Atoms, Ions and MoleculesChapter 2
Daltons Atomic Theory (1808) Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in simple whole-number ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged All matter is composed of extremely small particles called atoms Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties
Atomic Theory of MatterAll matter is made of tiny indivisible particles called atoms.
This proposal has been verified experimentally. Single atoms of a variety of elements have been photographed with a scanning transmission electron microscope.
Atomic Theory of Matter2. Atoms of the same element are identical and atoms of a different element have different masses and chemical properties.
However you will learn that atoms of the same element can have different masses.
Atomic Theory of MatterAtoms of different elements combine in whole number ratios to form compounds
One molecule of water always consists of two atoms of hydrogen and one atom of oxygen.
Atomic Theory of Matter4. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.
Modern research has altered this proposal. Atoms are not indestructible and may lose their identity when split during nuclear reactions. However Daltons proposal remains true, for chemical reactions.
Law of Multiple ProportionsAtoms of two or more elements may combine in different ratios to produce more than one compound.
A compound always contains two or more elements combined in a definite proportion by mass.Law of Constant Composition
Law of Conservation of MassThe total mass of materials present after a chemical reaction is the same as the total mass before the reaction
This is the basis for which postulate?
Parts of an AtomJ. J. Thomson - English physicist. 1897
Made a piece of equipment called a cathode ray tube.
Determined the charge to mass ratio
It is a vacuum tube - all the air has been pumped out.
Thomsons Experiment-+Vacuum tubeMetal Disks
Thomson ExperimentPassing an electric current makes a beam appear to move from the negative to the positive end-+
-By adding an electric field
Thomsons Experiment+-By adding an electric field he found that the moving pieces were negative
What Did Thomson Demonstrate?Cathode rays:
Travel in straight lines
Are negatively charged
Are deflected by electric and magnetic fields
Thomsons ModelFound the electron
Said the atom was like plum pudding
A bunch of positive stuff, with the electrons able to be removed
Robert MillikanAmerican Scientist
Goal: Determine the charge on the electron to determine its mass
Chapter 2The Discovery of Atomic StructureCathode Rays and ElectronsConsider the following experiment:Oil drops are sprayed above a positively charged plate containing a small hole. As the oil drops fall through the hole, they are given a negative charge.Gravity forces the drops downward. The applied electric field forces the drops upward.When a drop is perfectly balanced, the weight of the drop is equal to the electrostatic force of attraction between the drop and the positive plate.
Millikans ExperimentX-raysX-rays give some drops a charge.
Millikans ExperimentSome drops would hoverFrom the mass of the drop and the charge on the plates, he calculated the mass of an electron
RadioactivityDiscovered by accidentFrench scientist Henri BequerelStudying pitchblende (oxides of uranium)Discovered that it spontaneously emits high energy radiationThree types alpha- helium nucleus (+2 charge, large mass)beta- high speed electrongamma- high energy light
Rutherfords ExperimentErnest Rutherford English physicist. (1910)
Believed in the plum pudding model of the atom.
Used uranium to produce alpha particles.
Rutherfords ExperimentAimed alpha particles at gold foil by drilling hole in lead block.
Since the mass is evenly distributed in gold atoms alpha particles should go straight through.
Used gold foil because it could be made atoms thin.
Rutherfords ExperimentWhen the alpha particles hit a florescent screen, it glows.
Rutherfords ExperimentLead blockUraniumGold FoilFlorescent Screen
What he expected
Why ??The alpha particles would pass through without changing direction very much
The negative charges were spread out evenly. Alone they were not enough to stop the alpha particles
What he got
How He Explained ItAtom is mostly empty
Small dense, positive piece at center
Alpha particles are deflected by it if they get close enough
How He Explained It
Density and the AtomSince most of the particles went through, it was mostly empty.
Because the pieces turned so much, the positive pieces were heavy.
Small volume, big mass, big density
This small dense positive area is the nucleus
Discovery of the NeutronJames Chadwick bombarded beryllium-9 with alpha particles, carbon-12 atoms were formed, and neutrons were emitted.+Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 764*Walter Boethe
Modern ViewThe atom is mostly empty space
Nucleus- protons and neutrons
Electron cloud- region where you might find an electron
Structure of AtomThere are two regions:The nucleus: with protons and neutrons Almost all the mass
Electron cloud- Most of the volume of an atomThe region where the electron can be found
Size of AtomAtoms are small.
Measured in picometers, 10-12 meters
Hydrogen atom, 32 pm radius
Size of AtomNucleus tiny compared to atom
IF the atom was the size of a stadium, the nucleus would be the size of a marble.
Radius of the nucleus near 10-15m.
Density near 1014 g/cm3
Subatomic ParticlesNUCLEUSELECTRONSPROTONSNEUTRONSNegative ChargePositiveChargeNeutralChargeATOM
Subatomic particlesElectronProtonNeutronNameSymbolChargeRelative mass Actual mass (g)e-p+no-1+101/1840119.11 x 10-281.67 x 10-241.67 x 10-24
Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutronsIsotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei
SymbolsFind the number of protons
number of neutrons
number of electrons
SymbolsFind the number of protonsnumber of neutronsnumber of electronsAtomic numberMass NumberBr80 35
Symbolsif an element has an atomic number of 34 and a mass number of 78 what is the number of protonsnumber of neutronsnumber of electronsComplete symbol
Atomic MassHow heavy is an atom of oxygen?There are different kinds of oxygen atoms.More concerned with average atomic mass.Based on abundance of each element in nature.Dont use grams because the numbers would be too small
Measuring Atomic MassUnit is the Atomic Mass Unit (amu)One twelfth the mass of a carbon-12 atom. Each isotope has its own atomic mass we need the average from percent abundance.
Calculating averagesYou have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks?
Total mass = 4 x 50 + 1 x 60 = 260 g
Average mass = 4 x 50 + 1 x 60 = 260 g 5 5
Calculating averagesAverage mass = 4 x 50 + 1 x 60 = 260 g 5 5 5Average mass = .8 x 50 + .2 x 6080% of the rocks were 50 grams20% of the rocks were 60 gramsAverage = % as decimal x mass + % as decimal x mass + % as decimal x mass +
Atomic MassCalculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu.
100908070605040302010034353637AbundanceMassMass spectrum of chlorine. Elemental chlorine (Cl2) contains only two isotopes: 34.97 amu (75.53%) and 36.97 (24.47%)AAM = (34.97 amu)(0.7553) + (36.97 amu)(0.2447)AAM = (26.412841 amu) + (9.046559 amu)AAM = 35.4594 amuCl-35Cl-37
Atomic MassMagnesium has three isotopes. 78.99% magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 26 with a mass of 25.9826 amu. What is the atomic mass of magnesium?
If not told otherwise, the mass of the isotope is the mass number in amu
The Periodic Table
Atoms and ionsAtoms are electrically neutral.
Same number of protons and electrons.
Ions are atoms, or groups of atoms, with a charge.
Atoms and IonsDifferent numbers of protons and electrons.
Only electrons can move.
Gain or lose electrons.
AnionA negative ion.Has gained electrons.Non metals can gain electrons.Charge is written as a super script on the right.F-1Has gained one electronO-2Has gained two electrons
CationsPositive ions.Formed by losing electrons.More protons than electrons.Metals form cations.K+1Has lost one electronCa+2Has lost two electrons