CHEM 217 Lecture Notes - Lecture 2: Molar Mass, Mass Spectrum, Nonmetal

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24 Jun 2018
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Unit 2
2 . 1 I M A G I N G A N D M O V I N G I N D I V I D U A L A T O M S
oScanning Tunneling microscopy (STM) is a technique that can imageand even move individual atoms and
molecules
oSharp electrode (electrical conductor) over a surface and measuring the resulting tunneling
current, the electrical current that flows between the tip of electrode & surgace even though the
two aren’t in physical contact
oTunnelling current is extremely sensitive to distance between tip and surface so it’s possible to
maintain a precise separation of appr. 2 atomic diameters between tip and surface by moving the
tip so as to keep the current constant – if the current starts to drop the tip is moved down
towards surface to increase the current & vice versa
oWhen the tip goes over an atom, it must move away from the surface to maintain constant
current.
oBy measuring the up and down movement of the tip as it scans a surface, the microscope creates
an image, showing the location of individual atoms on the surface
oLater found that STM can pick up and move atoms allowing structures and patterns to be made
one atom at a time
2 . 2 E A R L Y I D E A S
oLeucippus & Democritus : atoms ‘nothing exists except atoms and empty space; everything else is opinion
oPlato/Aristotle didn’t agree – thought everything was composed of fire air earth and water – their
influence made this the main way of thinking
oScientific reevaluation – evidence to support atomic ideas cam eabout
2 . 3 M O D E R N A T O M I C T H E O R Y & L A W S T H A T L E D T O I T
oThree important laws
o1. Law of conservation of Mass
oin a chemical reaction, matter is neither created nor destroyed – mass of reactants = mass of
products
o2. Law of definite proportions
oProust. Elements composing of a given compound always occur in fixed, definite proportions in al
samples of that compound
oIn contrast, components of a mixture could be in any proportion
oSummary
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All samples of a given compound, regardless of their source or how they were
prepared, have the same proportions of their constituent elements
Ex. decomposition of 18g water results in 16 g of O and 2g H or an o2 to h mass ratio of
8:1
This ratio holds for any sample of pure water
oLaw applies to any compound, hints at the idea that matter is composed of atoms. Definite
proportions of mass = definite proportions of atoms
o3. Law of multiple proportions
oJohn Dalton: when two elements (A,B) form two different compoinds, the masses of element B
that combine with 1g of element A can be expressed as a ratio of small whole numbers
oDalton suspected matter was composed of atoms so that when two elements A&B combined to
form more than 1 compound, an atom of A combined with 1,2,3, or more atoms (AB, AB2, AB3,
etc.)
oTherefore, the ratio of the masses of B that reacts with a fixed mass of A would always be small
whole number
oEx. CO2 & CO
CO2 – O to C 2.67:1
CO – O to C 1.33:1
The ratio of these two masses itself is a small whole number
o2.67/1.33 = 2
oWith the help of molecular models we can see why the ratio is 2:1 – CO2 has two O for every
carbon while CO only has 1
JOHN DALTON AND ATOMIC THEORY
oExplained the laws through atomic theory including
o1. Each element is composed of atoms
oall atoms of a given element have the same mass and physical/chemical properties that
distinguish them from the atoms of other elements
oatoms combine in simple, whole number ratios to form compounds
oatoms of one element can’t change into atoms of another. In a chemical reaction, atoms only
change their configuration
2 . 4 A T O M I C S T R U C T U R E
oatoms themselves are composed of even smaller more fundamental particles
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THE DISCOVERY OF ELECTRONS
oThomson. Experiments to prove properties of cathode rays.
oConstructed a tube (cathode ray tube) that was partially evacuated of air
oHigh electrical voltage between two electrodes and found a beam of particles (cathode rays) travelled
from the negatively charged cathode and the positively charged anode.
oParticles had the following properties:
oTravelled in straight lines
oIndependent of the composition from the material from which they originated (the cathode) and
they carried negative electrical charge
Electrical charge is a fundamental property of some of the particles that compose atoms
and results in attractive and repulsive forces called electrostatic forces between those
particles. The area around a charged particle where thses forces exist is called an
electric field
[+] -> <- [-]
[+] <- -> [+]
[-] <- -> [-]
[+] + [-] -> + -
(+1) + (-1) = 0
opositive and negative charges of exactly the same magnitude sum to
zero when combined
oThomson measured charge to mass ratio of the cathode ray particles by deflecting them using
electric and magnetic fields.
oThe value he measured -1.76 x 108 coulombs © per gram, implies that the cathode ray particle
was about 2000 times lighter (less massive) than hydrogen (lightest atom)
Had discovered (e-), a negatively charged low-mass particle present in all atoms.
oMillikan’s oil drop experiment
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Document Summary

1 i m a g i n g a n d m o v i n g i n d i v i d u a l a t o m s o. Scanning tunneling microscopy (stm) is a technique that can imageand even move individual atoms and molecules o o. Sharp electrode (electrical conductor) over a surface and measuring the resulting tunneling current, the electrical current that flows between the tip of electrode & surgace even though the two aren"t in physical contact. Tunnelling current is extremely sensitive to distance between tip and surface so it"s possible to maintain a precise separation of appr. By measuring the up and down movement of the tip as it scans a surface, the microscope creates an image, showing the location of individual atoms on the surface. Later found that stm can pick up and move atoms allowing structures and patterns to be made one atom at a time.