl –17p+, 17e-17p+, 18e-chloride ion• Anions are named by taking the root name of theatom and changing the ending to –ideBr -bromide ion O2-oxide ion S2-sulfide ion
A monatomic ion contains only one atomA polyatomic ion contains more than one atom2.5Na+, Cl-, Ca2+, O2-, Al3+, N3-OH-, CN-, NH4+, NO3-
13 protons, 10 (13 – 3) electrons34 protons, 36 (34 + 2) electronsDo You Understand Ions?2.5How many protons and electrons are in ?Al27133+How many protons and electrons are in ?Se78342-
Ionic Compounds• Compounds formed from the strongelectrostatic attraction between oppositelycharged ions (a cation and an anion)• The strong electrostatic attraction betweenoppositely charged ions is called an ionic bondie. Na++ Cl-→ NaClSodium ChlorideFormula represents the simplest ratio of cation to anion present in sodiumchloride. It is called a formula unit.
Formula Units• For Ca+2and Cl-For every Ca +2ion two Cl-ions are neededCaCl2 is the formula unit for calcium chloride
For ionic compounds, the formula is always the same as theempirical formulaAn empirical formula shows the simplest whole-number ratio of the atoms in a substance• the sum of the charges on the cation(s) and anion(s) in eachformula unit must equal zeroThe ionic compound NaCl2.6
Polyatomic Ions• Groups of atoms with a chargeNH4+SO42-NO3-
Metals vs. Nonmetals• Metals have a tendancyto lose electrons andform cations• Most Non-metals(except the NobleGases) gain electronsand form anions** The Periodic Table can tell us:The charge of the cation formed by a metalandThe charge of the anion formed by a non-metal
Group Numbers and Ion Charge• Group I Metals- all form cations with a +1charge• Group II Metals- all form cations with a +2charge• Group III Metals- all form cations with a+3 charge For Groups I-III charges of cation formed areidentical to the Group Number
Transition Metals• All the transition metals form cations withvarious positive charges• There is no easy way to predict the charge ofthe cation that will be formed• Common charges of the transition metals arefound in Table 2.3 on pg 61 ….they must bememorized
Non-Metals• Non-metals form negative ions by gainingelectrons• Group VII- all gain one electron to form1-ions• Group VI- atoms of elements in this groupgain two electrons to form 2-ions• Group V- atoms gain 3 electrons forminganions with a 3-charge
It is important to Remember:• Isolated atoms do notform ions on their own• Ions form when metallicelements combine withnon-metallic elements toform compounds calledionic compounds• Chemical compoundsmust have a net charge ofzero Therefore if a compoundcontains ions:1. There must be bothcations and anions present2. the numbers of cations andanions must be such thatthe net charge is zero**These simple rules are integral to your ability to writeFormulas for Ionic Compounds
Formulas of Metal / Non-metal BinaryIonic Compounds• When writing the formula for a binary compound it isimportant to remember:Total charge + total charge → cmpd with zeroon cation on anion net chargeie. Magnesium + chlorine → ?Plan: use periodic table to find charges on ions of elements• Metal and nonmetal combine to neutralize charge• Always write the metal first and the non-metal second• Use subscripts to indicate the relative number of ions or atoms• Consider - Mg+2, Cl1-– cross multiply charges– Mg2++ 2Cl1-= MgCl2magnesium chloride
More Examples ofMetal /Nonmetal Compounds• Barium + Oxygen → ?– barium an alkaline earth metal - +2– Ba - +2– O - -2• Ba+2+ O-2→ BaO (barium oxide)• Lithium + Nitrogen → ?- lithium an alkali metal - +1- Li- +1- N- +3• Li +1+ N-3→ Li3N (lithium nitride)